Want a son? Pack on the calories. Biologist Fiona Mathews of the University of Exeter in England and her colleagues surveyed 740 first-time mothers on their pre-pregnancy eating habits and found that 56 percent of those on high-calorie diets had sons, compared with 45 percent of those on leaner menus.

But it wasn't only calories that contributed; specific foods also appear to play a role, say researchers. "Prior to pregnancy, breakfast cereal, but no other item, was strongly associated with infant sex," the researchers write in the journal Proceedings of the Royal Society B. "Women producing male infants consumed more breakfast cereal than those with female infants."

The reason is a mystery, but Mathews speculates that glucose may be key. This type of sugar, converted by the human body into energy, is a by-product of the breakdown of carbohydrates such as those in breakfast cereal. Women who do not eat breakfast tend to have low levels of glucose, and other studies have shown that glucose enhances the growth of male fetuses in vitro.

Matthews notes that low glucose levels may indicate to the body that food is scarce, signaling that it would be more prudent to produce a female fetus, which has been found to need less energy to grow. Scientists have also found that cows, deer and horses produce more male offspring when they have bountiful diets.

The researchers took into consideration variables such as the education, size, weight and age of the would-be moms. But there are other variables, such as temperature, hormone levels, and even frequency of sex, that may contribute. "It remains to be seen whether women with greater nutritional intakes, and higher frequency of breakfast cereal consumption, prior to conception are also those with more active sex lives," the researchers write.

But the finding may explain a persistent and puzzling drop in the ratio of male to female births in well-fed industrialized nations, a fact that Mathews ascribes to the decline in the proportion of women eating breakfast. She notes that the number of adolescent girls eating breakfast in the U.S. dropped from 85 to 65 percent between 1965 and 1991.

Such an intriguing finding will need to be replicated in other populations, such as those in developing nations who are chronically underfed, before any definitive conclusions are reached, according to biologist Virpi Lummaa of the University of Sheffield in England, who was not involved in this research but studies human evolution. Researchers in the future might also survey women about their diets before they get pregnant (while their eating patterns are fresh on their minds) rather than after the fact in their quest to prove whether we are, indeed, what our mothers ate.