Maternal health
Does being overweight in pregnancy raise the risk of complications?
Open access · cc by · source: Europe PMC
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.
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.
Key findings
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.
Methodology
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.
Limitations
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.
How this study connects
Role on claims
Each row is a claim on a concept or method page where this paper supports, challenges, or qualifies the statement. Roles are hand-checked — not a model guess.
The heavier a woman is going into her first pregnancy, the more likely she is to develop high blood pressure in pregnancy.
Among first-time mothers, pre-eclampsia and gestational hypertension rise steadily with pre-pregnancy BMI: in a Scottish cohort of about 24,000 women, morbidly obese women had about seven times the odds of pre-eclampsia, and in an Australian cohort of 1,661 women obese women had roughly three times the risk (RR 2.99).
Evidence for the claim as stated.
Obesity changes how labour goes and how big the baby grows, not just the mother's blood pressure.
Higher maternal BMI is also linked to more inductions and caesarean sections and to larger babies; in the Australian cohort obese women's babies were about twice as likely to be large for gestational age and over four times as likely to weigh 4.5 kg or more, and gestational diabetes was about twice as common.
Evidence for the claim as stated.
Maternal BMI clearly shifts hypertensive and delivery outcomes, but neither BMI cohort found a clear link with overall preterm birth: the Scottish study saw no difference in preterm birth before 37 weeks after adjustment (though more very preterm births in obese women), and the Australian composite of serious outcomes and preterm birth did not differ. Smoking, by contrast, is tied to preterm birth in the US data.
Evidence for the claim as stated.
The BMI cohorts studied first-time mothers with singleton pregnancies in Scotland and Australia, while the WHO growth study deliberately restricted to healthy women with BMI 18-30 who did not smoke, so its charts describe low-risk pregnancies rather than the obese or smoking mothers studied elsewhere.
Evidence for the claim as stated.
Open questions
Tensions this paper is part of
From concept pages' “where studies disagree.” Disagreement means the same question; scope means different assays, populations, or outcomes.
Maternal BMI clearly shifts hypertensive and delivery outcomes, but neither BMI cohort found a clear link with overall preterm birth: the Scottish study saw no difference in preterm birth before 37 weeks after adjustment (though more very preterm births in obese women), and the Australian composite of serious outcomes and preterm birth did not differ. Smoking, by contrast, is tied to preterm birth in the US data.
The BMI cohorts studied first-time mothers with singleton pregnancies in Scotland and Australia, while the WHO growth study deliberately restricted to healthy women with BMI 18-30 who did not smoke, so its charts describe low-risk pregnancies rather than the obese or smoking mothers studied elsewhere.
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