The impact of body mass index on maternal and neonatal outcomes: a retrospective study in a UK obstetric population, 2004–2011

Authors


Correspondence: Dr V Holmes, Centre for Public Health, School of Medicine, Dentistry and Biomedical Sciences, Queen's University, Belfast, ICS Block B, Grosvenor Road, Belfast, BT12 6BA, UK. Email v.holmes@qub.ac.uk

Abstract

Objective

To assess the prevalence of overweight and obesity, and the impact of body mass index (BMI) on maternal and neonatal outcomes, in a UK obstetric population.

Design

Retrospective study.

Setting

A tertiary referral unit in Northern Ireland.

Population

A total of 30 298 singleton pregnancies over an 8-year period, 2004–2011.

Methods

Women were categorised according to World Health Organization classification: underweight (BMI < 18.50 kg/m2); normal weight (BMI 18.50–24.99 kg/m2; reference group); overweight (BMI 25.00–29.99 kg/m2); obese class I (BMI 30.00–34.99 kg/m2); obese class II (BMI 35–39.99 kg/m2); and obese class III (BMI ≥ 40 kg/m2). Maternal and neonatal outcomes were examined using logistic regression, adjusted for confounding variables.

Main outcome measures

Maternal and neonatal outcomes.

Results

Compared with women of normal weight, women who were overweight or obese class I were at significantly increased risk of hypertensive disorders of pregnancy (OR 1.9, 99% CI 1.7–2.3; OR 3.5, 99% CI 2.9–4.2); gestational diabetes mellitus (OR 1.7, 99% CI 1.3–2.3; OR 3.7, 99% CI 2.8–5.0); induction of labour (OR 1.2, 99% CI 1.1–1.3; OR 1.3, 99% CI 1.2–1.5); caesarean section (OR 1.4, 99% CI 1.3–1.5; OR 1.8, 99% CI 1.6–2.0); postpartum haemorrhage (OR 1.4, 99% CI 1.3–1.5; OR 1.8, 1.6–2.0); and macrosomia (OR 1.5, 99% CI 1.3–1.6; OR 1.9, 99% CI 1.6–2.2), with the risks increasing for obese classes II and III. Women in obese class III were at increased risk of preterm delivery (OR 1.6, 99% CI 1.1–2.5), stillbirth (OR 3.0, 99% CI 1.0–9.3), postnatal stay > 5 days (OR 2.1, 99% CI 1.5–3.1), and infant requiring admission to a neonatal unit (OR 1.6, 99% CI 1.0–2.6).

Conclusions

By categorising women into overweight and obesity subclassifications (classes I –III), this study clearly demonstrates an increasing risk of adverse outcomes across BMI categories, with women who are overweight also at significant risk.

Introduction

Obesity has become an epidemic throughout the world. Worldwide, obesity rates have doubled in the last 30 years,[1] with rates also increasing among pregnant women.[2, 3] Maternal obesity has significant health implications, contributing to increased morbidity and mortality for both mother and baby. A higher proportion of women who die in pregnancy/postpartum are obese.[4, 5]

Antenatally, obesity increases the risk of miscarriage, gestational diabetes mellitus (GDM), gestational hypertension, thromboembolism, and pre-eclampsia.[6] Obesity is associated with poor labour outcomes, with obese women less likely to go into labour spontaneously, more likely to have prolonged pregnancies and have their labour induced, and less likely to achieve a normal delivery, being at increased risk of caesarean section.[2, 7-13] Postnatally, obese women are less likely to breastfeed successfully, have a longer postnatal stay in hospital, and are at risk of postnatal infections.[7-10, 14] Obesity is also associated with a higher risk of adverse neonatal outcomes, including stillbirth, congenital anomalies, neonatal intensive care admission, and neonatal death.[2, 7-9]

In addition, there are long-term consequences of obesity in pregnancy. Obese women tend to be heavier with each subsequent pregnancy.[15] These women are more likely to remain obese adults, with all the associated increased risks of obesity.[16, 17] Furthermore, long-term studies demonstrate that having an obese mother increases the risk of a child growing up to be obese themselves.[18, 19] The impact that obesity in pregnancy has on the long-term health of society as a whole, is therefore immeasurable.

