London ? 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.

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.