Outcome of pregnancy in a woman with an increased body mass index


Dr T. S. Usha Kiran, 66 Cefn Graig, Rhwibina, Cardiff CF14 6SX, UK.


Objective  To show the increased risk of adverse outcomes in labour and fetomaternal morbidity in obese women (BMI > 30).

Design  A population-based observational study.

Setting  University Hospital of Wales. The study sample was drawn from the Cardiff Births Survey, a population-based database comprising of a total of 60,167 deliveries in the South Glamorgan area between 1990 and 1999.

Population  Primigravid women with a singleton uncomplicated pregnancy with cephalic presentation of 37 or more weeks of gestation with accurate information regarding height and weight recorded at the booking visit (measured by the midwives) were included in the study.

Methods  Comparisons were made between women with a body mass index of 20–30 and those with more than 30. SPSS version 10 was used for statistical analysis. Student's t test, χ2 and Fisher's exact tests were used wherever appropriate.

Main outcome measures  Labour outcomes assessed were risk of postdates, induction of labour, mode of delivery, failed instrumental delivery, macrosomia and shoulder dystocia. Maternal adverse outcomes assessed were postpartum haemorrhage, blood transfusion, uterine and wound infection, urinary tract infection, evacuation of uterus, thromboembolism and third- or fourth-degree perineal tears. Fetal wellbeing was assessed using Apgar <7 at 5 minutes, trauma and asphyxia, cord pH < 7.2, babies requiring neonatal ward admissions, tube feeding and incubator.

Results  We report an increased risk [quoted as odds ratio (OR) and confidence intervals CI)] of postdates, 1.4 (1.2–1.7); induction of labour, 1.6 (1.3–1.9); caesarean section, 1.6 (1.4–2); macrosomia, 2.1 (1.6–2.6); shoulder dystocia, 2.9 (1.4–5.8); failed instrumental delivery, 1.75 (1.1–2.9); increased maternal complications such as blood loss of more than 500 mL, 1.5 (1.2–1.8); urinary tract infections, 1.9 (1.1–3.4); and increased neonatal admissions with complications such as neonatal trauma, feeding difficulties and incubator requirement.

Conclusion  Obese women appear to be at risk of intrapartum and postpartum complications. Induction of labour appears to be the starting point in the cascade of events. They should be considered as high risk and counselled accordingly.


Obesity has assumed epidemic proportions in the developed world and the UK is leading Europe with 57% of men and 48% of women being overweight.1 An increased association of morbidity and mortality with obesity is well established in both pregnant and non-pregnant women.2–6 Maternal obesity has been reported as a risk factor for various antenatal, intrapartum, postpartum and neonatal complications such as postdates, induction of labour, macrosomia, shoulder dystocia, prolonged duration of labour, increased blood loss, caesarean section rates and neonatal admissions.3–6

The majority of the published research is from the United States4,5,7 and is based on sample populations with confounding variables such as diabetes, pre-eclampsia and chronic hypertension. Research in the British population is limited to one large study, which again included confounding variables.3 The Cardiff Birth Survey data set allows a population-based study on the outcome of singleton pregnancy according to the BMI in otherwise uncomplicated singleton primigravid women.


A population-based study was designed including all deliveries to women living within the catchment area and excluding all deliveries to women living elsewhere. The study sample was drawn from the Cardiff Births Survey, a population-based database comprising of a total of 60,167 pregnancies in the South Glamorgan area between 1990 and 1999. The database includes all births registered since 1965, is robust and is regularly audited. These data are validated and for the purposes of this study was used anonymously.

Primigravid women with a singleton uncomplicated pregnancy with cephalic presentation of 37 or more weeks of gestation with accurate information regarding height and weight recorded at the booking visit (measured by the midwives) were included in the study. Exclusions included any babies with any congenital abnormalities, pre-eclampsia, gestational diabetes and women with medical disorders such as diabetes mellitus, chronic hypertension, cardiac or endocrine disorders and surgical conditions. This allowed reduction of confounding factors to a minimum.

The resulting sample was stratified into two groups as follows: Group 1 with body mass index (BMI) between 20 and 30, which was considered as the comparison group; and Group 2 with BMI of more than 30, which was the study group.

