New research from the UK suggests that a baby’s sex is linked to his or her mother’s diet around the time of conception and the finding may explain why fewer boys are born nowadays in the industrialized world, including the UK and the US.

The study is the work of researchers at the Universities of Exeter and Oxford and is published today in the Proceedings of the Royal Society B: Biological Sciences.

The researchers found a strong link between the consumption of a high energy diet around the time of conception and giving birth to sons.

Over the last four decades the birth rate for boys has been declining steadily in industrialized countries including the UK, the US and Canada. The decline is small but consistent, at around one in 1,000 births a year, said the researchers.

For the study, lead author Dr Fiona Mathews of the University of Exeter and colleagues investigated the diets of 740 first time mothers living in the UK who did not know the sex of their unborn child. The mothers to be gave information about their eating habits before and around conception and during the early months of their pregnancy.

The participants were grouped according to their daily calorie intake at around conception and the results compared with the sex of their baby.

The results showed that:

  • 56 per cent of the women in the highest energy intake group had sons.
  • This compared with only 45 per cent of the women in the lowest energy intake group.
  • Women who had sons not only had higher energy diets but they were also more likely to have eaten a wider range and higher amount of nutrients such as potassium, calcium, plus vitamins C, E and B12.
  • There was also a strong association between eating breakfast cereals and having sons.

The women were representative of the UK average according to weight, health and lifestyle, and there was no link between the mother’s body mass index (BMI) and smoking and caffeine intake before pregnancy and the sex of the babies.

Other studies have found that average energy intake in the developed world has reduced, and that the obesity epidemic is driven by increased sedentary living and changes in food quality and eating habits. In the developed world many people now skip breakfast; in the US the percentage of adults who had breakfast fell from 86 per cent in 1965 to 75 per cent in 1991, said the authors.

Mathews suggested that:

“This research may help to explain why in developed countries, where many young women choose to have low calorie diets, the proportion of boys born is falling.”

She said the discovery is evidence of a natural way where women are already using diet to determine the sex of their baby, which is interesting when contrasted with the current debate on whether legislation should be introduced to regulate gender clinics that allow parents to choose the sex of their baby, for non medical reasons.

Biologists have noticed that many animal species produce more sons when resources are abundant or the mother is high ranking (meaning she gets the best food). This is commonly observed in invertebrates and mammals as well, such as horses, some types of deer and cows. The phenomenon is often explained as an evolved survival strategy.

“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,” said Mathews, adding that on the other hand, females tend to 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,” she explained.

Although the father, through sperm, determines the sex of the fetus, mothers appear to have influence too. While poorly understood in mammals, IVF research suggests that high levels of maternal blood glucose inhibit female embryos but encourage male embryos to grow.

Perhaps the increasing tendency for humans to skip breakfast, which depresses glucose levels, is giving the body the impression that resources are low and food is scarce, suggested the authors.

Click here for Proceedings of the Royal Society B: Biological Sciences.

Sources: University of Exeter press release.

Written by: Catharine Paddock, PhD