The impact of obesity on outcomes of midwife-led pregnancy and childbirth in a primary care population: a prospective cohort study

Authors

  • DOA Daemers,

    Corresponding author
    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
    • Correspondence: DOA Daemers, Faculty of Midwifery Education and Studies (AVM), PO Box 1256, 6201 BG Maastricht, the Netherlands. Email d.daemers@av-m.nl

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  • HAA Wijnen,

    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
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  • EBM van Limbeek,

    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
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  • LM Budé,

    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
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  • MJ Nieuwenhuijze,

    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
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  • MEA Spaanderman,

    1. Department of Obstetrics and Gynaecology, University Hospital Maastricht, Maastricht, the Netherlands
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  • RG de Vries

    1. Faculty of Midwifery Education and Studies Maastricht, Research Centre for Midwifery Science, Zuyd University, Maastricht, the Netherlands
    2. Caphri School for Public Health and Primary Care, Maastricht University, Maastricht, the Netherlands
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Abstract

Objective

To assess the impact of obesity on the likelihood of remaining in midwife-led care throughout pregnancy and childbirth.

Design

Secondary analysis of data from a prospective cohort study.

Setting

Dutch midwife-led practices.

Population

A cohort of 1369 women eligible for midwife-led care after their first antenatal visit.

Methods

First-trimester body mass index (BMI) was calculated as weight measured at booking divided by height squared. Obstetric data were retrieved from medical records. Multiple logistic regressions were performed to examine the effects of BMI classification on midwife-led pregnancies and childbirths.

Main outcome measures

Percentages of women remaining in midwife-led care throughout pregnancy and throughout childbirth.

Results

Of women in obesity classes II and III, 55% remained in midwife-led care throughout pregnancy and 30% remained in midwife-led care throughout birth. Compared with women of normal weight, women in obesity classes II and III had fewer midwife-led pregnancies (OR 0.38, 95% CI 0.21–0.69), and women who were overweight or in obesity class I had fewer midwife-led childbirths (OR 0.63, 95% CI 0.44–0.90; OR 0.49, 95% CI 0.29–0.84, respectively). Compared with women of normal weight, women who were obese had higher referral rates for hypertensive disorders (4 versus 14%), prolonged labour (4.6 versus 10.4%), and intrapartum pain relief (4 versus 10.4%). The women who were eligible for midwife-led birth and who were overweight or obese, had no more urgent referrals than women of normal weight. Women who were obese and who completed a midwife-led birth had no more adverse outcomes than women of normal weight, with the exception of higher rates of large for gestational age (LGA) babies (>97.7 centile; 12.1%, versus 1.9% in normal weight and versus 3.3% in overweight women).

Conclusions

Although fewer women who were obese remain in midwife-led care during pregnancy and childbirth, there was no increased risk of unfavourable birth outcomes for women who were obese and eligible for a midwife-led birth when compared with women of normal weight. This indicates that when primary care midwives use a risk assessment tool throughout pregnancy and childbirth they are able to safely assign women who are obese to either midwife-led or obstetrician-led care.

Introduction

Managing obesity in pregnancy and childbirth is a challenge for professionals who provide maternity care. Reported rates of obesity (body mass index, BMI ≥30 kg/m2) in pregnancy range from 1.8 to 25.3%.[1] Systematic reviews and meta-analyses indicate an association between maternal obesity and higher risks of adverse perinatal outcomes.[2-9]

In the Netherlands, maternal care is primarily provided by midwives. If risk factors arise or complications occur the midwife refers the woman to obstetrician-led care. This continuing risk assessment is based on the Obstetric Indication List (OIL), a national guideline that specifies indications for referral based on evidence and/or the consensus of professionals.[10] Obesity has not yet been included in the OIL, but class–III obesity (BMI ≥40 kg/m2) is mostly considered a medical condition that requires obstetrician-led care.[11] Given the evidence, the question arises as to whether obesity (BMI ≥30 kg/m2) should always be treated as a high-risk situation that requires obstetrician-led care.

Restraint should be exercised in extrapolating research results from hospital-based populations to primary care populations, however. The generalisability of study results depends on the resemblance of study settings and participants between the underlying studies and the target population.[12] Published reviews on obesity may therefore not be applicable to midwife-led care populations: the women in the included studies are, for the most part, recruited from those admitted to hospitals or the data are sampled from large hospital-based databases, relevant co-morbidities are not always excluded or corrected for, and the characteristics of the women and the care they received may differ. Not taking the above aspects into account could lead to an overestimation of the risks of obesity for women in primary care. Furthermore, as a solitary risk factor, (a high) BMI has not proven to be a useful single predictor for complications such as pre-eclampsia or abnormal fetal growth.[13, 14] Considering this, obesity can be viewed as a risk modifier of obstetric pathology (e.g. hypertensive disorders, gestational diabetes) that can be detected in a timely way in midwife-led care using an agreed-upon risk assessment tool.