Demographic details such as maternal age, ethnicity and smoking habit were collected. Outcome measures assessed included postdates, induction of labour, macrosomia (macrosomia was defined as birthweight of more than 4000 g), mean duration of labour, delay in the second stage of labour, operative delivery and maternal and perinatal morbidity. A clinically useful definition of macrosomia would be a birthweight above the 90th centile; however, we were unable to obtain centile birthweights from the database. Moreover, the definition of macrosomia in the literature varies between >4000 and >4500 g; accordingly, our view is that birthweight >4000 g is an adequate definition of macrosomia for the purposes of this study. Postdates were defined as a pregnancy over 41 weeks of gestation, usually confirmed by early ultrasound scan. This definition was used, as it is the unit policy to review women at 41 weeks of gestation to book them for induction of labour between 40 weeks + 10 days and 42 weeks. As the gestational age is recorded in completed weeks of gestation in the database, we were unable to obtain information about the number of days of gestation. Maternal complications such as thromboembolism, wound infection, urinary tract infection, perineal tear, excessive blood loss and blood transfusion were considered in assessing maternal morbidity. A blood loss of more than 500 mL was considered to be excessive as per the definition of postpartum haemorrhage by the World Health Organisation (WHO).8 Perinatal outcome measures included Apgar scores at 5 minutes of less than seven neonatal unit admissions, trauma and asphyxia. Trauma included all types of injuries like superficial skin cuts, grazes, bruises to deeper ones like fractures, muscle haematomas, dislocations, cephalhaematomas and nerve palsies. Asphyxia was based on the neonatologist's clinical impression of the baby depending on factors like Apgar scores, respiratory difficulty, blood pressure, pulse, muscle tone and coma if present. The effect of weight and height individually on these abovementioned outcome measures was analysed separately by using an independent sample t test.

SPSS version 10 was used for statistical analysis. Student's t test, χ2 and Fisher's exact tests were used wherever appropriate. A P value of less than 0.05 was considered to be significant. Odds ratios (ORs) with confidence intervals (CIs) were used to quantify the risk.


A total of 60,107 pregnancies were recorded in the database between 1990 and 1999. After strict application of inclusion and exclusion criteria, the study sample consisted of 8350 pregnancies (Fig. 1).

Figure 1.

Study sample.

The incidence of obese pregnancies has more than doubled (3.2–8.9%) in the past 10 years. The incidence of pregnancies with a booking BMI > 30 was 8.1% (n= 677). Morbidly obese (BMI > 40) pregnancies constituted only 0.4% (n= 40).

The demographic features such as mean age (26.1 [5.3] vs 26.2 [5.3] years) and percentage of smokers [n= 1781 (23%) vs 159 (23%)] were not significantly different in the comparison group and those with a BMI of more than 30, respectively. The majority were Caucasians [n= 7008 (96%) and 636 (98%)] in both groups.

As it is evident from Table 1, women with a BMI > 30 were at increased risk of postdates pregnancy and more likely to require induction of labour. They were also less likely to be delivered with a cervical sweep/ARM alone [18 (7.4%) vs 295 (15%), OR 0.3 (0.2–0.6)], more likely to need syntocinon [75 (31%) vs 431 (22%), OR 1.2 (1–1.6)] and more likely to require all three methods of induction (prostin, syntocinon and ARM) [26 (11%) vs 105 (5.4%), OR 1.8 (1.1–2.9)]. Mean durations of first stage of labour (hours) and second stages of labour (in minutes) were not significantly different between the groups. Similarly prolonged second stage (>2 hours) was also not significantly different. Although there did not appear to be an increased risk of assisted vaginal delivery, the group of women with a raised BMI was found to be at 1.6 times the risk of caesarean section versus a spontaneous vaginal delivery. We also noted a statistically increased risk of failed trial of instrumental delivery in the raised BMI group, although this was potentially clinically less significant. Among those who underwent caesarean section, emergency procedures were more likely than elective procedures in women with BMI > 30 compared with those with a normal BMI. However, induction of labour did not significantly influence the caesarean section rate in obese or normal BMI groups. There was also a doubling of the risk of a macrosomic baby in those with BMI > 30 and this group was also four times more likely to have shoulder dystocia, although they were not at any higher risk of third- or fourth-degree perineal tears (Table 2).

