Expectant mothers have bigger appetites on average if they are carrying a boy rather than a girl, suggests a study of pregnant US women.
The eating habits of 244 pregnant women attending a Boston hospital revealed that those who gave birth to baby boys had a 10 per cent higher daily calorie intake than those who bore girls. Boys tend to be about 100 grams heavier than girls at birth, but no study has really explained why, says the team of international researchers.
The latest findings may add to the understanding of human evolution, believes Dimitrios Trichopoulos, an epidemiologist at Harvard University and the study leader.
“Humans, like several other animals, have sexual dimorphism – where the males are bigger than females,” he says. “This might make evolutionary sense because until recently men used to compete for women – bigger males beat smaller males. Apparently this starts in the womb.”
But he adds that the difference in appetite is not striking enough to predict with accuracy the gender of a baby from what the expectant mum eats. “It seems to be biologically very important – but whether it has practical implications for modern society I’m not quite sure,” he told New Scientist.
Trichopoulos says the team’s results suggest that the male fetus somehow signals to its mother to eat more. He says one obvious way the male fetus might do this is by the secretion of testosterone – which tends to increase metabolism – from the fetal testicles. This would reach the mother through the placenta.
Previously scientists were unsure if baby boys are bigger at birth because a pregnant woman utilises energy more efficiently when she is carrying a boy, or whether she simply eats more.
Women carrying a boy had an average daily energy intake of 9025 kilojoules, compared with 8259 for women carrying a girl – a difference of about 200 calories a day.
The higher energy intake was “across the board”, says Trichopoulos. Pregnant women carrying male fetuses had an 8.0 per cent higher intake of protein, 9.2 per cent higher carbohydrate intake, a 10.9 per cent higher intake of animal fats and a 14.9 per cent higher intake of vegetable fats.
He speculates that the findings could suggest the “cravings” pregnant women occasionally experience may be more common for those carrying a boy than a girl.
Journal reference: British Medical Journal (vol 326, p 1245)