A woman’s diet around the time of conception may influence the gender of her baby, research suggests.
The study suggests a high-calorie diet at this time - and regular breakfasts - might increase the odds of a boy.
The researchers say the modern trend to opt for low calorie diets might explain why the proportion of boys is falling in developed countries.
The study, by the Universities of Exeter and Oxford, appears in the Royal Society journal Biological Sciences.
The study focused on 740 first-time pregnant mothers in the UK, who were asked to provide records of their eating habits before and during the early stages of pregnancy.
The researchers found 56% of women with the highest energy intake around the time of conception had boys, compared to just 45% among women with the lowest energy intake.
The average calorie intake for women who had sons was 2,413 a day, compared to 2,283 calories a day for women who had girls.
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.
Over the last 40 years 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.
Previous research has also shown a reduction in the average energy intake in the developed world, and there is also evidence that more people now skip breakfast.
Scientists already know that in many animals, more males are produced when a mother has plentiful resources or is high ranking.
The phenomenon has been most extensively studied in invertebrates, but is also seen in horses, cows and some species of deer.
The explanation is thought to lie with the evolutionary drive to produce descendants.
Lead researcher Dr Fiona Mathews said: “Potentially, males of most species can father more offspring than females, but this can be strongly influenced by the size or social status of the male, with poor quality males failing to breed at all.
“Females, on the other hand, reproduce more consistently.
“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.
“However, in leaner times having a daughter is a safer bet.”
It is known from IVF research that high levels of glucose encourage the growth and development of male embryos while inhibiting female embryos.
In humans, skipping breakfast depresses glucose levels and so may be interpreted by the body as indicating poor environmental conditions and low food availability.
Dr Allan Pacey, an expert in fertility at the University of Sheffield, said there was good evidence that nature had subtle ways of changing the sex ratio of a population in response to a variety of circumstances.
However, he said: “I would urge women to not to start starving themselves in order to try influence the sex of their baby.
“It has been observed in some animal studies that even small changes in female diet can affect the life long health of the offspring, so it is important that the mother has appropriate nutrition at the time of conception and throughout her pregnancy.”
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