British researchers say they have found a link between what a mother eats at the time of conception and the sex of her child.

The study is published in the journal Proceedings of the Royal Society B: Biological Sciences.

The study of 740 first-time mothers tracked eating habits at the time of conception and in the early phase of their pregnancies.

The participants were divided into three groups based on their caloric consumption. Of those women who consumed the most calories per day around the time of conception, 56 per cent had sons.

In the group that consumed the fewest calories per day around the time of conception, 45 per cent had sons.

Women who conceived sons also consumed a wider range of nutrients, such as potassium, calcium and vitamins C, E and B12.

The researchers were attempting to find a cause for the decline in the number of boys born annually in countries like the UK, Canada and the U.S.

The researchers theorize that fewer boys are being born because more women are opting for low-calorie diets.

"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," said Fiona Matthews, lead author of the paper and a researcher at the University of Exeter.

Though the researchers acknowledge that men's sperm determine the sex of a child, they believe the woman does play a role in the process.

The researchers point out that in the animal kingdom, in animals such as horses, cows and some deer species, more sons are produced when a mother has access to plenty of food and is of high standing.

"If a mother has plentiful resources, then it can make sense to invest in producing a son because he is likely to produce more grandchildren than would a daughter," said Matthews. "However, in leaner times having a daughter is a safer bet."