Maternal health
Do babies grow the same way in the womb around the world?
Open access · cc by · source: Europe PMC
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.
Study at a glance
- Design
- Cohort — Prospective 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
- 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.
- Population
- 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.
- Outcome
- 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.
Structured fields used in claim comparison tables when every cited study has a complete layer.
Key findings
Country significantly affected every estimated-weight percentile even after adjusting for maternal characteristics and fetal sex. Near term the 10th percentile of estimated weight was about 3,400 g in Norway but 2,700 g in India, against a global value of about 3,100 g, so the same fetus could be 'small' in one country and normal in another. Male fetuses were a few percent heavier than female ones, and older, taller, heavier and parous mothers had somewhat larger fetuses. The weight distribution became increasingly skewed towards heavier fetuses late in pregnancy, and removing pregnancies with complications barely changed the charts.
Methodology
WHO recruited low-risk pregnant women early in pregnancy in ten countries in Africa, Asia, Europe and South America, confirmed gestational age by ultrasound, and scanned each fetus about every four weeks with identical machines and trained sonographers. From these repeated measurements they calculated estimated fetal weight and built percentile charts with quantile regression, a method that makes no assumption about the shape of the distribution. They then tested how much country, fetal sex and maternal age, height, weight and parity shifted the percentiles.
Limitations
Ten sites, one per country, are a thin sample of global diversity, especially for Africa, and the site samples may not represent their national populations. The study was not designed to explain why countries differ, so it cannot separate genetic, dietary, socioeconomic or measurement contributions, and ethnicity was only partly recorded. Gestational age appeared on screen during scans, which could in principle have influenced care, and pooled charts only partly reflect each country, so the authors advise testing and possibly customising them locally.
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.
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.
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.
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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