Poor nutrition and health, not race or ethnicity, cause most of the current wide disparities in foetal growth and newborn size, researchers have found.
Babies' growth in the womb and their size at birth, especially their length, are strikingly similar the world over when babies are born to healthy, well-educated and well-nourished mothers, they said.
The landmark international study led by Oxford University researchers, involved almost 60,000 pregnancies in eight defined urban areas in Brazil, China, India, Italy, Kenya, Oman, the UK and US.
Worldwide there are wide disparities in the average size of babies at birth. This has significant consequences for future health, as small for gestational age babies who are already undernourished at birth often face severe short- and long-term health consequences.
It has previously been suggested that 'race' and 'ethnicity' are largely responsible for differences in the size of babies born in different populations and countries.
These new results show that what matters more is the educational, health and nutritional status of the mothers, and care provided during pregnancy.
The researchers carried out ultrasound scans from early pregnancy to delivery to measure babies' bone growth in the womb, using identical methods in all countries and the same ultrasound machines. They also measured the length and head circumference of all babies at birth.
They demonstrated that if mothers' educational, health and nutritional status and care during pregnancy are equally good, babies will have equal chances of healthy growth in the womb and future good health.
"Currently we are not all equal at birth. But we can be," said the lead author Professor Jose Villar of the Nuffield Department of Obstetrics & Gynaecology, University of Oxford.
"We can create a similar start for all by making sure mothers are well educated and nourished, by treating infection and by providing adequate antenatal care.
"Don't tell us nothing can be done. Don't say that women in some parts of the world have small children because they are predestined to do so. It's simply not true," said Villar.
The study found that babies' bone growth in the womb and their length and head circumference at birth are strikingly similar the world over - when babies are born to educated, healthy and well-nourished mothers.
Overall, no more than 4 per cent of the total difference in foetal growth and birth size could be attributed to differences between the eight populations in the study.
The findings were published in The Lancet, Diabetes & Endocrinology.