Last Updated: 2003-06-06 1:00:17 -0400 (Reuters Health)

LONDON (Reuters) - Swedish and American researchers have solved the puzzle of why baby boys are so much bigger at birth than girls -- their mothers eat more during pregnancy.

Women carrying male embryos consume about 10 percent more calories, eight percent more protein and have a higher intake of carbohydrates and animal and vegetable fats, according to research published on Friday.

"It is widely accepted that on average newborn boys are heavier than newborn girls. The findings give us a better understanding of why that is the case," said Rulla Tamimi, an epidemiologist at the Harvard School of Public Health.

Tamimi and colleagues at Harvard and the Karolinska Institute in Sweden studied the diets of 244 American women during their second trimester of pregnancy. They believe women carrying boys eat more because they have a higher energy requirement, which could be due to testosterone secreted by the foetal testicles.

But although they produce bigger babies, mothers of boys do not put on more weight than other women during pregnancy because the gender of the baby had no effect on maternal weight.

"These data suggest that in utero boys are already more demanding than girls," said Harvard's Dimitrios Trichopoulos, a co-author of the study published in the British Medical Journal.