OLD wives' tales are full of advice for mothers-to-be on how to determine the sex of their future child, from what to eat before conception to the best sexual positions for making boys or girls. The Chinese even developed a conception calendar, or Shengyubiao, based on a woman's age in lunar years and on the month of conception, for couples to choose the sex of their offspring. Such methods sound more like superstition than science. But a study published in the April issue of Human Reproduction argues that the sex of a child may indeed vary according to the time of year that he or she is conceived.

Angelo Cagnacci and his colleagues at the Policlinico of Modena, in Italy, have examined the records of more than 14,000 children born at the town's Institute of Obstetrics and Gynaecology between 1995 and 2001. The group found, as others have elsewhere, that there is no significantly “high” season for delivering boys versus girls. But Dr Cagnacci reasoned that such seasonal differences might be masked by variations in the length of pregnancy. So he and his team analysed the dates of conception of the children, which the institute had established by taking ultrasound snapshots of the babies as fetuses.

Using these data, they found that there were significant seasonal variations in the number of conceptions, and also in the proportion of boys and girls conceived, at particular times of the year. According to Dr Cagnacci, a normal sex ratio is 511 males and 489 females out of every 1,000 conceptions that yield a live birth. But the study showed that from September to November there were both more conceptions and a higher ratio of boys to girls conceived than would be expected or could be chalked up to chance. From March to May, the converse was true.

Why should this be? Unlike many species, humans are not seasonal breeders. Couples' reproductive urges seem to have more to do with champagne and roses than daily temperature or hours of sunlight at certain times of the year. But Dr Cagnacci argues that the seasonal variation he finds in the number of conceptions may allow children to be born in more favourable conditions.

If so, it makes sense that more boys than girls would be conceived at the times of the year when they have a better chance of survival. This is because in biological, if not literary, terms, males are the weaker sex. They are more likely than females to perish at every stage of life, from pregnancy to old age. Other studies have shown that fewer boys than girls are born when their parents are stressed by age, smoking, exposure to pollution or even earthquakes. Dr Cagnacci speculates that nature therefore tips the sexual balance in conception—perhaps by influencing hormone levels that affect semen or the implantation of male as opposed to female embryos—so that more boys are created in biologically favourable conditions.

Still, many unknowns remain. There is, as yet, little evidence to show that children conceived at certain times of the year are healthier than others on the basis of season alone. Nor have the researchers ruled out the possibility that the seasonal effects are due to other factors, such as smoking patterns. Nor do they have evidence for any biochemical mechanism to explain their findings. Until these questions are resolved, conceiving a boy or a girl through old-fashioned sex will remain something of a shot in the dark.