Several UK studies have looked at adverse outcomes in pregnancy in relation to obesity.[2, 10, 20-25] However, none of these studies have looked at adverse outcomes in relation to each of the World Health Organisation (WHO) body mass index (BMI) classifications, where obesity is subdivided into obese class I, II, and III (morbid obesity). The objective of this study was to assess the prevalence of overweight and obesity, and to investigate the impact of rising BMI using the WHO classification on maternal and neonatal outcomes.

Methods

This retrospective study used data from births between January 2004 and December 2011 within a tertiary referral unit, with over 5000 births per year in Northern Ireland. This study was designed as a clinical audit and therefore did not require approval from a Research Ethics Committee. Local audit committee approval was obtained to use the data routinely collected using the Northern Ireland Maternity System (NIMATS), a computerised clinical database for recording information on an individual pregnant women's medical history and pregnancy outcomes, including the antenatal, intranatal, and immediate postnatal (until discharge from hospital) periods.

Anonymised data on 43 267 babies were collated with data retrieved relating to each baby delivered within the 8-year study period. Exclusion criteria were: births at less than 24 weeks of gestation (n = 99); multiple pregnancies (n = 1724); BMI recorded after 16 weeks of gestation (n = 8986); and no BMI recorded (2160). The final cohort consisted of 30 298 cases (Figure 1).

Figure 1.

Cohort selection.

Data from NIMATS were transferred into spss 17. The BMIs (kg/m²) were calculated from the heights and weights measured during the antenatal booking visits. Women were categorised using the WHO classification: underweight (BMI < 18.50 kg/m²); normal weight (BMI 18.50–24.99 kg/m²; reference group); overweight (BMI 25.00–29.99 kg/m²); obese class I (BMI 30–34.99 kg/m²); obese class II (BMI 35–39.99 kg/m²); and obese class III (BMI ≥ 40 kg/m²).[26] Social deprivation scores were calculated using the Northern Ireland Multiple Deprivation Measure,[27] with women in the bottom third decile considered to be socially deprived. Data are expressed as frequency (n) and percentages (%) or means and standard deviations (SDs). Logistic regression was used to calculate odds ratios (ORs) for categorical variables, with the normal BMI group as the standard reference population. A confidence interval of 99% (99% CI) was used, and P < 0.01 was considered significant. All variables were adjusted for age, parity, year of birth, social deprivation, and smoking. In addition, induction of labour, emergency caesarean section, elective caesarean section, and preterm and post-term birth were adjusted for pre-gestational diabetes mellitus and essential hypertension; birthweight was adjusted for gestational age and gender.

Results

An early pregnancy BMI (at ≤16 weeks of gestation) was available for 93.3% of women who met other inclusion criteria. Within this cohort, women were categorised as underweight (2.8%), normal weight (52.5%), overweight (27.8%), obese class I (11.0%), obese class II (3.9%), and obese class III (1.9%). Demographic and clinical characteristics are outlined in Table 1. Compared with women of normal weight, a higher proportion of underweight women were younger, nulliparous, unmarried, smokers, and socially deprived. By contrast, as BMI increased, so did maternal age and parity.

Table 1. Demographic and clinical characteristics by BMI category (kg/m2)
 Underweight BMI < 18.50 n = 862Normal BMI 18.50–24.99 n = 15 908Overweight BMI 25.00–29.99 n = 8415Obese class I BMI 30.00–34.99 n = 3333Obese class II BMI 35.00–39.99 n = 1194Obese class III BMI ≥ 40.00 n = 586
  1. Data are n (%), unless otherwise indicated.

Age, mean (SD), years26.2 (6.2)29.7 (6.0)30.6 (5.7)30.4 (5.7)30.5 (5.5)31.1 (5.5)
Socially deprived477 (55.3)6096 (38.3)3259 (38.7)1501 (45.0)563 (47.2)262 (44.7)
Planned pregnancy395 (45.8)10 665 (67.0)5853 (69.6)2267 (68.0)808 (67.7)361 (61.6)
Married327 (37.9)8917 (56.1)4903 (58.3)1864 (55.9)642 (53.8)304 (51.9)
Nulliparous511 (59.3)7600 (47.8)3398 (40.4)1281 (38.4)418 (35.0)215 (36.7)
Smoker274 (31.8)3122 (19.6)1625 (19.3)699 (21.0)267 (22.4)120 (20.5)
Alcohol during pregnancy36 (4.2)709 (4.5)372 (4.4)135 (4.1)41 (3.4)18 (3.1)
Pre-existing diabetes0 (0)84 (0.5)112 (1.3)54 (1.6)35 (2.9)26 (4.4)
Essential hypertension1 (0.1)73 (0.5)78 (0.9)65 (2.0)39 (3.3)30 (5.1)