Table 1.  Labour outcomes. The values are presented as n (%) or mean [SD].
OutcomeGroup 1 (7673), BMI 20–30Group 2 (677), BMI > 30P/OR (CI)
Postdates2490 (32.5)278 (41)1.4 (1.2–1.7)
Induction1955 (25.5)244 (36)1.6 (1.3–1.9)
Mean duration of labour
First stage (hours)7.7 [4.0]8.09 [4.2]P= 0.06
Second stage (minutes)88.3 [67.6]92.5 [7.6]P= 0.2
Second stage (>2 hours)1808 (24)166 (25)1.0 (0.8–1.2)
Macrosomia583 (7.6)100 (14.8)2.1 (1.6–2.6)
Shoulder dystocia39 (0.5)10 (1.5)2.9 (1.4–5.8)
Mode of delivery
Spontaneous vaginal delivery4304 (58)325 (48) 
Assisted vaginal delivery1981 (25.8)169 (25)P= 0.6
Caesarean section1388 (18)183 (27)1.6 (1.4–2)
 Emergency1156 (83.3)167 (91.3)2.0 (1.2–3.5)
 Elective232 (16.7)16 (8.7) 
 Induced686 (49.5)100 (54.6)P= 0.11
 Not induced699 (50.5)83 (45.4) 
Failed instrumental delivery111 (1.4)17 (2.5)1.75 (1.1–2.9)
Table 2.  Maternal and neonatal complications. Values are presented as n (%). Odds ratios are included when significant.
ComplicationsGroup 1 (BMI 20–30)Group 2 (BMI > 30)OR (CI)
Blood loss (>500 mL)1185 (15.5)150 (22.2)1.5 (1.2–1.8)
Urinary tract infection86 (1.1)15 (2.2)1.9 (1.1–3.4)
Uterine and wound infection109 (1.7)10 (1.6) 
Third- or fourth-degree perineal tear53 (0.7)5 (0.8) 
Apgar at 5 minutes (<7)69 (0.9)8 (1.2) 
Neonatal trauma356 (4.6)47 (6.9)1.5 (1.1–2.1)
Asphyxia8 (0.1)2 (0.3) 
Neonatal unit admissions195 (2.5)26 (3.8)1.5 (1.09–2.3)
Cord (pH < 7.2)53 (0.7)7 (1) 
Tube feeding323 (4.2)42 (6.3)1.5 (1.08–2.0)
Incubator requirement140 (1.8)20 (3)1.6 (1.01–2.6)

Table 2 shows data on maternal morbidity. Blood loss was significantly greater in the group of women with increased BMI, but the secondary postpartum haemorrhage [13 (1.9%) vs 111 (1.5%)] and blood transfusion [27 (4%) vs 233 (3%)] rates and number of women needing evacuation of uterus [3 (0.4%) vs 53 (0.7%)] were not significantly different. Apart from an increased risk of urinary tract infection, these women were not prone to any other postnatal complications. In those who underwent caesarean section, the wound infection rates were not significantly different.

Babies born to women with a BMI of more than 30 appeared to be at an increased risk of birth trauma, and more of them required admission to the neonatal unit and were more likely to require assistance with feeding and maintenance of body temperature.

The effect of weight and height was considered independently as two separate variables and subjected to an independent t test. Women who had postdate pregnancies, induction of labour or macrosomia were significantly heavier than those who did not. Delivery by caesarean section was also significantly more frequently performed in those with increased weight. Height was significantly different in those requiring induction of labour and macrosomia.


Obesity is a growing epidemic and its effect on the outcome of pregnancy and delivery in the healthy UK population has not hitherto been extensively studied. This study drawn from a population-based set of uncomplicated, primigravid women aims to report the effect of maternal obesity on pregnancy complications with minimal confounding bias.

An increasing trend towards obesity was found over the last 10 years, and at present, one-third of our pregnant population has a BMI greater than 30. This is similar to the national trend (prevalence rate increased by 1.6% between 1993 and 1996) documented in the fifth Confidential Enquiries into Maternal Deaths.9 A previous UK study reported a higher incidence of obesity (10.9%) in their population.3 However, inclusion of pregnancies with medical complications may account for the higher rate reported in this study.

Although the mean age in those who were obese did not significantly differ from the comparison group in this study, it has been reported that increasing age is an added risk factor for obesity.10–12 This is evident from the Confidential Enquiries into Maternal Deaths 1997–1999, which reported more pregnancies occurring in women of more than 25 years of age in 1997–1999 compared with 1988–1990.9 We were not able to evaluate the association of race in this study, as the majority of our population was Caucasian. However, it has been previously reported that South Asian and Afro-Caribbean women are more likely to be obese (OR 1.83 and 3.01, respectively) compared with the native British population.13

