Newborn babies born to healthy, well nourished mothers are strikingly similar in size the world over, scientists have shown.
On average, they have a body length of 49.4 centimetres (19.45 inches), an international study found.
Previously it was suggested that ethnicity was largely responsible for the widespread variation seen in the size of babies born around the world.
The new research suggests race and ethnicity contribute little to baby size. What matters more is the education, health and nutrition of mothers, and the care they receive during pregnancy.
Overall no more than 4% of differences in foetal growth and birth size could be attributed to population differences.
Scientists taking part in the Intergrowth-21st study looked at almost 60,000 pregnancies in urban areas of the UK, US, Brazil, China, India, Italy, Kenya and Oman.
Ultrasound scans were carried out to assess babies’ bone growth in the womb, and at birth every infant had its body length and head circumference measured.
Lead researcher Professor Jose Villar, from the Nuffield Department of Obstetrics and Gynaecology at Oxford University, said: “Currently we are not all equal at birth, but we can be. 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 say that women in some parts of the world have small children because they are predestined to do so. It’s simply not true.”
Findings from the study, funded by the Bill & Melinda Gates Foundation, are reported in the journal The Lancet Diabetes & Endocrinology.
In 2010, an estimated 32.4 million babies were born undernourished in low-to-middle-income countries, representing 27% of all live births globally.
Small size at birth is associated with infant death and illness, as well as increased risks of diabetes, high blood pressure and heart disease in adult life.
Currently foetal growth and the size of newborns are evaluated in clinics around the world using at least 100 different growth charts.
“This is very confusing for doctors and mothers and makes no biological sense,” said Professor Stephen Kennedy, another member of the Oxford team. “How can a foetus or a newborn be judged small in one clinic or hospital and treated accordingly, only for the mother to go to another city or country, and be told that her baby is growing normally?”
The ultimate aim of Intergrowth-21st is to provide international standards describing the ideal growth of a baby in the womb and from birth to five years of age.
Co-author Professor Zulfiqar Bhutta, from the Aga Khan University in Karachi, Pakistan and the Hospital for Sick Children in Toronto, Canada, said: “The fact that when mothers are in good health, babies grow in the womb in very similar ways the world over is a tremendously positive message of hope for all women and their families.
“But there is a challenge as well. There are implications in terms of the way we think about public health: This is about the health and life chances of future citizens everywhere on the planet. All those who are responsible for health care will have to think about providing the best possible maternal and child health.”