Pregnant women carrying boys tend to eat more than those carrying girls, research has found.
Babies need good nutrition in the womb
The finding supports the theory that a developing male foetus demands more energy from its mother -and may be more likely to develop problems if it does not receive an adequate supply of nutrition.
This is a phenomenon that continues after birth - men are more vulnerable than women to most diseases and environmental risks throughout life.
Researchers analysed the diets of 244 pregnant women attending a large hospital in Boston, US.
They found that overall those carrying boys had a 10% higher energy intake.
The assumption here is that a woman's diet is affected because she is having a male baby, but it might be that she is having a male baby because of her diet
On average, they had an 8% higher intake of protein, a 9% higher intake of carbohydrates, an 11% higher intake of animal fats, and a 15% higher intake of vegetable fats.
The researchers, from the Harvard School of Public Health, suggest that a male foetus may secrete a chemical from its developing testicles that stimulates its mother to step up her energy intake.
Lead researcher Professor Dimitrios Trichopoulos said the reason for the difference appeared simply to be that male babies tend to grow bigger in the womb.
Boys, on average, tend to weigh abut 100g more than girls at birth.
Professor Trichopoulos said: "Sad to say, but there is discrimination in nature.
"For evolutionary reasons - such as having to compete among themselves to gain the favours of women - males have to be bigger than females and this phenomenon has its origins in the womb."
He added that all pregnant women should try to eat a balanced diet.
However, they should not worry about consciously modifying their diet to take account of the sex of their child.
"It is the embryo that signals the mother and not the other way round."
Chicken and egg argument
Professor Gordon Smith, of the Department of Obstetrics and Gynaecology at Cambridge University, said it possible the researchers had come up with the wrong conclusion.
He told BBC News Online: "The assumption here is that a woman's diet is affected because she is having a male baby, but it might be that she is having a male baby because of her diet.
"There is some evidence to show that diet can affect the sex ratio of live births."
Professor Smith said studies on mice had shown that those given a high fat diet were more likely to have male offspring.
Similarly, women with coeliac disease, which can cause digestive problems, had been found to be more likely to have girls.
Professor Smith said female babies were more robust, and so more likely than their male counterparts to survive to full term when nutrition was limited.
The research is published in the British Medical Journal.