A mother-to-be carrying a male fetus eats significantly more than one carrying a female because the male fetus somehow sends out signals requiring more calories, says a report in the June 7 issue of the British Medical Journal.

"Boys are more demanding, so women suffer more," says study leader Dr. Dimitrios Trichopoulos, a professor of epidemiology at the Harvard School of Public Health.

A meal-by-meal survey of 244 women seen during pregnancy at Beth Israel Hospital in Boston shows that those carrying a male embryo had an 8 percent higher intake of protein, a 9 percent higher intake of carbohydrates, an 11 percent higher intake of animal fats, and a 15 percent higher intake of vegetable fats than those carrying a female, the report says.

The finding helps explain why boy babies are generally larger than girls, says Rulla Tamimi, an epidemiologist at Harvard and a member of the research team. "In almost all populations, they average about 100 grams [roughly four ounces] heavier," she says.

It's not clear how the male fetus makes its demands on the mother, Tamimi says. "One possible mechanism is that baby boys secrete testosterone, which is a signal that the mother should eat more," she says. Testosterone is the male sex hormone, secreted by the testicles.

Trichopoulos speculates the root cause is a survival-of-the-fittest thing, going back to prehistoric times.

"There were too many men competing for select women," he says. "Competition for those women was important in those days. Power depended on weight, so selection favored men who were bigger."

While the difference in food intake is scientifically significant, it isn't great enough to tell a woman the sex of the unborn child, Trichopoulos says. As for nutrition during pregnancy, "it certainly makes sense for a woman to eat more," he adds.

More information

Guidance on good nutrition during pregnancy is offered by the National Women's Health Information Center. Meanwhile, the March of Dimes has plenty of tips on how to take care of yourself during pregnancy.