Does being overweight in pregnancy raise the risk of complications?
First-time mothers who were obese in early pregnancy had roughly triple the risk of pre-eclampsia and high blood pressure, and were more likely to need a caesarean or have a very large baby.
Source
The risk of adverse pregnancy outcomes in women who are overweight or obese
Study at a glance
- Design
- Cohort — Secondary observational analysis of the ACTS vitamin C/E pre-eclampsia trial (arms pooled because the trial was null); women grouped by BMI at first antenatal visit and compared with the normal-BMI group using log-binomial relative risks.
- N
- N=1661 · 1661 of the 1877 trial participants who had a BMI recorded at their first antenatal visit (943 normal, 446 overweight, 272 obese).
- Population
- Nulliparous women with a singleton pregnancy, normotensive at 14-22 weeks' gestation, recruited at Australian centres 2001-2005.
- Outcome
- Maternal outcomes (pregnancy-induced hypertension, pre-eclampsia, gestational diabetes, induction, caesarean, postpartum infection) and neonatal outcomes (birthweight, large-for-gestational-age, macrosomia, composite serious morbidity).
Structured fields used in claim comparison tables when every cited study has a complete layer.
What they did
The researchers used data from a large Australian trial of vitamin C and E supplements in first-time mothers, which had found no benefit, so both arms were combined. Women were grouped as normal weight, overweight or obese from the BMI recorded at their first antenatal visit. Each group's rates of pregnancy, birth and newborn complications were compared with the normal-weight group as relative risks.
What they found
Of 1661 women, 446 were overweight and 272 obese. Obese women had roughly triple the risk of pre-eclampsia (RR 2.99) and pregnancy-induced hypertension (RR 3.19), roughly double the risk of gestational diabetes (RR 2.10), and more inductions and caesareans (caesarean RR 1.63; 1.42 for overweight women). Their babies were twice as likely to be large for gestational age (RR 2.08) and their risk of weighing 4.5 kg or more was more than quadrupled (RR 4.54). The composite of serious maternal or infant outcomes and preterm birth did not differ significantly between groups.
The limits
What it doesn't show
This is an observational comparison, so it cannot show that excess weight itself causes the complications; the paper does not clearly describe adjusting the relative risks for differences such as socioeconomic status, education and ethnicity, which differed by BMI group. The sample was too small to detect effects on rare serious outcomes or reliably separate the overweight group from normal weight for pre-eclampsia and gestational diabetes, as the authors acknowledge. Only first-time mothers enrolled in a trial were studied, and the same BMI cut-offs were applied to Asian women, who carry more fat at a given BMI.
Key terms
- Body mass index (BMI)
- Weight in kilograms divided by height in metres squared; here normal was 18.5-24.9, overweight 25-29.9 and obese 30 or more.
- Pre-eclampsia
- A pregnancy disorder after 20 weeks combining high blood pressure with signs of organ involvement such as protein in the urine or liver or kidney problems.
- Relative risk (RR)
- The risk of an outcome in one group divided by the risk in a reference group; an RR of 3 means three times as likely.
- Macrosomia
- A newborn weighing 4.5 kg or more in this study, which raises the risk of difficult delivery.
- Nulliparous
- Describes a woman who has not given birth before.
- Composite outcome
- A single endpoint counted if any one of several serious events occurs, used to gain statistical power for rare events.
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Quiz yourself
What was the relative risk of pre-eclampsia for obese versus normal-weight women?
Common questions
Why could the authors combine the vitamin and placebo groups?
The original trial found no difference in outcomes between antioxidant and placebo, so treatment group was not expected to distort the comparison by BMI.
Why was no difference found in the serious composite outcome?
Serious complications are rare, and with a few hundred women per group the study lacked power to detect differences; the authors note that larger studies have found higher morbidity and mortality with maternal obesity.
Does this mean losing weight during pregnancy would prevent these problems?
No. The study only shows associations with weight at the start of pregnancy; the authors call for randomised trials of prevention strategies before and during pregnancy.
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