Does a mother's weight change the risks of a first pregnancy?
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.
Source
Effect of Body Mass Index on pregnancy outcomes in nulliparous women delivering singleton 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.
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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Body Mass Index (BMI)
- Weight in kilograms divided by height in metres squared; used here to group women from underweight to morbidly obese.
- Pre-eclampsia
- A pregnancy disorder of new high blood pressure plus signs of organ involvement such as protein in the urine; a major cause of maternal and fetal harm.
- Adjusted odds ratio
- The odds of an outcome in one group relative to a reference group after statistically accounting for other variables such as age and smoking.
- Retrospective cohort
- A study that looks back at records of an already-defined group to compare outcomes between people with different exposures.
- Macrosomia
- A baby that is unusually large at birth, which raises the risk of difficult delivery.
- Primigravida
- A woman who is pregnant for the first time.
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What type of study design was used?
Common questions
Why study only first-time mothers with one baby?
Previous pregnancies and twins independently change the risk of many complications, so restricting the sample removes these as confounders and makes the BMI comparison cleaner.
Does this mean being underweight is healthy in pregnancy?
Not simply. Underweight women had fewer blood-pressure disorders and caesareans, but their babies were more often of low birthweight, which carries its own risks.
Why didn't every outcome stay significant for the morbidly obese group?
That group was very small, so confidence intervals were wide, and the authors suggest that the heavy burden of hypertension and interventions in those women may also mask other effects.
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