Antenatal outcomes are outlined in Table 2. The risk for GDM increased across the overweight and obese categories, to an OR of 8.5 (99% CI 5.7–12.9) for women classified as obese class III. Likewise, the risk of hypertensive disorders of pregnancy also increased in relation to an increase in BMI classification, to an OR of 6.6 (99% CI 4.9–8.9) for women in obese class III. Only the women who were underweight were at increased risk of anaemia (OR 1.3, 99% CI 1.0–1.7), and there was no statistically significant association between being underweight and any other antenatal outcome. There was no statistically significant association between any of the BMI categories and the following antenatal outcomes: placenta praevia, antepartum haemorrhage, placental abruption, or thromboembolism.

Table 2. Maternal outcomes by BMI category (kg/m2)
 Underweight BMI <18.50 n = 862Normal BMI 18.50–24.99 n = 15 908Overweight BMI 25.00–29.99 n = 8415Obese class I BMI 30.00–34.99 n = 3333Obese class II BMI 35.00–39.99 n = 1194Obese class III BMI ≥ 40.00 n = 586P (unadjusted/adjusted)
  1. All variables are adjusted for age, parity, social deprivation, smoking, and year of birth. Values presented as OR (99% CI), with < 0.01 considered to be significant (shown in bold). See Table S1 for data presented with 95% CIs.

  2. a

    Analysis of vaginal births only (= 20 604).

  3. b

    Wound problems following caesarean section (= 9694).

