Male fetuses more demanding

The birth weight of boys is about 100 g more than the birth weight of girls, and this seems to be consistent across populations. Researchers have attempted to solve the puzzle as to why baby boys are so much bigger at birth than baby girls and why their mothers eat more during pregnancy. It is widely accepted that amongst newborns, boys are heavier than girls. Researchers at the Harvard School of Public Health, USA and the Karolinska Institute in Sweden studied the diets of 244 women during their second trimester of pregnancy. They calculated the intake of energy and energy generating nutrients from the dietary data. The sex of the fetus was correlated with the dietary intake of the mothers.Women who were carrying a male fetus had a higher daily energy intake than women carrying a female fetus. Mothers of male fetuses had a higher intake of protein 8.0%, carbohydrates 9.2%, fats of animal origin 10.9% and fats of vegetable origin 14.9% as compared to mothers of female fetuses. They did not observe any effect of sex of the fetus on mother's weight gain, even though weight gain during pregnancy is positively associated with birth weight.

Women carrying boys eat more because they have a higher energy requirement, which could be due to testosterone secreted by the fetal testicles. 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 mother's weight. The study found that the energy intake of pregnant women is about 10% higher when they are carrying a boy rather than a girl. The findings support the fact that women carrying male fetuses have higher energy requirements and that male fetuses may be more susceptible to energy restriction. The chances of having a boy rather than a girl is higher by about 35% when mother's food intake is higher.

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