Obese mums 'more likely to be induced'


“Obese pregnant women have more complicated births” The Daily Telegraph reported. It said that a study has found that obese women are more likely to have a longer pregnancy, need to have labour induced artificially...

“Obese pregnant women have more complicated births,” The Daily Telegraph reported. It said that a study has found that obese women are more likely to have a longer pregnancy, need their labour artificially induced and then to need a caesarean section.

This study found that as women’s body mass index (BMI) increased, so did their risk of a prolonged pregnancy and need to be induced. Obese women also had a higher rate of caesarean section following induction compared with normal-weight women. However, most obese women who were induced (more than 70%) still managed a successful vaginal delivery. Rates of other delivery or neonatal complications were also comparable between obese women and normal-weight women. The authors say that induced labour for prolonged pregnancy appears to be a “reasonable and safe management option” for obese women.

Overweight and obesity are associated with other adverse effects on the health of the mother and the developing baby. However, dieting while pregnant is not recommended. It is advisable for women to try and obtain a healthy weight prior to becoming pregnant.

Where did the story come from?

This study was carried out by researchers from the University of Liverpool and University of Warwick. It was reported that the lead author received funding from the Wellcome Trust. The study was published in the peer-reviewed British Journal of Obstetrics and Gynaecology .

What kind of research was this?

The study investigated whether women who were obese were more likely to have a prolonged pregnancy and would therefore also be more likely to require (artificial) induction of labour. It also investigated whether obese women who were induced had increased risk of complications during delivery and in the newborn child. Several previous studies have demonstrated that obesity is a risk factor for a prolonged pregnancy.

This is a retrospective cohort study, an appropriate method for assessing how a prior exposure (in this case obesity) affects the likelihood of an outcome (in this case, complications following the induction of labour). Where possible, studies need to account for other confounding factors that could affect any associations that are made, such as medical conditions linked with both obesity and the likelihood of delivery complications. This study relied on routinely collected data from obstetric records. This is a potential weakness for the study in that the data was not specifically collected, raising the risk that some data is missing, or that there may be differences in how the data is recorded and outcomes assessed.

What did the research involve?

A total of 29,224 women gave birth to single babies in Liverpool Women’s Hospital between 2004 and 2008. The anonymous medical records included information on the women’s ethnicity, age, weight, height, lifestyle habits and all details relating to the labour and delivery outcome. The researchers were mainly interested in 3,076 of these women who required induction of labour due to prolonged pregnancy (pregnancy above 41 weeks and three days duration [290 days]). The hospital protocol for labour induction was the same in all the women.

The researchers were mainly interested in how the type of delivery (vaginal or caesarean) and delivery-related complications (e.g. excessive blood loss, vaginal tear) differed between obese and non-obese pregnant women. They also looked at newborn complications, including shoulder dystocia (one of the shoulders getting stuck on delivery), Apgar score (the test used to give a quick evaluation of the physical health of the baby immediately following birth) and stillbirth. These associations were adjusted for the potential confounders of age, ethnicity, previous children, smoking status, high blood pressure and diabetes.

What were the basic results?

An analysis of all 29,224 women showed a trend for a slighter longer pregnancy coinciding with increasing BMI at the start of pregnancy. The average pregnancy duration ranged from 281 days for underweight women to 287 days for morbidly obese women. Prolonged pregnancy was observed in 30% of all obese women (32.4% of very obese and 39.4% of morbidly obese women) compared with 22.3% of normal-weight women. Compared to normal-weight women, obese women were about 50% more likely to have a prolonged pregnancy (odds ratio 1.52, 95% CI 1.37 to 1.70). Increasing age and first pregnancy were also associated with increased likelihood of prolonged pregnancy, while smoking was associated with prematurity.

Of the 3,076 women who had induced labour, 22% were obese, 29% were overweight, 43% were normal weight and 6% were underweight. About three quarters of the women (2,351; 76.4%) had a vaginal delivery, with the remainder, about a quarter, requiring a caesarean. When categorised according to BMI, 28.8% of women having a caesarean were obese and 18.9% were of normal weight.

Women with a higher BMI were at greater risk of needing a caesarean section, and the risk increased if it was their first baby (38.7% of obese women having their first baby required a caesarean compared with 23.8% of normal-weight women having their first baby). Obese women having a second or subsequent baby had a lower risk (9.9% and 7.9% respectively).

BMI had no association with the length of first stage of labour, postpartum haemorrhage, third-degree tear, rate of low cord blood pH, low Apgar scores and shoulder dystocia.

How did the researchers interpret the results?

The researchers conclude that higher maternal BMI at the start of pregnancy is associated with a greater risk of prolonged pregnancy requiring induced labour. However, they say that despite this, more than 60% of obese women having their first baby still achieved vaginal delivery, as did more than 90% of second or subsequent-time obese mothers.

Complications of labour in women with prolonged pregnancies were “largely comparable” between obese and normal-weight women giving birth.


This study has strengths in that it examined a large cohort of 29,224 women having a single baby, and a reasonably large sub-cohort of 3,076 of these women who had prolonged pregnancy and required induced labour. This large sample size meant that when the women were categorised according to their BMI or delivery methods there were still enough numbers in each group for comparison.

The study relied on data from medical records. However, it is a reasonable assumption that height and weight would have been objectively measured (i.e. not the woman’s self-report) and that other pregnancy and labour-related information would have been accurately recorded.

One weakness is that some women had to be excluded due to missing data, which the researchers acknowledge. It should also be noted that this cohort of women were all cared for in a single, specialist women’s hospital, and findings may differ in other locations. Additionally, the researchers were unable to assess the full decision-making process for each woman (i.e. what individual factors contributed to the doctor’s decision to induce, perform a caesarean, etc).

The researchers observed that increasing BMI was associated with a slightly higher risk of a prolonged pregnancy and the need for induced labour. There were also more caesarean sections following induced labour in obese women compared with normal-weight women, but most (more than 70%) still managed a successful vaginal delivery. Reassuringly, the rate of other complications during delivery for obese women and in the newborn was comparable with the rate in normal-weight women.

The authors say that induced labour for prolonged pregnancy appears to be a “reasonable and safe management option” for obese women, and this seems appropriate given their findings.

Obesity has been associated with other problems in pregnancy, such as gestational diabetes, which this study did not assess. It is recommended that women are a healthy weight before they become pregnant.

Article Metadata Date Published: Mon, 21 Aug 2017
Author: Zana Technologies GmbH
NHS Choices