Gestational diabetes mellitus6680.3 (0.4–1.6) 0.0601.00 1.7 (1.3–2.3) <0.001 3.7 (2.8–5.0) <0.001 6.0 (4.2–8.5) <0.001 8.5 (5.7–12.9) <0.001 <0.001/<0.001
Hypertensive disorders of pregnancy18090.9 (0.5–1.5) 0.5101.00 1.9 (1.7–2.3) <0.001 3.5 (2.9–4.2) <0.001 5.0 (4.0–6.4) <0.001 6.6 (4.9–8.9) <0.001 <0.001/<0.001
Anaemia3981 1.3 (1.0–1.7) 0.007 1.000.9 (0.8–1.0) 0.0791.0 (0.8–1.1) 0.6180.8 (0.7–1.1) 0.0730.8 (0.5–1.1) 0.050 <0.001/<0.001
Placenta praevia1611.1 (0.3–4.2) 0.8351.000.9 (0.5–1.4) 0.3830.8 (0.4–1.6) 0.4520.8 (0.3–2.4) 0.6110.227/0.993
Other antepartum haemorrhage4010.9 (0.4–1.9) 0.7721.000.9 (0.6–1.2) 0.2170.7 (0.4–1.1) 0.0540.6 (0.2–1.2) 0.0310.6 (0.2–1.8) 0.2180.002/0.096
Placental abruption731.9 (0.5–7.5) 0.2201.001.1 (0.5–2.2) 0.7371.3 (0.5–3.2) 0.4980.7 (0.1–4.5) 0.6110.7 (0.1–11.5) 0.7220.855/0.793
Pulmonary embolism/deep vein thrombosis711.2 (0.2–8.0) 0.8071.001.3 (0.6–2.7) 0.3541.7 (0.7–4.1) 0.1521.2 (0.3–6.0) 0.7150.8 (0.1–6.1) 0.8570.347/0.788
Induction of labour87090.9 (0.7–1.1) 0.1091.00 1.2 (1.1–1.3) <0.001 1.3 (1.2–1.5) <0.001 1.4 (1.2–1.7) <0.001 1.6 (1.3–2.0) <0.001 <0.001/<0.001
Normal delivery16 5121.0 (0.9–1.3) 0.5521.00 0.8 (0.8–0.9) <0.001 0.7 (0.6–0.8) <0.001 0.6 (0.5–0.6) <0.001 0.5 (0.4–0.6) <0.001 <0.001/<0.001
Instrumental delivery40271.2 (0.9–1.5) 0.1411.00 0.8 (0.7–0.9) <0.001 0.7 (0.6–0.8) <0.001 0.5 (0.3–0.6) <0.001 0.5 (0.3–0.7) <0.001 <0.001/<0.001
Caesarean section96940.8 (0.7–1.0) 0.0241.00 1.4 (1.3–1.5) <0.001 1.8 (1.6–2.0) <0.001 2.5 (2.1–2.9) <0.001 2.8 (2.4–3.5) <0.001 <0.001/<0.001
Elective caesarean section46150.9 (0.7–1.3) 0.5531.00 1.3 (1.2–1.4) <0.001 1.6 (1.4–1.9) <0.001 2.4 (2.0–2.9) <0.001 2.6 (2.0–3.3) <0.001 <0.001/<0.001
Emergency caesarean section50790.8 (0.6–1.0) 0.0111.00 1.4 (1.2–1.5) <0.001 1.6 (1.4–1.8) <0.001 1.8 (1.5–2.2) <0.001 1.9 (1.4–2.5) <0.001 <0.001/<0.001
Shoulder dystociaa2090.4 (0.1–2.6 0.2191.001.5 (1.0–2.3) 0.0121.3 (0.7–2.5) 0.2222.0 (0.8–4.7) 0.0462.1 (0.6–6.8) 0.1220.001/0.031
Third- or fourth-degree perineal teara3041.1 (0.6–2.4) 0.6281.000.9 (0.7–1.3) 0.6221.2 (0.8–1.9) 0.2200.9 (0.4–2.3) 0.8661.3 (0.4–3.8) 0.5780.671/0.755
Postpartum haemorrhage97940.9 (0.7–1.1) 0.0861.00 1.4 (1.3–1.5) <0.001 1.8 (1.6–2.0) <0.001 2.4 (2.0–2.8) <0.001 2.7 (2.2–3.4) <0.001 <0.001/<0.001
Postpartum haemorrhage, excluding caesarean sectiona15101.0 (0.6–1.5) 0.9081.00 1.2 (1.0–1.4) 0.006 1.3 (1.0–1.7) 0.002 1.5 (1.0–2.2) 0.011 1.4 (0.8–2.5) 0.096 0.003/0.001
Breastfed13 5400.9 (0.7–1.1) 0.0661.00 0.8 (0.7–0.8) <0.001 0.6 (0.6–0.7) <0.001 0.5 (0.4–0.6) <0.001 0.4 (0.3–0.5) <0.001 <0.001/<0.001
Wound problemb1710.4 (0.0–5.3) 0.3541.001.2 (0.7–2.1) 0.2941.6 (0.9–3.0) 0.044 3.5 (1.8–6.7) <0.001 6.0 (3.0–12.1) <0.001 <0.001/<0.001
Length of postnatal stay18541.1 (0.8–1.6) 0.5001.001.0 (0.9–1.2) 0.6221.3 (1.0–1.5) 0.0031.4 (1.0–1.9) 0.004 2.1 (1.5–3.1) <0.001 <0.001/<0.001

The intranatal outcomes are outlined in Table 2. Women who were overweight or obese were more likely to have their labour induced, and this risk increased with each BMI category, to an OR of 1.6 (99% CI 1.3–2.0) for women in obese class III. Likewise, women who were overweight were at increased risk of caesarean section, and this risk increased with an increase in BMI to an OR of 2.8 (99% CI 2.4–3.5) for women in obese class III. This increased risk exists for both emergency caesarean section (OR 1.9, 99% CI 1.4–2.5) and elective caesarean section (OR 2.6, 99% CI 2.0–3.3). Conversely, overweight and obese women were less likely to have a normal delivery or an instrumental delivery. The odds ratio for normal delivery and for instrumental delivery decreased with an increase in BMI to an OR of 0.5 (99% CI 0.4–0.6) and 0.5 (99% CI 0.3–0.7), respectively, for women in obese class III. The risk of postpartum haemorrhage (PPH) increased as BMI increased (OR 2.7, 99% CI 2.2–3.4). There was no statistically significant risk of shoulder dystocia or third- or fourth-degree perineal tear in relation to BMI.

