Concept
Maternal weight and smoking as pregnancy risk factors
4 studiesEvidence last moved Sep 24, 2026
Some risks to mother and baby come from factors that can, at least in principle, change before or during pregnancy, especially body weight at the start of pregnancy and cigarette smoking. This page draws on large cohort studies from Scotland, Australia and the United States, plus a multinational fetal growth study that shows how the yardstick for a 'small' baby is itself set.
Students often lump every bad pregnancy outcome together as though one risk factor raises all of them equally. These studies show that high BMI mainly raises hypertensive disorders, diabetes, big babies and caesareans, while smoking is tied to preterm birth, and that the timing of quitting matters.
Studies
4
Findings
5
7 supporting · 0 challenging · 1 qualifying citations
Open tensions
2
Latest change
Concept page published
Maternal weight and smoking as pregnancy risk factors
Currently
What we know
- The heavier a woman is going into her first pregnancy, the more likely she is to develop high blood pressure in pregnancy.
- Obesity changes how labour goes and how big the baby grows, not just the mother's blood pressure.
- Risk does not simply rise with weight; low BMI trades one set of problems for another.
- There is no clearly safe low level of smoking in pregnancy, and quitting before conceiving is what brings risk back to baseline.
- Birthweight-based outcomes such as low birthweight or large-for-age rest on reference charts that vary by population.
Largest unresolved question
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.
Common misconceptions
Cutting down to a few cigarettes a day during pregnancy makes smoking harmless.
In the 25-million-pair US analysis even 1-2 cigarettes a day in the first trimester was linked to higher preterm-birth odds, and only quitting before pregnancy brought risk to never-smoker levels.
Maternal obesity raises the risk of every pregnancy complication, including preterm birth.
Both BMI cohorts found strong links with hypertension, diabetes, caesarean and large babies, but overall preterm birth did not clearly differ by BMI after adjustment.
A single universal cut-off tells you whether any fetus is too small.
The WHO multinational study found fetal weight percentiles differed substantially between countries even among healthy low-risk pregnancies, so the same fetus could be classed as small in one setting and normal in another.
Related
Claim ledger
What the evidence shows
Drawn from 4 studies in this library. Mix labels say which citation roles are present; they are not a strength score. Supports means evidence for a finding; Challenges means evidence against a stated position; Qualifies marks scope.
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).
- Does a mother's weight change the risks of a first pregnancy?
- Does being overweight in pregnancy raise the risk of complications?
Study Role Design N Population Outcome Does a mother's weight change the risks of a first pregnancy? Supports CohortRetrospective population-based cohort using routinely recorded booking height and weight; logistic regression comparing four BMI bands against the normal-BMI reference group. 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. Nulliparous women with singleton pregnancies in a geographically defined Scottish population. Pre-eclampsia, gestational hypertension, induction of labour, elective and emergency caesarean section, postpartum haemorrhage, preterm delivery, stillbirth, low birthweight and macrosomia. Does being overweight in pregnancy raise the risk of complications? Supports CohortSecondary 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=1661 · 1661 of the 1877 trial participants who had a BMI recorded at their first antenatal visit (943 normal, 446 overweight, 272 obese). Nulliparous women with a singleton pregnancy, normotensive at 14-22 weeks' gestation, recruited at Australian centres 2001-2005. 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). 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.
- Does being overweight in pregnancy raise the risk of complications?
- Does a mother's weight change the risks of a first pregnancy?
Study Role Design N Population Outcome Does being overweight in pregnancy raise the risk of complications? Supports CohortSecondary 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=1661 · 1661 of the 1877 trial participants who had a BMI recorded at their first antenatal visit (943 normal, 446 overweight, 272 obese). Nulliparous women with a singleton pregnancy, normotensive at 14-22 weeks' gestation, recruited at Australian centres 2001-2005. 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). Does a mother's weight change the risks of a first pregnancy? Supports CohortRetrospective population-based cohort using routinely recorded booking height and weight; logistic regression comparing four BMI bands against the normal-BMI reference group. 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. Nulliparous women with singleton pregnancies in a geographically defined Scottish population. Pre-eclampsia, gestational hypertension, induction of labour, elective and emergency caesarean section, postpartum haemorrhage, preterm delivery, stillbirth, low birthweight and macrosomia. 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.
There is no clearly safe low level of smoking in pregnancy, and quitting before conceiving is what brings risk back to baseline.
In US birth records covering about 25 million mother-infant pairs, first-trimester smoking was linked to higher odds of preterm birth (odds ratios about 1.31 to 1.53 across intensities, even at 1-2 cigarettes a day); women who quit before pregnancy had risk similar to never-smokers, while quitting only after conception did not fully remove the excess.
- Is any prenatal smoking safe for preterm birth?— Observational birth-certificate data with self-reported smoking; associations, not proven causal effects.
Birthweight-based outcomes such as low birthweight or large-for-age rest on reference charts that vary by population.
Whether a fetus counts as 'small' depends on the chart used: in the WHO multinational study of low-risk pregnancies, the near-term 10th percentile of estimated fetal weight was about 3,400 g in Norway but about 2,700 g in India, and country affected every percentile even after adjusting for maternal characteristics.
Debates
Tensions and limits
Some items are genuine disagreements on the same question. Others mark 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.
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.
- Does a mother's weight change the risks of a first pregnancy?
