Child's sex is determined by mother's diet during pregnancy and before

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."

Although sex is genetically determined by fathers, mothers therefore appear able to favour the development of one sex of infant rather than another. The mechanism is not yet understood in mammals, but 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.

The group of women taking part in the study was representative of the UK average in terms of the weight, health and lifestyle. The findings showed no evidence of a link between a mother smoking and drinking caffeine prior to pregnancy and the gender of her baby. There was also no correlation between the body mass index (BMI) of a mother and the sex of her child. Although this research provides the first link between a human mother's diet and the sex of her offspring, there is still no evidence that diet during pregnancy, rather than around the time of conception, plays any role in the sex of a fetus.