Postnatal outcomes are included in Table 2. Women who were overweight or obese were less likely to breastfeed, and the OR for this variable decreased as BMI increased, such that women who were morbidly obese had an OR of 0.4 (99% CI 0.3–0.5). There was an increased risk of wound problems following caesarean section for women in obese class II (OR 3.5, 99% CI 1.8–6.7) and in obese class III (OR 6.0, 99% CI 3.0–12.1). Women in obese class III were at higher risk of a postnatal stay greater than 5 days (OR 2.1, 99% CI 1.5–3.1). Maternal antenatal, intranatal, and postnatal outcomes, using a confidence interval 95%, are presented in Table S1.

Neonatal outcomes are presented in Table 3. In relation to spontaneous preterm birth, all BMI groups were at slightly increased risk compared with women of normal weight; however, this was only statistically significant for obese class I (OR 1.3, 99% CI 1.0–1.6) and obese class III (OR 1.6, 99% CI 1.1–2.5). When adjusted for gestational age and gender, the underweight group was at increased risk of delivering a baby of low birthweight (OR 1.6, 99% CI 1.0–2.4), with borderline significance (P = 0.01). However, all three obese groups were less likely to have a baby of low birthweight (<2.5 kg), and this risk decreased as BMI increased, such that women in obese class III had an OR of 0.5 (99% CI 0.2–1.0), just below the level of significance (P = 0.011). There was a statistically significant association between macrosomia (birthweight ≥ 4.0 kg) and BMI categories. The underweight group was least likely to deliver a macrosomic baby (OR 0.5, 99% CI 0.3–0.7), whereas women in obese class III were most likely to deliver a macrosomic baby (OR 3.2, 99% CI 2.4–4.1). The risk of stillbirth was of borderline significance for women in obese class III (OR 3.0, 99% CI 1.0–9.3; P = 0.01). Neural tube defects were only statistically significant for women in obese class II (OR 7.5, 99% CI 1.2–46.5). Only women in obese class III had a statistically significant association with low Apgar score at 5 minutes (OR 2.0, 99% CI 1.1–3.6). After adjusting for pre-gestational diabetes and preterm delivery, the risk for admission to the neonatal unit was still statistically significant for the three obese groups: obese class I, OR 1.3 (99% CI 1.1–1.7); obese class II, OR 1.6 (99% CI 1.2–2.2); and obese class III, OR 1.6 (1.0–2.6). Neonatal outcomes using a confidence interval of 95% are presented in Table S2.

Table 3. Neonatal outcomes by BMI category (kg/m2)
 Underweight BMI <18.50 n = 862Normal BMI 18.50–24.99 n = 15 908Overweight BMI 25.00–29.99 n = 8415OBESE CLASS I BMI 30.00–34.99 n = 3333OBESE CLASS II BMI 35.00–39.99 n = 1194OBESE CLASS III BMI ≥ 40.00 n = 586P (unadjusted/adjusted)
  1. All variables are adjusted for age, parity, social deprivation, smoking, and year of birth. Values presented as OR (99% CI), with < 0.01 considered to be significant (shown in bold). See Table S2 for data presented with 95% CIs.

  2. a

    Preterm and post-term also adjusted for elective caesarean section and induction of labour.

  3. b

    Low birthweight and macrosomia also adjusted for gender and gestational age.

  4. c

    Admission to neonatal unit also adjusted for preterm delivery and both pre-existing and gestational diabetes.