- Does being overweight in pregnancy raise the risk of complications?
- Is any prenatal smoking safe for preterm birth?
Study Role Design N Population Outcome Does a mother's weight change the risks of a first pregnancy? Supports CohortRetrospective population-based cohort using routinely recorded booking height and weight; logistic regression comparing four BMI bands against the normal-BMI reference group. 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. Nulliparous women with singleton pregnancies in a geographically defined Scottish population. Pre-eclampsia, gestational hypertension, induction of labour, elective and emergency caesarean section, postpartum haemorrhage, preterm delivery, stillbirth, low birthweight and macrosomia. Does being overweight in pregnancy raise the risk of complications? Supports CohortSecondary 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=1661 · 1661 of the 1877 trial participants who had a BMI recorded at their first antenatal visit (943 normal, 446 overweight, 272 obese). Nulliparous women with a singleton pregnancy, normotensive at 14-22 weeks' gestation, recruited at Australian centres 2001-2005. 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). Is any prenatal smoking safe for preterm birth? Supports CohortMaternal smoking before and during pregnancy vs preterm birth Perinatal cohort; single analytic N not extracted Pregnant women with recorded smoking status before and during pregnancy Preterm-birth risk associated with maternal cigarette smoking
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
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.
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.
- Do babies grow the same way in the womb around the world?
- Does a mother's weight change the risks of a first pregnancy?
- Does being overweight in pregnancy raise the risk of complications?
Study Role Design N Population Outcome Do babies grow the same way in the womb around the world? Supports CohortProspective multinational longitudinal cohort: low-risk women recruited in the first trimester in ten countries had standardised serial ultrasound biometry at roughly 4-weekly visits; percentiles built with quantile regression and tested for effects of country, fetal sex and maternal characteristics. N=1362 · 1,362 women contributing ultrasound data to the growth curves, from 1,439 enrolled (52 withdrew consent); 8,203 scan sessions were analysable. Healthy pregnant women aged 18-40 with BMI 18-30, singleton pregnancies, no smoking or chronic disease, living below 1,500 m in Argentina, Brazil, D. R. Congo, Denmark, Egypt, France, Germany, India, Norway and Thailand. Percentile reference curves for estimated fetal weight and biometric measures (biparietal diameter, head and abdominal circumference, femur and humerus length), and the influence of country, fetal sex and maternal factors on them. Does a mother's weight change the risks of a first pregnancy? Supports CohortRetrospective population-based cohort using routinely recorded booking height and weight; logistic regression comparing four BMI bands against the normal-BMI reference group. 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. Nulliparous women with singleton pregnancies in a geographically defined Scottish population. Pre-eclampsia, gestational hypertension, induction of labour, elective and emergency caesarean section, postpartum haemorrhage, preterm delivery, stillbirth, low birthweight and macrosomia. Does being overweight in pregnancy raise the risk of complications? Supports CohortSecondary 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=1661 · 1661 of the 1877 trial participants who had a BMI recorded at their first antenatal visit (943 normal, 446 overweight, 272 obese). Nulliparous women with a singleton pregnancy, normotensive at 14-22 weeks' gestation, recruited at Australian centres 2001-2005. 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).
PaperFren reads this as a limit on how far one study travels — different assays, populations, or outcomes — not a forced fight between papers.
Timeline
How understanding moved
Study years are when the paper was published. Evidence edits are dated changes to this page's claims. Explanations are when PaperFren added a Discovery — not a claim that the science happened that day.
2026
Concept page published
Maternal weight and smoking as pregnancy risk factors
Change log
What changed
Dated edits to this page's evidence: studies added or removed from a claim, claims added or withdrawn, and new explanations tagged here. Rewordings are not listed.
- Concept page published
Papers
4 studies in this library bear on Maternal weight and smoking as pregnancy risk factors, ordered by citations.
- Do babies grow the same way in the womb around the world?
Even in carefully selected healthy pregnancies, fetal growth differed markedly between ten countries, so a single global growth chart can misclassify babies as too small or too large.
- 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.
- 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.
- Is any prenatal smoking safe for preterm birth?
Among 25.6 million US births, even 1–2 cigarettes/day in the first or second trimester raised preterm-birth odds; quitting before pregnancy matched nonsmoker risk.
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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.
Ask PaperFren about Maternal weight and smoking as pregnancy risk factors
Study this conceptflashcards and short-answer questions
Which pregnancy outcomes are most consistently associated with maternal obesity, and which are not?
Cohorts of first-time mothers in Scotland and Australia both found that obesity sharply raised the risk of pre-eclampsia and gestational hypertension, with roughly three- to sevenfold increases at the highest BMIs. Obese women also had more inductions, caesareans, gestational diabetes and large babies. Overall preterm birth, however, did not clearly differ by BMI after adjustment, and underweight women instead had more low-birthweight babies. Because these are observational cohorts, they show association rather than proving weight itself is the cause.
What does the evidence say about the timing of quitting smoking in relation to pregnancy?
An analysis of about 25 million US mother-infant pairs found that smoking in the first trimester raised preterm-birth odds at every intensity, even a couple of cigarettes a day. Women who quit before pregnancy had risk comparable to never-smokers, but quitting only after conception did not fully normalise risk. This suggests cessation support should target women before they conceive. The data are observational and rely on recorded smoking status, so residual confounding is possible.
Flashcards
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