Maternal health
Does a mother's weight change the risks of a first pregnancy?
Open access · cc by · source: Europe PMC
The heavier a first-time mother was early in pregnancy, the more likely she was to develop high blood pressure disorders and need induction or an emergency caesarean, while underweight women had fewer of these complications but more small babies.
Study at a glance
- Design
- Cohort — Retrospective population-based cohort using routinely recorded booking height and weight; logistic regression comparing four BMI bands against the normal-BMI reference group.
- N
- N=24241 · 24,241 first-time mothers (primigravidae) delivering a single baby after 24 weeks in Aberdeen city and district between 1976 and 2005 who booked by 16 weeks.
- Population
- Nulliparous women with singleton pregnancies in a geographically defined Scottish population.
- Outcome
- Pre-eclampsia, gestational hypertension, induction of labour, elective and emergency caesarean section, postpartum haemorrhage, preterm delivery, stillbirth, low birthweight and macrosomia.
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
Pre-eclampsia and gestational hypertension rose steadily with BMI; morbidly obese women had about seven times the odds of pre-eclampsia and three times the odds of gestational hypertension compared with normal-weight women. Induction of labour and emergency caesarean section also rose with BMI, and obese women had more very preterm births, stillbirths and large babies. Underweight women had lower odds of pre-eclampsia, gestational hypertension and emergency caesarean, but higher odds of a low-birthweight baby. Overall preterm birth before 37 weeks did not differ by BMI after adjustment.
Methodology
The researchers pulled records for every first-time mother having a single baby in Aberdeen over three decades, using height and weight measured at the first antenatal visit (by 16 weeks) to calculate BMI. Women were split into underweight, normal, overweight, obese and morbidly obese groups, and each group's rate of pregnancy, labour and birth complications was compared with the normal-BMI group after adjusting for factors such as age, social class, smoking and year of delivery.
Limitations
This is observational, so it shows associations rather than proof that weight itself causes the complications; unmeasured factors such as diet, gestational diabetes or care practices could contribute. BMI came from early-pregnancy measurements, which only approximate pre-pregnancy weight, and clinical practice changed over the 30-year window (the authors adjusted for year but cannot remove this fully). The morbidly obese group was small, so estimates for rare outcomes like stillbirth are imprecise, and the paper is inconsistent about the morbid-obesity cut-off (above 35 in the methods, above 40 in the discussion). Results cover first-time mothers in one Scottish region only.
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.
Risk does not simply rise with weight; low BMI trades one set of problems for another.
Underweight carries a different pattern: in the Scottish cohort underweight women had lower odds of pre-eclampsia and emergency caesarean but higher odds of a low-birthweight baby.
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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