Gestation <37 weeks (preterm)a17501.2 (0.9–1.8) 0.1501.001.1 (1.0–1.3) 0.036 1.3 (1.0–1.6) 0.004 1.3 (0.9–1.7) 0.079 1.6 (1.1–2.5) 0.003 0.012/0.002
Gestation >41 weeksa9070.5 (0.2–1.0) 0.0161.000.9 (0.7–1.1) 0.1700.8 (0.5–1.1) 0.0470.9 (0.5–1.6) 0.6810.8 (0.4–1.7) 0.3960.693/0.077
Low birthweight (<2.5 kg)b1491 1.6 (1.0–2.4) 0.010 1.00 0.8 (0.6–1.0) 0.010 0.7 (0.5–1.0) 0.007 0.5 (0.3–0.9) 0.002 0.5 (0.2–1.0) 0.011 0.004/<0.001
Macrosomia (>4.0 kg)b4391 0.5 (0.3–0.7) 0.001 1.00 1.5 (1.3–1.6) <0.001 1.9 (1.6–2.2) <0.001 2.1 (1.7–2.6) <0.001 3.2 (2.4–4.1) <0.001 <0.001/<0.001
Stillbirth1262.0 (0.6–6.0) 0.1251.001.5 (0.9–2.5) 0.0540.8 (0.3–2.0) 0.5282.2 (0.9–5.7) 0.027 3.0 (1.0–9.3) 0.010 0.055/0.013
Cardiac defect541.1 (0.1–16.6) 0.9031.000.8 (0.3–2.3) 0.5740.5 (0.1–3.5) 0.4003.7 (1.0–14.2) 0.0142.1 (0.2–19.2) 0.4060.069/0.105
Neural tube defect251.001.6 (0.4–7.1) 0.4081.1 (0.1–10.2) 0.904 7.5 (1.2–46.5) 0.004 0.036/0.127
Apgar <7 at 5 minutes6231.4 (0.8–2.4) 0.1181.001.1 (0.9–1.4) 0.2411.0 (0.7–1.4) 0.9850.9 (0.5–1.6) 0.537 2.0 (1.1–3.6) 0.002 0.458/0.024
Admission to NNUc16751.1 (0.7–1.6) 0.7511.001.1 (0.9–13) 0.269 1.3 (1.1–1.7) 0.001 1.6 (1.2–2.2) 0.001 1.6 (1.0–2.6) 0.008 <0.001/<0.001
Infant stay >5 days38671.0 (0.7–1.3) 0.7541.001.0 (0.9–1.1) 0.5101.0 (0.9–1.1) 0.9020.9 (0.7–1.1) 0.1871.3 (1.0–1.7) 0.0430.952/0.237

Discussion

Main findings

This large retrospective study clearly demonstrates that being overweight or obese increases the risk of adverse maternal and neonatal outcomes. In particular, by categorising women into subclassifications of obesity this study highlights a relationship between increasing BMI (from overweight to obese class III) and increasing risk of adverse outcomes, including gestational diabetes mellitus (GDM), hypertensive disorders of pregnancy, caesarean section, macrosomia, and neonatal unit admission, with women in the highest obesity group at risk of additional adverse outcomes, including stillbirth, a longer postnatal stay, and wound problems following caesarean section. Importantly, as BMI increases women were less likely to achieve a normal delivery and were less likely to breastfeed.

Current UK guidelines recommend that women with a BMI > 30 should be offered a glucose tolerance test antenatally, and that those with a BMI > 35 should have additional monitoring for pre-eclampsia.[28] In the study reported here, obese women were at an increased risk of GDM and hypertensive disorders of pregnancy, and this risk increased as BMI increased, a finding consistent with other studies.[2, 7, 9, 11-13, 21, 23] However, women who were overweight were also at increased risk of hypertensive disorders of pregnancy and GDM, and therefore ‘at risk’ women who are overweight or obese (class I) may not be offered appropriate antenatal screening under the current guidelines. Intranatally, obesity contributes to poorer outcomes. As found in other studies, in the current study women who were obese were more likely to have their labour induced, were less likely to have a vaginal delivery, and were at increased risk of PPH.[2, 8, 9, 11, 23, 29, 30] To the best of the authors' knowledge, no studies to date have investigated the role of intranatal management on outcomes for women who are overweight or obese, and thus further research is now needed to elucidate the optimal intranatal management for women who are overweight or obese. Postnatally, in this study, women who were obese were less likely to breastfeed successfully, which has been reported elsewhere.[14] This has long-term implications for health, in particular with regard to obesity, as breastfeeding has been associated with women losing more weight postnatally, and breastfed babies are less likely to become obese.[31, 32]

In terms of neonatal outcomes, maternal BMI clearly influenced birthweight in the current study, with women who were underweight being more likely to deliver a baby of low birthweight, and women in obese class III being more likely to have a macrosomic baby. In agreement with the recent study by Wloch et al., obesity was associated with risk of wound infection among the population reported here, with risk increasing with BMI category.[33] In addition, in this study, women in obese class III were at higher risk in relation to stillbirth, as has been demonstrated in other larger studies.[8, 34]

