Obesity 'is risk in pregnancy'

“Fat mums at risk” warns the Daily Mirror , saying that obese pregnant women and their babies are at risk of serious birth problems. These include premature or underweight birth, or a five-fold increased risk of pre-eclampsia, a blood pressure problem found in pregnancy. 

The claims come from a study investigating the risk of birth problems in obese first-time mothers, comparing their body mass index (BMI) to rates of negative birth outcomes. Researchers compared the rates to expected national averages for obese pregnant women, regardless of whether they had had previous births or not. Researchers found that as the women’s BMI increased so did their risk of having birth problems such as pre-eclampsia. They also found that being a first-time mother raised the risk in obese women.

While confirming overall that the women studied had higher rates of pre-eclampsia and negative outcomes for their children, this research must be interpreted in the context of its unusual study design. The women in this study were not actually compared to mothers of a healthy weight or obese mothers with previous children, as neither group was included in the study. Also, the women were sourced from a previous study which included women "at risk" of pre-eclampsia. As such they would have had a higher-than-usual risk of the problem.

Where did the story come from? 

Dr Rajasingam and colleagues from King’s College London carried out this study, funded by the Wellcome Trust. Some of the authors receive financial support from the National Institute for Health Research (NIHR) and Tommy’s, the baby charity.

The study is to be published in the American Journal of Obstetrics and Gynecology, a peer-reviewed medical journal. It is accompanied by a press release issued by Tommy’s.

What kind of scientific study was this? 

This study investigated the birth outcomes and the risk of pre-eclampsia in obese first-time mothers.

It reports results from a subgroup of women who were originally involved in another study (a randomised controlled trial) examining the effects of vitamin supplementation on risk of pre-eclampsia in women at risk of the condition.

The subgroup of women used in this subsequent study consisted of 385 women from the previous randomised controlled trial. This included only those women who were obese (a BMI of 30 or more), who had been in the placebo arm of the trial and who were pregnant for the first time.

To set the context for their study, the researchers report that maternal obesity carries well established risks of complications, including for gestational diabetes (high blood glucose during pregnancy), pre-eclampsia (high blood pressure during pregnancy), high birth weight babies and stillbirths.

However, it is not known how maternal obesity specifically affects women who are pregnant for the first time. In this study, the researchers were able to explore what the risk of poor pregnancy outcomes was in obese women who were pregnant for the first time.

The researchers wanted to assess biochemical measures of stress in the women studied. This was done using blood samples provided by 208 of the women before they started the original randomised controlled trial on vitamin supplements.

Birth outcomes were recorded in a special database, which held details on complications, mode of delivery, delivery complications, hospital stay, birth weight and neonatal admissions. Pre-eclampsia was defined according to the International Society for the Study of Hypertension in Pregnancy as new hypertension after 20 weeks’ gestation and evidence of protein in urine (proteinuria).

If women already had high blood pressure or proteinuria, then pre-eclampsia was defined as increased severity of proteinuria or high blood pressure accordingly.

Statistical models were used to assess the relationship between pregnancy outcomes for both the mother and child and maternal BMI, which was categorised as moderately obese, severely obese and morbidly obese.

The researchers analysed their results, adjusting for factors that may affect the outcome (confounders), namely age, ethnicity, smoking, housing, employment, education and gestational age.

What were the results of the study?  

The researchers found a significant link between severity of obesity and pre-eclampsia, reporting that 19% of those who were morbidly obese had pre-eclampsia compared with 8.3% of moderately obese women. This represented a three-fold increase in risk of pre-eclampsia in morbidly obese first-time mothers compared with moderately obese first-time mothers. They also say that pre-eclampsia was more common in this group of women than in populations of women who are obese and have other children. 

The researchers say that gestational hypertension and preterm delivery were not linked to BMI. However, they say that a larger than expected number of first-time pregnant obese women delivered preterm, i.e. 11% compared with the national average of 6% in obese women who are mixed parity (a mix of first-time pregnancies and women with previous children).

The authors of this study report a number of other findings. They report that:

  • They found no link between BMI and birth weight, but say increasing BMI was associated with more caesarean sections.
  • There were high rates of babies who were small for gestational age (19% of births) and of those who were large for gestational age (13%). This is in comparison to a 7% national average for small for gestational age babies in women who are obese and have other children.
  • In two thirds of cases, those babies born small for their gestational age were not born to women with pre-eclampsia, which is a risk factor for a small baby.
  • Increasing BMI was also linked with significantly longer hospital stay and antenatal complications.
  • There were few significant links between BMI and measures of stress markers in the blood, although they did find that the mother’s BMI was linked to the level of a form of vitamin E (which has antioxidant and anti-inflammatory properties) in the blood in the second trimester.

What interpretations did the researchers draw from these results?

The researchers conclude that obesity in first-time pregnancies is associated with an increased risk of negative pregnancy outcome, which includes being small for age and increased preterm birth.

They say that first-time pregnancy is a factor that adds to the risk “imposed by obesity alone”.

The authors say that the finding that more than 50% of babies with restricted growth were born to mothers without pre-eclampsia is new, and contrasts with findings from another study, which instead found that obesity protects against babies being small for gestational age.

What does the NHS Knowledge Service make of this study?  

This cohort study has confirmed a link between obesity (BMI) and some poor pregnancy outcomes within a group of obese women who originally participated in a separate randomised controlled trial. The results apply specifically to women who were pregnant for the first time, and researchers have been able to compare the effects of increasing BMI on pre-eclampsia and negative outcomes for the baby.

The researchers found that in their group of women, increasing BMI was linked with a greater risk of pre-eclampsia. Those who were morbidly obese were about three times more likely to have pre-eclampsia than those who were moderately obese. When they did the same analysis for birth weight (whether the child was small or large for gestational age), they found no such relationship with BMI.

The findings of the study are difficult to interpret because of the lack of a comparison group. In a study questioning whether obesity is a risk factor for something, it is usual to have a non-obese comparison group. Equally, in a study questioning whether first-time pregnancy in obese women is more risky than other pregnancies, it is usual to see first-time mothers compared with mothers with one or more previous pregnancies.

Instead of using standard comparison groups the researchers compared the rate of adverse outcomes in a population of obese first-time mothers with the rates of adverse outcomes they would expect to find (according to national averages) in obese pregnant women with or without previous births. 
This is an unusually designed study, and it is not clear how comparable the women in this study are with the general population. This is particularly relevant here since the trial from which these participants were selected was in women “at risk of pre-eclampsia” (including obesity or hypertension). On this basis it is not entirely unexpected that higher rates of pre-eclampsia were found in this group.

NHS Attribution