How much a mother eats at the time of conception may influence whether she gives birth to a boy or a girl, a new report shows.

The report, from researchers at Oxford and the University of Exeter in England, is said to be the first evidence that a child’s sex is associated with a mother’s diet. Although sex is genetically determined by whether sperm from the father supplies an X or Y chromosome, it appears that a mother’s body can favor the successful development of a male or female embryo.

The study, published in the journal Proceedings of the Royal Society B: Biological Sciences, shows a link between higher energy intake around the time of conception and the birth of sons. The difference is not huge, but it may be enough to help explain the falling birthrate of boys in industrialized countries, including the United States and Britain.

The reason food intake may influence the development of one sex of infant rather than another isn’t fully understood. However, in vitro fertilization studies show that high levels of glucose encourage the growth of male embryos while inhibiting female embryos.

It may be that male embryos are less viable in women who regularly limit food intake, such as skipping breakfast, which is known to depress glucose levels. A low glucose level may be interpreted by the body as indicating poor environmental conditions and low food availability, the researchers said.

The findings are based on a study of 740 first-time pregnant mothers in Britain who didn’t know the sex of their fetus. They provided records of their eating habits before and during the early stages of pregnancy, and researchers analyzed the data based on estimated calorie intake at the time of conception. Among women who ate the most, 56 percent had sons, compared with 45 percent among women who ate the least. As well as consuming more calories, women who had sons were more likely to have eaten a higher quantity and wider range of nutrients, including potassium, calcium and vitamins C, E and B12. There was also a strong correlation between women eating breakfast cereals and producing sons.

The data are limited by the fact that they are based on self-reported food intake, which can be unreliable. However, the consistency of the trend offers an explanation for the small but consistent decline in the proportion of boys born in industrialized countries over the last 40 years, where even though women in general appear to be consuming more, eating habits have changed.

In the United States, for instance, the proportion of adults eating breakfast fell from 86 percent to 75 percent between 1965 and 1991. And although women may be eating more overall, a nutrient-poor diet could be less favorable to a male embryo. Glucose levels may also fluctuate in women who are dieting and trying to lose weight prior to pregnancy. In animals, more sons are produced when a mother ranks high in the group or has plentiful food resources.