Limitations and strengths

One of the major strengths of this study is the categorisation of women into all WHO BMI classification categories, including the three categories of obesity. In fact, to our knowledge, this is the only UK study to look at each separate category of obesity, thereby enabling the observation of a much clearer association in terms of risk and obesity for several outcomes. Because of the relatively large cohort in this study, it was possible to examine the outcomes for each BMI category, be selective about the deliveries included, and adjust for potentially confounding variables, yet still obtain statistically significant results for several important outcomes. Another strength of this study is the availability of BMIs for 93.3% of women, with BMIs recorded in early pregnancy (before 16 weeks of gestation), and therefore more likely to reflect pre-pregnancy BMIs, in line with current recommendations.[28]

As with any study, there are some limitations. As a result of the data available on the NIMATS database, this study was only able to look at hypertensive disorders of pregnancy, rather than clearly distinguishing between conditions such as pre-eclampsia or gestational hypertension, and thus could not specifically assess the individual risks for these conditions. Although weight gained during pregnancy can have an impact on risk,[17, 29, 35, 36] the current study was unable to adjust for pregnancy weight gain, as women were not routinely re-weighed during pregnancy. Furthermore, data were missing on NIMATS with regards to the ethnic background of the women; however, Northern Ireland still has a relatively homogenous white background population(99.15%), so it is unlikely that ethnic background would have been significantly different between the groups,[27] making this study generalisable to a largely metropolitan white population. For outcomes where only a limited number of cases are available, such as neural tube defects and stillbirth, false negatives are possible, and the results should be interpreted with caution. Data was not available to take into account clustering of births, and thus it is possible that a woman may have contributed more than one birth over the time period. Finally, it must be highlighted that although the current study provides a comprehensive analysis of antenatal, intranatal, and neonatal outcomes across the BMI categories over an 8-year period, this has resulted in multiple comparisons, and, given the large number of outcomes considered, further studies are needed to confirm the findings.

Interpretation

This study infers that women who are obese are more likely to require specialist medical care during their pregnancy, as a result of the increased risks associated with obesity. Women who were overweight or obese were less likely to labour without medical intervention, and were more likely to need a caesarean section, increasing the level of medical input, with cost implications for intranatal care. Although women in the highest BMI category were at the highest risk for an adverse outcome, these women, as expected, represented the smallest group in this study (1.9%). The largest ‘at risk’ groups were women who were overweight or in obese class I, representing 38.8% of the cohort studied. As national guidelines currently focus primarily on women within the highest BMI groups, and given resource allocation pressures within the health service, women who are overweight or in obese class I may not receive additional screening or management. Admittedly, these women may not have the same level of risk as women with the highest BMI; however, they are still at increased risk of several adverse outcomes, as highlighted in this study. This provides a challenge for healthcare professionals, as a substantial proportion of women they care for will be ‘at risk’ as a result of being overweight or obese, yet may not be identified as such, according to local policy and national guidelines.

In summary, being overweight or obese has a significant adverse impact on maternal and neonatal outcomes, with risk increasing across BMI categories. These risks have obvious implications for the management of these women during their pregnancy, labour, and postnatal period. It is important when planning care for women who are overweight or obese that resources are allocated appropriately in order to minimise the risk factors for these women. While current guidelines consider women who are obese, women who are overweight are also at an increased risk, and should therefore also be monitored closely during pregnancy and delivery to ensure optimum outcomes for women and their babies.

Disclosure of interests

None of the authors have any potential conflicts of interest to declare.

Contribution to authorship

DS and VAH conceived and designed the study, with input from RS, AH, and CC. DS and VAH obtained audit committee approval and acquired the data. RS undertook the analysis and interpretation of the data, with input from CC, DS, and VAH. RS wrote the first draft of the article. AH provided obstetrical expertise. All authors participated in the editing of this article and approved the final version for publication.

Ethics approval

This study was designed as an audit, and thus did not require ethics committee approval. Local audit committee approval was granted. The data provided to the researchers in this study were anonymised. No identifiable data were available to the researchers. The study was performed in an ethical manner.

Funding

This study received no external funding.

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