The trend to skip breakfast could be altering the male/female balance in the population, scientists say.
A study of 740 first-time mothers has shown that women who ate breakfast cereals around the time of conception were more likely to produce sons.
Higher calorie consumption favours the birth of boys and both have declined in industrialised countries over the past 40 years. The growth in obesity in recent decades is linked with reduced exercise, not increased calorie consumption.
Researchers at the universities of Exeter and Oxford, who carried out the study published in Proceedings of the Royal Society B, say it is the first direct evidence that a child's sex is associated with their mother's diet.
Dr Fiona Matthews, of the University of Exeter, who led the study, said: "This research may help explain why, in developed countries where many young women choose to have low-calorie diets, the proportion of boys born is falling."
It is known from IVF research that high levels of glucose encourage the growth of male embryos while inhibiting the growth of female embryos. Skipping breakfast depresses glucose levels.
The decline in the birth of boys has been small but consistent – at about one per 1,000 births annually. Dr Matthews said the finding put the debate over sex selection by IVF clinics in a new light. "Here we have evidence of a natural mechanism that means that women appear to be already controlling the sex of their offspring by their diet," she said.