The aim of this study is to examine how many women continue to receive midwife-led care throughout pregnancy and childbirth, and to gain insight into the likelihood of, and reasons for, referral to obstetrician-led care in relation to first-trimester BMI in a primary care population. In addition, we evaluate the effect of the current risk assessment tools on the number of urgent referrals and birth outcomes after midwife-led birth for women who are obese, compared with women of normal weight.

Methods

Study and participants

This study analysed data from the Kempen V study, a prospective cohort study on aspects of maternal wellbeing and obstetrical outcomes in relation to gestational thyroid function.[15] Women were included between July 2002 and November 2004. White, pregnant women in five midwife-led practices, living in and around the city of Eindhoven (the Netherlands), who were eligible for midwife-led care after their first antenatal visit at 10–12 weeks of gestation, were invited to participate. This study excluded women who subsequently had a miscarriage after the first antenatal booking, women who were immediately referred to obstetrician-led care because the pregnancy was considered to be of high risk at the outset (according to the OIL), and women referred to obstetrician-led care around 36 weeks of gestation because of a presumed increased delivery risk based on their obstetric history (e.g. for a previous caesarean section).[10] These criteria for referral to obstetrician-led care were agreed on at the outset of the pregnancy, and were not influenced by the maternal BMI. We also excluded women who were underweight because our focus was on obesity and because of the differences in pathophysiology between overweight and underweight in obstetrics.

Setting of the study

In the Netherlands, midwives are authorised to supervise physiological pregnancy, childbirth, and puerperium (primary care). Continued risk assessment by the midwife plays a crucial role in the maternity care system. This assessment starts at the first antenatal booking appointment, with an examination of a woman's medical, psychosocial, and obstetric history, and is continued throughout the perinatal period.[16] If an obstetric risk factor, as defined by the OIL, is identified during the pregnancy, the woman is then referred to obstetrician-led care. If not, she is eligible for primary care in a midwife-led practice. A woman can only choose for a midwife-led home or hospital birth if her pregnancy remains healthy up to the beginning of labour. During a midwife-led birth, the maternal and fetal condition and progress of the birth process are observed. Again, if risk factors arise or complications occur, these women are then referred to secondary care. The great majority of referrals during childbirth are not urgent because they are not life-threatening for the mother or the child, and often the family's own transport is used to reach an obstetric hospital.[17] In the event of an urgent referral, professional transport is available.

During the study period, obesity was not an indication for referral to obstetrician-led care in the OIL or in any other national or regional guideline. Therefore, the obstetric outcomes from this study reflect the impact of obesity itself. In the participating region, a selective screening protocol was used for gestational diabetes mellitus (GDM): this protocol includes indications for macrosomia and GDM in the obstetric history, maternal weight ≥90 kg at the initial antenatal check–up, the suspicion of a large for gestational age (LGA) fetus, high gestational weight gain (GWG), and glycosuria. If indications were present, an O'Sullivan test was performed at 24–28 weeks of gestation (or later in pregnancy), followed by an oral glucose tolerance test (OGTT) in the case of a value ≥7.8 mmol/l.

Data collection

Data on maternal characteristics (age, education, partner, alcohol, and smoking) were taken from questionnaires completed by women after their first antenatal booking. Maternal weight at 10–12 weeks of gestation was measured by the midwife using calibrated scales. Maternal height was self-reported. The first-trimester BMI was then calculated as weight at the first antenatal booking divided by height squared (kg/m2). BMI was classified according to the WHO classification: underweight (<18.5 kg/m2), normal weight (18.5–24.99 kg/m2), overweight (25–29.99 kg/m2), obese class I (30–34.99 kg/m2), obese class II (35–39.99 kg/m2), obese class III (≥40.00 kg/m2).[18]

All other data were retrieved from the midwife's obstetrical records and/or hospital records (in cases of referral to obstetrician-led care during pregnancy, childbirth, or the puerperium).

Outcomes

Primary outcomes were the completion of a ‘midwife-led pregnancy’ and a ‘midwife-led childbirth’. A midwife-led pregnancy is defined as a pregnancy supervised by a midwife without any referral to secondary care before the onset of term labour (defined as contractions and/or rupture of the membranes between 37 and 41+6 weeks of gestation). A midwife-led birth is a birth supervised by a midwife, either at home or in the hospital, without any referral to secondary care from the onset of labour until 2 hours after the birth of the placenta.

