Oh Boy—Is Mom Hungry!

June 10, 2003 at 1:24 pm

At birth, boys tend to weigh about 100 grams (3.5 ounces) more than girls. An international research team wondered whether that meant that boys’ moms ate more during pregnancy. In data published this week, the scientists now confirm that’s exactly what happens.

Though women eat more when carrying a boy, they don’t gain more weight during pregnancy than do women who have girls, researchers find.

Dimitrios Trichopoulos of the Harvard School of Public Health said his group was interested in factors during and shortly after pregnancy that “might have long-term consequence on [a baby’s] cancer risk, on coronary disease, and so on.” As a first step, the researchers wanted to investigate characteristics of moms that might affect birth weight.

Trichopoulos and his colleagues recruited several hundred Boston-area women into a study when they made their first prenatal doctor’s visit. Just before each woman’s next prenatal visit, the epidemiologists mailed out a dietary survey.

Scientifically, Trichopoulos says, this was a fishing expedition: He and his team really didn’t know what they’d find. To their surprise, he says, they discovered that among the 244 women who answered the detailed questions about what they were eating, those who were carrying boys ate food delivering 9.6 percent more energy each day than women carrying girls did–an average difference of 190 calories. The difference held across the various types of nutrients the moms ate. Mothers-to-be carrying sons ate 8 percent more protein, 9.2 percent more carbohydrates, 10.9 percent more animal fats, and 14.9 percent more vegetable fats. Trichopoulos and his colleagues present their findings in the June 7 British Medical Journal.

Interestingly, the researchers found that the women who were eating more during their pregnancy with a boy weren’t gaining more weight than moms of girls were. What this implies is that in the womb, “boys are already more demanding than girls,” Trichopoulos says.

Although the researchers don’t know how a developing male signals his need for more energy, the scientists speculate that it could be via testosterone produced by his fetal testes.

The researchers speculate that the signal tells a mother’s body that it has extra work to do to bear a son. Studies have shown that during fetal development, “the male sex is clearly the more fragile one,” according to Bruce B. Allan, an obstetrician-gynecologist in Calgary, Alberta. He acknowledges that this idea probably elicits a hearty laugh from parents who have seen their rambunctious sons dive over furniture and survive daily bruising while cavorting with pals.

Junior commandos might not seem fragile, but before birth, boys are more at risk than girls. Though some 125 males are conceived for every 100 females, only about 106 boys are born for every 100 girls. In other words, Allan observes, stillbirths and miscarriages disproportionately cull boys.

Trichopoulos and his colleagues suspect that the higher energy demands of boys during fetal development may represent some evolutionary adaptation that increases their odds of survival.