Women on low-calorie diets or who skip breakfast at the time of conception are more likely to give birth to girls than boys, British scientists said on Wednesday.
New research by the universities of Exeter and Oxford provides the first evidence that a child's sex is associated with the mother's diet, and higher energy intake is linked to males.
"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," said Fiona Mathews of the University of Exeter.
There has been a small but consistent decline, of about one per 1,000 births annually, in the proportion of boys being born in industrialized countries over the past 40 years.
In humans, going without breakfast may be interpreted by the body as signaling low food availability, since it depresses levels of blood sugar.
Although sex is genetically determined by fathers, it is known that high levels of glucose encourage the growth and development of male embryos while inhibiting female ones, although the exact mechanism is unclear.
Mathews and colleagues studied 740 first-time pregnant mothers in Britain and found 56 percent of those in the group with the highest energy intake at conception had sons, compared with 45 percent in the lowest group.
In evolutionary terms, this correlation may make sense.
Males' breeding potential is strongly influenced by fitness, while females breed 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," Mathews said.
Some researchers have been sounding alarms for years over the change in sex ratios in developed countries and have in the past blamed pollutants and synthetic chemicals such as those found in some pesticides which disrupt human hormones.
The latest findings were published in the journal Proceedings of the Royal Society B: Biological Sciences.