The secondary outcomes of this study are outlined in Figure 1. These include indications for referral during pregnancy and childbirth and birth outcomes in midwife-led and obstetric-led care. In this study, the referrals registered in the midwives’ records were based on the procedures of the Perinatal Registration in the Netherlands (PRN), which records only the main reason for referral. An indication for referral does not always reflect the actual outcome. For example, a referral for a small for gestational age (SGA) fetus during pregnancy does not always result in a neonate actually being born SGA. Concerning referrals during childbirth, we distinguished between referrals with and without urgency based on the list presented by Amelink et al.[17] The following referrals were coded as urgent: suspected fetal distress, placental problems (blood loss during birth, placenta praevia, placental abruption, vasa praevia, retained placenta), abnormal fetal presentation together with ruptured membranes (cord prolapse included), postpartum haemorrhage >1000 ml, intrapartum fetal death, Apgar score <7 at 5 minutes, respiratory problems (including meconium aspiration), and congenital malformations requiring immediate care. The authors have also added preterm labour in the active phase as an indication for an urgent referral. Urgent referrals were registered in both midwife-led home births and midwife-led hospital births.

Figure 1.

Outline of secondary outcomes in the study.

Birth outcome measures were stillbirth, induction of labour, use of pain relief, instrumental delivery (ventouse or forceps), caesarean section, shoulder dystocia, Apgar score <7 at 5 minutes, birth trauma, SGA <2.3 centile (SGA <2.3), LGA more than 97.7 centile (LGA >97.7), congenital malformations, and other neonatal problems within 24 hours postpartum. Stillbirth was defined as intrauterine death or death immediately postpartum. Use of pain relief comprised any analgesic or sedative medicine (oral, intramuscular, intravenous, spinal, or epidural) administered during labour. The definition of shoulder dystocia was a birth requiring more than one manoeuvre to deliver the infant's shoulders. Birth trauma covered Erb's Palsy, cephalic haematoma, clavicle fracture, and ‘other birth trauma’. SGA <2.3 and LGA >97.7 were determined according to the ‘Kloosterman centiles’, adjusted for gestational age at delivery, parity, and sex of the infant.[19] Other neonatal problems within 24 hours postpartum included problems or events concerning the neonate during the first day postpartum, as registered by the midwives (e.g. routine paediatric examination after instrumental delivery or symptoms of infection), except for Apgar score <7 at 5 minutes, birth trauma, and congenital malformations.

Analysis

To assess the influence of BMI category on midwife-led pregnancies and childbirths, two separate multiple logistic regressions were run using a backward selection method with the likelihood ratio criterion for exclusion. Maternal education, age, parity, smoking, alcohol use, having a partner, and the interactions between each BMI category and parity were included as covariates in the model on midwife-led pregnancy. In the model on midwife-led childbirth the same covariates were included, except for the interactions between BMI category and parity, because of the small numbers in the obese categories. Dummy variables were made for all of the (categorical) independent variables except for age, which was recorded as a continuous variable. A multiple linear regression was performed in order to study the association between BMI as a continuous variable and moment of referral in pregnancy, including the covariates as described above. To test the effect of BMI classification on the indications for referral and the various birth outcomes, we performed chi–square or Fisher's exact tests, where appropriate. Chi-square and Fisher's exact tests were also used to determine differences in the distribution of particular birth outcomes over the BMI categories, between those who were referred to obstetrician-led care during pregnancy and childbirth and those who were not; P < 0.05 was considered statistically significant. Odds ratios (ORs) with 95% confidence intervals (95% CIs) were used to quantify the risk. The analyses were conducted using spss 15.01.

Results

A total of 1601 (80.7%) women who met the inclusion criteria agreed to participate. After the exclusion of 97 women with missing data and an additional 135 women for a variety of other reasons, 1369 women were included in the analysis (Figure 2).

Figure 2.

Flow chart for study population.

Of all women in our study, 55.2% (n = 756) were of normal weight, 30.1% (n = 412) were overweight, 10.0% (n = 137) were in obese class I, and 4.7% (= 64) were in obese classes II and III, according to their first-trimester BMIs. Maternal characteristics were equally distributed over these BMI categories, with the exception of maternal education. Women in the obese categories tended to have a lower level of education than those in the normal weight and overweight categories (= 0.01; Table 1).

Table 1. Maternal characteristics of women with a normal and increased body mass index (BMI).
CharacteristicsNormal weight (n = 756) Mean (SD)/n (%)Overweight (n = 412) Mean (SD)/n (%)Obese class I (n = 137) Mean (SD)/n (%)Obese classes II–III (n = 64) Mean (SD)/n (%)Total (n = 1369) Mean (SD)/n (%)P-value
  1. Education: low – primary education or secondary education, not completed; middle – secondary education completed; high – degree at university level.

  2. GWG: gestational weight gain, defined as weight at 36 weeks of gestation minus weight at 12 weeks of gestation.

  3. Total: missing values for age, n = 259; weight gain, n = 81; education, n = 255; partner and smoking in pregnancy, n = 252; and alcohol, n = 251.

