Hungry? Might be having a boy!

In the first study to examine why newborn boys are heavier than girls, researchers from the Harvard School of Public Health and the Karolinska Institutet in Stockholm, Sweden have found that women who were carrying a male embryo ate on average 10 percent more calories than women carrying a female embryo. The study appears in the June 7, 2003 issue of the British Medical Journal.

The researchers assessed the diets of 244 American women via a food frequency questionnaire during the second trimester. They found that women expecting a boy had an eight percent higher intake of protein, a nine percent higher intake of carbohydrates, an 11 percent higher intake of animal fats, and a 15 percent higher intake of vegetable fats than women who were carrying a female embryo. The gender of the newborn had no effect on maternal weight gain, even though weight gain is linked to birth weight.

These findings support the theory that women carrying male rather than female embryos may have higher energy requirements and that male embryos may be more susceptible to energy restriction, say the authors.

The researchers hypothesize that the signal from the fetus responsible for the higher energy intake of women carrying a boy could be related to the strongly anabolic testosterone secreted by the fetal testicles; however, further research is needed.

Rulla Tamimi, lead author and a researcher in the Department of Epidemiology at the Harvard School of Public Health commented, "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." Senior author Dimitrios Trichopoulos adds, "These data suggest that in utero boys are already more demanding than girls."