Women who eat a full breakfast and a high fat diet at the time of conception are more likely to have a boy, scientists claim.
A low fat diet with periods of long fasts favours girls, the researchers have found.
The findings seem to give some credence to to old wives' tales such as eat "bacon for boys".
Dr Cheryl Rosenfeld, of the University of Missouri, and colleagues said: "High calorie diets generally favour birth of males over females, whereas low calorie diets tend to favour females over males.
Researchers analysed the genes in placentas of pregnant mice fed diets high in fat or carbohydrates and low calorie diets and found each one had a distinctive effect compared with a third group given normal soybean meal-based food.
As well as the changes in sex, female foetuses were more sensitive to their mother's diet and there genes were more likely to be affected or altered, it was found.
After 12 days – just over half the animals' pregnancy term – there were differences in almost 2,000 genes including those involved in kidney function and smell.
They concluded gene expression in the mouse placenta is "adaptive and shaped by maternal diet" with the biggest effect on the placentas of females.
The research, published in Proceedings of the National Academy of Sciences, follows a study two years ago that found a woman's diet around the time of conception may influence the gender of her baby.
A high-calorie diet at this time – and regular breakfasts – might increase the odds of a boy while women with a lower energy intake were likelier to give birth to a girl.
The research shows a higher calorie intake around the time of conception can shift the odds of having a son from ten to 11 boys in every 20 births.
Sons and daughters are also at different risk for diseases later in life, apparently related to either the mother's diet or body condition while pregnant.
For instance, sons of obese mothers are more likely than daughters to become obese and develop diabetes as they get older, even though no differences in birth weight may be evident.
The researchers added: "The reason why a maternal high fat, low carbohydrate diet favours survival of sons and a maternal low fat, high carbohydrate diet results in more daughters continues to elude us.
"The effect was such that the more women ate the more likely she was to have a boy.
"Women who had sons were also more likely to have eaten a higher quantity and wider range of nutrients including potassium, calcium and vitamins C, E and B12. They were also more likely to have eaten breakfast cereals."
There has been a small but consistent decline of about one per 1,000 births annually in the proportion of boys being born in industrialised countries, including the UK, over the last 40 years.
It could be because women have been consuming low fat foods and skipping breakfast, among other things.
Source:telegraph