  4. a

    P < 0.05.

  5. b

    Chi-square test.

  6. c

    Analysis of variance (anova).

  7. d

    Conditions chi-square not fulfilled.

Demographic
Age30.10 (3.86)30.26 (3.38)30.40 (3.45)29.87 (4.06)30.17 (3.68)0.76c
Height1.70 (0.06)1.70 (0.06)1.69 (0.06)1.69 (0.06)1.70 (0.06)0.27c
No partner11 (1.8)4 (1.2)3 (2.5)0 (0.0)18 (1.6) d
Education
Low98 (16.2)52 (15.4)27 (22.7)15 (28.3)192 (17.2)0.01a,b
Middle276 (45.7)157 (46.4)61 (51.3)28 (52.8)522 (46.9) 
High23 (38.1)129 (38.2)31 (26.1)10 (18.9)410 (35.9) 
Lifestyle habits
Smoking74 (12.2)36 (10.7)17 (14.3)9 (16.7)136 (12.2)0.53b
Alcohol87 (14.3)48 (14.2)9 (7.6)5 (9.3)149 (13.3)0.18b
Obstetric
Parity
Nulliparous384 (50.8)194 (47.1)68 (49.6)30 (46.9)676 (49.4)0.65b
Multiparous372 (49.2)218 (52.9)69 (50.4)34 (53.1)693 (50.6) 
First-trimester weight64.80 (6.34)77.66 (6.94)91.46 (7.21)108.90 (9.78)73.40 (13.57)0.00a,c
First-trimester BMI22.40 (1.55)27.01 (1.40)31.99 (1.34)37.92 (2.65)25.48 (4.45)0.00a,c
GWG12.05 (3.66)11.14 (4.31)10.05 (4.67)6.54 (4.62)11.32 (4.20)0.00a,c

Of the 1369 women who were eligible for primary midwife-led care, 44.3% (n = 607) had a pregnancy, childbirth, and puerperium without the need for specialised obstetric care (Figure 3). During pregnancy, 27.8% (n = 380) of women were referred to secondary care, leaving 72.2% (n = 989) of women with an uncomplicated pregnancy. During labour, 38% of women (n = 376) were referred to secondary care, resulting in almost half of the initial sample having a midwife-led childbirth either at home or in the hospital (44.8%; n = 613). Another six (1%) referrals occurred in the puerperium because of neonatal indications.

Figure 3.

Number of midwife-led pregnancies, childbirths, and puerperia.

Midwife-led pregnancy and childbirth

The association between BMI category and the likelihood of remaining in midwife-led care for pregnancy and childbirth is presented in Table 2. Of the women of normal weight who started their pregnancy in primary care, 75% had a midwife-led pregnancy. This percentage decreased with increasing BMI, to 55% of women in obese classes II and III. Multiple logistic regression analyses showed that only women in obese classes II and III had fewer midwife-led pregnancies compared with women of normal weight (adjusted OR, aOR, 0.38, 95% CI 0.21–0.69). The proportion of variance explained by the model was 6.7% (Nagelkerke). BMI category alone explained 1.5% of the variance. The interaction between parity and BMI was tested but did not reach statistical significance. This means that the effect of multiparity on having a midwife-led pregnancy was the same for all BMI categories.

Table 2. Multiple logistic regression: predictor variables on midwife-led pregnancy and childbirth.
Predictor variablesMidwife-led pregnancy (n = 989)Midwife-led childbirth (n = 613)
n (%)AOR (95% CI)n (%)AOR (95% CI)
  1. Covariates included in the model were parity, age, alcohol, smoking, maternal education, having a partner, BMI classes and the interactions between each BMI class and parity (interactions only in model midwife-led pregnancy).

  2. AOR (adjusted odds ratio) >1 means a favourable effect on midwife-led pregnancy/childbirth.

  3. a

    Used as reference in log regression.

Multiparous (vs. nulliparous)   2.35 (1.76–3.15)   6.18 (4.39–8.70)
Age  0.98 (0.94–1.02) 0.99 (0.95–1.04)
Partner   2.23 (0.65–7.67)
Education
Low 1a 
Middle  1.45 (1.01–2.10)  
High  1.72 (1.15–2.55)  
BMI
Normal weight (n = 756)570 (75.4)1a365 (48.3)1a
Overweight (n = 412)294 (71.4)0.81 (0.60–1.10)182 (44.2) 0.63 (0.44–0.90)
Obese class I (n = 137)90 (65.7)0.66 (0.43–1.01)47 (34.3) 0.49 (0.29–0.84)
Obese classes II–III (n = 64)35 (54.7) 0.38 (0.21–0.69) 19 (29.7)0.48 (0.21–1.12)

In the group of women who started their labour in primary care, the likelihood of remaining in midwife-led care during childbirth decreased with increasing BMI categories. Compared with women of normal weight, women who were overweight or in obese class I had a lower likelihood of having a midwife-led childbirth: aOR 0.63 (95% CI 0.44–0.90) and aOR 0.49 (95% CI 0.29–0.84), respectively. Of women in obese classes II and III, 30% had a midwife-led childbirth. The proportion of variance explained by this model was 22.7% (Nagelkerke). BMI category alone explained 1.7% of the variation in the occurrence of a midwife-led birth.

