Women pregnant with boys tend to eat about 10 percent more calories a day than those carrying girls but don't gain more weight, new research indicates.
The study, published this week in the British Medical Journal, appears to explain — at least in part — why newborn boys are heavier than girls and suggests that signals between the fetus and the mother drive the appetite during pregnancy.
Boys are on the average 3.5 ounces heavier at birth than girls. The study by researchers from the Harvard School of Public Health and the Karolinska Institute in Stockholm, Sweden, is the first to examine whether that difference could be due to the mother eating more.
The scientists assessed the diets of 244 American women one week before they came to the hospital for a routine prenatal checkup at 27 weeks of pregnancy. All the women later gave birth to normal-weight babies at full term.
The researchers found that women who gave birth to boys were consuming about 10 percent, or 200, more calories per day than those who went on to bear girls.
Yet the amount of weight mothers gained during pregnancy did not differ between those who had girls and those who had boys.
"This sounds undoubtedly driven by the fetus," said Kent Thornburg, a fetal physiologist at Oregon Health Sciences University who was not connected with the study.
Thornburg said the findings do not necessarily mean that boys are heavier solely because their mothers eat more.
"That would lead to the conclusion that the more a pregnant woman eats the bigger her baby will be and that female babies would be larger if only their mothers ate more," he said. "A more realistic hypothesis is that fetuses stimulate the appetite in their mothers in proportion to their requirement for optimal growth."
Scientists do not understand exactly what causes appetite to increase during pregnancy, but the study's findings suggest there is a chemical communication between mother and fetus so that males can grow faster than females, with the mother being signaled to eat more to enable that growth, Thornburg said.
Thornburg said the findings could be relevant to the recently discovered relationship between growth in the womb and the risk in adulthood of illnesses such as heart disease and diabetes.
"A decade ago, we thought that the primary risk for chronic disease in any apparently healthy baby was solely the result of genetic endowment from parents," Thornburg said. "We now know that the access to nutrients by the fetus is important in determining prenatal growth rate and thus lifelong health."
The study's authors said their results indicate that male fetuses may be more vulnerable than female ones to problems linked to fetal nutrition.