This study detected a higher incidence of postdates, induction of labour, caesarean section, macrosomia and shoulder dystocia in the group of women with increased BMI. Macrosomia has been consistently reported by many researchers (with OR ranging between 1.5 and 2.2).3–7 Similarly, the increased risk of shoulder dystocia has also been reported.4,11,14,15 However, increased risks of factors leading to perinatal morbidity such as neonatal trauma and admission to special care unit have been reported in only a few studies.16–18 Although an increased risk of postdates15,19,20 has not been previously demonstrated, these women do appear to require induction of labour more frequently (OR 2.14).3,10 This can probably be explained by the inclusion of women with other medical conditions such as diabetes, pre-eclampsia, and so forth, and therefore the need for induction for other indications. Our findings of an increased risk of caesarean section in women with a BMI > 30 is in accordance with other published work and the excess risk appears to be one to twofold.3–7 One study showed that even in low risk obese women with otherwise normal pregnancies who are managed by nurse midwives, the risk of caesarean section was higher.21 Unfortunately, in the UK National Sentinel Caesarean Section Audit, it was not possible to evaluate the effect of maternal weight on the rate of caesarean section, but it was noted that 34% of women who were delivered by caesarean section had a BMI greater than 30.22

It is possible that these complications could be a cascade of events with the starting point being postdates, and then a difficult induction leading to other interventions. Why there should be delay in onset of labour in these women is unknown. Surprisingly, no increase in the rate of third- or fourth-degree perineal tears was seen in this study, although it has been reported elsewhere,4 perhaps because caesarean section was resorted to more often than instrumental delivery.

It is likely that many of the maternal and fetal complications follow as a result of labour events. An increased caesarean section rate is likely to lead to increased blood loss, especially where emergency procedures for large babies are being undertaken. However, obese women who had vaginal delivery also had excessive blood loss over 500 mL when compared with those with a BMI of 20–30. Hence, increased rate of caesarean section might not be the only factor influencing the blood loss in this group. When women undergo vaginal delivery with a big baby, shoulder dystocia, maternal and neonatal trauma and thus increased admission to neonatal unit are more likely. Others3,4 have reported an increased risk of postpartum haemorrhage.

Most studies have noted an increased incidence of infection (e.g. genital, urinary and wound).3,6,23 In this study, the only notable finding was an increased incidence of urinary tract infection. The reason for this difference might be the exclusions in the study group (e.g. exclusion of women with diabetes or to the practice of giving antibiotic prophylaxis at caesarean section).

Previous studies have reported an increased incidence of pre-eclampsia and gestational diabetes,3–5 which are in themselves likely to lead to an increased intervention rate. These problems, however, were excluded from the study group, as our sample included only uncomplicated pregnancies, and yet the trend to increased interventions in labour in normal women with an increased BMI persisted, making them a group at increased risk.

Having identified the risks for this growing population of pregnant women in the UK, there is no evidence-based management plan for them. Once pregnant, the options appear limited. The Cochrane review revealed that energy/protein restriction has no clear effect on pregnancy-induced hypertension or pre-eclampsia, and its effect on other outcomes including maternal and perinatal morbidity and mortality has not been reported.24

This study, however, clearly demonstrates the increased risk associated with embarking upon a pregnancy when overweight and shows that this group of women need to be regarded as ‘high risk’ when counselling and risk assessment is done in the antenatal clinic. Many women who died in the triennium (1997–1999) were classified as obese.25 It has been recommended in the fifth Confidential Enquiries into Maternal Deaths that all mothers should have their BMI calculated at booking as a part of the full risk assessment and should be offered advice about sensible weight reduction, including diet and exercise and referral to a dietician where appropriate. It has also been recommended that the midwives should inform mothers who are obese about how to recognise early warning signs of complications.

In summary, this study demonstrates and confirms that an increased BMI increases the incidence of macrosomia, the need for obstetric interventions and consequent morbidity in mother and baby. Awareness of these findings needs to be increased in both women planning a pregnancy and their carers.

Further research is needed to explore pathophysiology of postdates in obese women and intervention studies of the management of obesity in pregnancy.

The following are the limitations of this study:

  • 1Our study population may not be representative of other inner city populations, as our immigrant and non-Caucasian population was low.
  • 2Data are not recent (1990–1999) but there is no reason to believe that the rate of obesity in the UK is decreasing.
  • 3As antibiotic and thromboprophylaxis protocols were in place, rates of thromboembolism and infection would be low anyway.


The authors would like to thank Dr Barry Nix and Dr Kiren Sidhu, University Hospital of Wales, Cardiff.

Accepted 31 July 2004