Reasons for referral

If women were referred to obstetrician-led care during pregnancy, linear regression showed that BMI as a continuous variable had no effect on when the referral occurred (regression coefficient = 0.07, = 0.17). The four most frequent reasons for referral in pregnancy were hypertensive disorders (n = 89, 23.4%), post-term gestation (n = 66, 17.4%), abnormal fetal positions at term pregnancy (n = 58, 15.3%), and preterm labour (n = 45, 11.8%), accounting for 68% of all referrals during pregnancy (n = 380). During childbirth, 71% of all referrals were for meconium-stained amniotic fluid (n = 105, 27.9%), prolonged labour (= 56, 14.9%), prelabour rupture of membranes (PROM) (n = 54, 14.4%), and requests for pain relief (n = 50, 13.3%). BMI was found to have a significant effect on three of the reasons for referral to obstetrician-led care during pregnancy and childbirth. Referrals for hypertensive disorders increased with increasing BMI category: 4% for women of normal weight, 7.5% for women who were overweight, and 14% for women in obesity classes I–III [χ2(2) = 26.94; < 0.001]. Referrals for prolonged labour [χ2(2) = 6.55; = 0.04] and requests for pain relief [χ2(2) = 8.73; = 0.01] doubled in women who were obese, compared with women who were normal weight or overweight. None of the referrals in the puerperium [respiratory problems (= 3), birth at 36 weeks of gestation (n = 1), and anal atresia (n = 2)] were for women who were obese.

Birth outcomes

Table 3 summarises the birth outcomes of the total study population eligible for midwife-led care after their first check–up, and irrespective of any referral during pregnancy and childbirth. Of all birth outcomes, there was a significant association between BMI classification and induction of labour [χ2(3) = 19.07; < 0.001], pain relief administered [χ2(3) = 24.99; < 0.001], caesarean section [χ2(3) = 9.10, < 0.05], and LGA >97.7 [χ2(3) = 25.11; < 0.001].

Table 3. Birth outcomes in study population in relation to BMI classification.
Birth outcomesNormal weight n = 756 n (%)Overweight n = 412 n (%)Obese class I n = 137 n (%)Obese class II–III n = 64 n (%)Total n = 1369 n (%)χ2P-value
  1. a

    < 0.001.

  2. b

    < 0.05.

  3. c

    Fisher's exact test obese vs. non-obese.

Stillbirth3 (0.4)3 (0.7)006 (0.4) c 0.39
Induction of labour78 (10.3)58 (14.1)30 (21.9)14 (21.9)180 (13.1)19.070.000a
Pain relief85 (11.2)55 (13.3)35 (25.5)15 (23.4)190 (13.9)24.990.000a
Instrumental delivery68 (9.0)34 (8.3)15 (10.9)8 (12.5)125 (9.1)1.820.61
C-section69 (9.1)42 (10.2)23 (16.8)10 (15.6)144 (10.5)9.100.03b
Shoulder dystocia12 (1.6)11 (2.7)3 (2.2)2 (3.1)28 (2.0) c 0.39
Apgar score <7 at 5 min10 (1.3)3 (0.7)01 (1.6)14 (1.0) c 0.37
Birth trauma1 (0.1)0001 (0.1) c 0.85
SGA < 2.36 (0.8)3 (0.7)2 (1.5)011 (0.8) c 0.50
LGA > 97.719 (2.5)17 (4.1)14 (10.2)7 (10.9)57 (4.2)25.110.000a
Cong malformations15 (2.0)9 (2.2)5 (3.6)029 (2.1) c 0.43
Neonatal problems < 24 hours57 (7.5)32 (7.8)8 (5.8)8 (12.5)105 (7.7)2.780.43

In order to acquire more in-depth information about the effect of BMI category on childbirth, we examined birth outcomes of women who were referred and who were not referred to secondary care in pregnancy by BMI category. In the group who experienced a referral during pregnancy and had obstetrician-led care during birth, induction of labour increased with increasing BMI categories [χ2(2) = 7.32; < 0.05]. Significantly more requests for intrapartum pain relief and more LGA >97.7 babies occurred in women from obesity classes I–III, compared with other BMI categories, in both the referred and non-referred women. Among the women not referred during pregnancy, a significant association was found between BMI category and caesarean section [χ2(2) = 6.75; < 0.05]. All six cases of stillbirth happened during pregnancy, none of them to women who were obese.

Unfavourable outcomes in midwife-led birth

Looking at the effect of risk assessment in relation to obesity, we specifically studied two unfavourable outcomes: urgent referrals during midwife-led birth and adverse birth outcomes after midwife-led birth. No significant effect was found for BMI category on the urgency of referrals during or immediately after childbirth [χ2(2) = 1.4; = 0.50].

Of all the birth outcomes of women with a midwife-led birth (i.e. women who were not referred, NR; Table 4), only LGA >97.7 was affected by BMI category [χ2(2) = 17.6, P < 0.001]. We thoroughly investigated the 41 caesarean sections among women referred during childbirth and found that none were performed for urgent fetal or maternal indications diagnosed in primary care. We found no significant difference in the distribution of the relevant birth outcomes over the BMI categories between the referred and non-referred groups [χ2**** in Table 4].

Table 4. Birth outcomes of women referred and not referred during birth, in relation to normal weight, overweight, and obesity
Birth outcomesNormal weight R (n = 205) NR (n = 365) n (%)Overweight R (n = 112) NR (n = 182) n (%)Obese classes I–III R (n = 59) NR (n = 66) n (%)Total R (n = 376) NR (n = 613) n (%)χ2c R NR P χ 2 d P
  1. a

    < 0.001.

  2. b

    Fisher's exact test for obese versus non-obese.

  3. c

    χ2 on association between BMI classification and particular birth outcome within R (women referred during childbirth to secondary obstetric care) and NR (women not referred during childbirth, i.e. midwife-led care) groups, respectively.

  4. d

    χ2 on the distribution of a particular birth outcome over BMI classes between R and NR groups, respectively.

Stillbirth
R0000
NR0000  
Intrapartum pain relief
R53 (25.9)36 (32.1)24 (40.7)113 (30.1)5.120.08
NR  
Instrumental delivery
R50 (24.4)28 (25.0)15 (25.4)93 (24.7)0.030.98
NR  
Caesarean section
R16 (7.8)16 (14.3)9 (15.3)41 (10.9)4.490.11
NR  
Shoulder dystocia
R5 (2.4)5 (4.5)1 (2.3)11 (2.9) b 0.46 b 0.38
NR4 (1.1)2 (1.1)2 (3.0)8 (1.3) b 0.21  
Apgar score <7 at 5 minutes
R8 (3.9)1 (0.9)1 (1.7)10 (2.7) b 0.52
NR0000  
Birth trauma
R1 (0.5)001 (0.3) b 0.84
NR0000  
SGA <2.3 centile
R1 (0.5)001 (0.3) b 0.84 b 0.83
NR3 (0.8)1 (0.5)1 (1.5)5 (0.8) b 0.44  
LGA >97.7 centile
R6 (2.9)6 (5.4)3 (5.1)15 (4.0) b 0.431.390.50
NR7 (1.9)6 (3.3)8 (12.1)21 (3.4)17.60.00a  
Congenital malformations
R1 (0.5)1 (0.9)02 (0.5) b 0.71 b 0.60
NR6 (1.6)6 (3.3)4 (6.1)16 (2.6) b 0.08  
Neonatal problems <24 hours
R16 (7.8)9 (8.0)7 (11.9)32 (8.5)1.020.60 b 0.18
NR6 (1.6)2 (1.1)08 (1.3) b 0.40  

Discussion

Main findings

In this study of 1369 women in midwife-led care, 55% of women in obesity classes II and III had no need for referral to secondary care during pregnancy, and 30% remained in midwife-led care throughout pregnancy and childbirth. Fewer women who were obese remained in midwife-led care throughout pregnancy and childbirth; however, no increased risk of unfavourable birth outcomes was noted for women who were obese and eligible for a midwife-led birth compared with women of normal weight when risk assessment occurred at regular intervals throughout pregnancy and childbirth, except for an increase in LGA babies with higher maternal BMI. Risk assessment was based on an agreed risk assessment tool without any particular guideline on maternal weight or BMI.

Strengths and limitations

We believe that this study offers new evidence on obesity in midwife-led care, a topic that has received only limited study to date. Our study measured weight (i.e. not self-reported), which enhances the accuracy of women's calculated first-trimester BMI. The timing of the data collection (2002–2005) could be seen as a strength given increasing obesity interventions between then and now. By using data from a period before the implementation of guidelines on obesity that now influence risk assessment and outcomes, we were able to examine the impact of obesity itself on the course of pregnancy and childbirth. Regarding the prevalence of obesity, these data can still be considered current: the prevalence of obesity among women of 20 years and older in the Netherlands has not changed between 2002 and 2011, according to the latest national figures (10.8%). Within this period, this figure has varied between 10.8 and 12.7%.[20, 21] The generalisability of our study is limited because only white women were included. Because some adverse outcomes occurred infrequently in this low-risk population, we were unable to study the association between these outcomes and BMI category.

In our study we combined obesity classes II and III (with prevalences of 10.0 and 4.7%, respectively) because of the small number of participants. Because normality decreases with increasing BMI class, caution should be taken when extrapolating our results to obese classes II and III (≥35 kg/m2) in populations with other proportions of obese women in class II versus class III. There is not a linear relationship between GWG and our primary outcomes: midwife-led pregnancy and childbirth. The outcome variables are composites that may have been negatively affected by both inadequate and excessive GWG (i.e. a U–shaped relationship). Therefore, we were unable to correct for GWG as a potential confounding factor in our logistic regression models.

Interpretation in light of other evidence

Although the decrease in midwife-led pregnancies and childbirths with increasing BMI categories could be expected based on the published literature,[2-9] we have established that a considerable number of women that were obese (BMI ≥30 kg/m2) had an uncomplicated pregnancy and/or childbirth, and could remain in primary care. The finding that the association between the obesity classes II-III and a lower likelihood of having a midwife-led birth did not reach statistical significance can be explained by the small number of participants in this obesity category.

Women who are overweight and obese have been reported to have higher risks of hypertensive disorders, prolonged labour, intrapartum pain relief, induction of labour, caesarean section, and LGA >97.7 centile, which is consistent with the findings from our study.[5, 7-9] In line with previous studies, our data show no association between BMI class and referral for (spontaneous) preterm birth.[7, 22, 23] Unlike the review by Heslehurst (2008), we found no association between BMI classes and meconium-stained amniotic fluid, post-term gestation, instrumental delivery, low Apgar score, and other neonatal problems. The difference between the conclusions of our study and those found by Heslehurst et al. are not the result of insufficient power in the studies. The groups of participants with meconium-stained amniotic fluid, post-term gestation, instrumental delivery, and ‘other neonatal problems’ were large enough to allow us to discover a statistically significant association with BMI category, but we found no such association. Very few neonates in our study had an Apgar score <7 at 5 minutes, but a Fisher's exact test showed no significant difference in the likelihood of low Apgar scores for neonates born to women who were obese and for those born to women who were not obese (in both the total population and in the referred group after midwife-led birth). The small number of low Apgar scores in our population is likely to result from the fact that women in our study were at low risk, and those that were referred had low rates of interventions (e.g. 10.5% caesarean sections and 9.1% instrumental delivery). Only seven women were referred to secondary care because of GDM, with the need for treatment with insulin (women with GDM treated only by diet were not referred). The frequency of a referral for GDM with insulin in this population was too small to be able to draw any conclusion about an association between BMI category and GDM as established in previous research.[4] The finding that there was no association between BMI and the timing of the referral limits the possibility of using BMI category as a possible proxy indicator for subsequent pregnancy complications.

The only unfavourable outcome after midwife-led birth was a higher risk of a neonate with LGA >97.7 centile among women who were obese. LGA and macrosomia (15% of the newborns in our study had birthweights >4000 g) are related to both short-term complications (e.g. mechanical birth problems, obstetric interventions, maternal/neonatal problems as a result of birth problems/interventions, and neonatal hypoglycaemia)[24-31] and long-term complications (e.g. metabolic syndrome in childhood,[32] childhood leukaemia,[33] and adult cancers).[34] Thus, prevention of LGA might be advisable from an obstetric and public health point of view. Two possible options should be considered for changing our risk assessment guidelines. First, a stricter GDM screening protocol, especially among women who are overweight or obese, should be considered. Motherhood GDM increases the risk of macrosomia/LGA in the offspring,[35] and exacerbates some of the short- and long-term problems related to LGA and macrosomia.[24, 27, 31, 32, 36] A stricter GDM screening protocol may diagnose more women with GDM, and the subsequent control of maternal glucose levels may result in fewer LGA infants.[37] In the Netherlands there is a continuing debate about using an unselective (all women) versus a selective (women who fulfil the relevant criteria) screening protocol for GDM.[38] In this study, a selective GDM screening protocol was used. Second, further research should be undertaken to test whether managing a healthy GWG could prevent LGA. Earlier research on this same study population has shown a higher risk of excessive GWG in women who are overweight or obese,[39] and we know that LGA is also related to excessive GWG.[40]

Conclusion

Although a higher maternal BMI category is associated with a reduced likelihood of remaining under midwife-led care in pregnancy and childbirth, our study indicates that a system of risk assessment throughout pregnancy and childbirth enables primary care midwives to select women with higher BMIs in a way that allows for appropriate care. Obesity is defined in the medical literature as a high-risk situation requiring the need for high-risk care. In our study, a considerable number of women who were obese remained in primary care throughout pregnancy and childbirth without unfavourable outcomes for mother and child. From a client's perspective, these are important results given the knowledge that labelling women as being at ‘high risk’ might result in more unnecessary interventions, negative psychological sequelae,[41] and reduced choices throughout pregnancy and around the birth.[42]

As there is increasing interest in the creation of maternal care pathways for women who are obese,[7, 43] our findings are also of interest for professionals outside the Netherlands. Although our study contributes to the body of knowledge necessary to underpin care for women who are obese, further research is needed. A larger study would enable a more detailed evaluation of infrequent birth outcomes and midwife-led risk assessment in higher BMI classes, and would allow adjustments for client characteristics such as parity, GWG (inadequate, adequate, and excessive), ethnicity, lifestyle, and social economic status. In order to prevent LGA, Dutch risk assessment guidelines should aim at a stricter screening protocol on GDM, and at optimising GWG in women who are overweight or obese.

Disclosure of interests

The authors declare that they have no conflicts of interest.

Contribution to authorship

D.O.A.D., H.A.A.W., E.B.M.v.L., and M.J.N. designed the study; H.A.A.W. collected the data; D.O.A.D. conducted the statistical analyses; L.M.B. and E.B.M.v.L. contributed to the planning and execution of the statistical analyses; D.O.A.D. performed the literature review, interpreted the results, and wrote the article. H.A.A.W., E.B.M.v.L., L.M.B., M.J.N., M.E.A.S., and R.G.d.V. contributed to the interpretation of the results and the drafting and revision of the article. All authors approved the final version.

Details of ethics approval

The Kempen V study was approved by the Medical Ethical Committee of the Máxima Medical Centre in Veldhoven, the Netherlands (no. 0116).

Funding

The work in this study was conducted with the support of the Faculty of Midwifery Education and Studies at Zuyd University, Maastricht, the Netherlands.

Acknowledgements

Many thanks to Professor Victor Pop for his contribution to the draft of this study and to Femke van Son, midwife, for her contribution in collecting the data.

Obesity substantially impacts low-risk pregnancies

  • MA Turrentine

  • Clinical Research, Obstetrics & Gynecology, Kelsey-Seybold Clinic and Baylor College of Medicine and UT-Health Medical School, Department of Obstetrics & Gynecology, Houston, TX, USA

Linked article This article is a mini commentary on Daemers DOA et al., pp 1403–13 in this issue. To view this article visithttp://dx.doi.org/10.1111/1471-0528.12684.

Maternal obesity is a significant risk factor for adverse outcomes during pregnancy. The problems related to the management of obesity in pregnancy are many. In this issue of BJOG, Daemers et al. present a secondary analysis of a multicentre, prospective cohort study evaluating low-risk women managed by midwife practices in the Netherlands in relation to gestational thyroid function and depression (Pop et al. Thyroid 2006;16:485–92). Data collected from this trial was used to estimate the effect of obesity on the woman's continuation of care and delivery in a minimal-risk setting. More than 1300 women were followed over a 2-year time span. A decreasing trend was noted in continued prenatal care under the supervision of a midwife alone as the body mass index (BMI) of women increased, but this was only significantly different from women of normal weight for BMIs ≥35 kg/m2 (aOR 0.38, 95% CI 0.21–0.69). However, the likelihood of having a midwife-managed birth significantly diminished if women were overweight (BMI 25–29.9 kg/m2, aOR 0.63, 95% CI 0.44–0.90) or obese (BMI 30–34.9 kg/m2, aOR 0.49, 95% CI 0.29–0.84), when compared with women of normal weight. A women's BMI had a significant interaction for referrals to physician providers, with a greater number of transfers for hypertension, prolonged labour, and request for intrapartum pain relief in women who were obese. Although 68% of women who were overweight and/or obese continued their care with a midwife provider, only 40% had their delivery performed in this setting.

Although the study was large, some of the subgroups were relatively small (e.g. women with a BMI ≥35 kg/m2), and therefore some important differences may not have been detected as a result of a lack of power in individual categories. In a subsequent study (Daemers et al. Midwifery 2013;29:535–41) using this data, the authors observed that 43% of overweight and obese women exceeded the 2009 Institute of Medicine guidelines for total maternal gestational weight gain; however, no stratification for this potential effect was able to be performed in the current study. Finally, no standardized screening for gestational diabetes mellitus was employed during the investigational period. One must be cautious when evaluating these observed associations. The causal implications of these data will need to be confirmed.

With a large number of women beginning pregnancy as either overweight or obese, and gaining more weight than suggested by recommended guidelines, the manifestation of select maternal morbidities will only become more prevalent. The current study suggests that the public health consequences of obesity during pregnancy in these low-risk settings are potentially substantial.

Disclosure of interests

I have no financial disclosures to declare.

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