Gender selection: From diet to chromosomes

A link between what women eat before conception and the sex of their baby has been found in research from the Universities of Exeter and Oxford in the UK.

The results of their study show a clear association between a high energy intake before conception and the birth of sons. As well as consuming more calories, women who had sons were more likely to have eaten a higher quantity and wider range of nutrients, such as potassium, calcium, and vitamins C, E, and B12 than women who had girls. There was also a strong correlation between women who ate breakfast cereals and the birth of male children.

The study's lead author, Dr Fiona Mathews from the University of Exeter, 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."

The study was performed in 740 nulliparous women with normal singleton pregnancies who kept a prospective food diary of their diet in early pregnancy and gave a retrospective report of their usual diet in the year prior to conception. Results showed that 56 percent of women in the highest third of preconceptional energy intake bore boys, compared with 45 percent of those in the lowest third. However, intakes during pregnancy were not associated with any gender differences, suggesting, say the authors, that the fetus does not manipulate maternal diet.

The results, the authors add, are relevant for two reasons: first, that changes in dietary habits (skipping breakfast, for example) may explain the falling proportion of male births in industrialized countries; and second, as more evidence in the debate about gender selection in fertility treatment. The latter represents a continuing ethical issue for those involved in assisted reproduction.

Gender selection in IVF has been rarely (and controversially) described for the purposes of "family balancing" and more routinely as a medical indication for couples at risk for passing on a sex-linked single gene defect to their offspring. Indeed, the first reported pregnancies following pre-implantation genetic diagnosis (PGD), from the Hammersmith Hospital in London in 1990, were in couples at risk of transmitting recessive X chromosome-linked diseases to their children. That risk was removed by the chromosomal detection of gender in each embryo (then done by polymerase chain reaction) and the transfer of only "female" embryos.

The latest report from the PGD Consortium of ESHRE (European Society of Human Reproduction and Embryology), the only group today collecting data on PGD, shows that, during the 6 years prior to the latest analysis (for 2004, with pregnancies into 2005), there were a total of 703 cycles of sexing for X-linked disease performed among the reporting centers. In 2004 alone, 113 cycles were reported, nearly all using fluorescent in situ hybridization (FISH) to identify gender.

The Consortium's 2004 analysis showed that, of the embryos successfully biopsied, 93 percent (564/608) gave a diagnostic result, of which only 32 percent (183/564) were transferable (female); only 67 percent of the started cycles (76/113) reached embryo transfer. A positive heartbeat was found in 20 cycles (18 percent), giving an implantation rate of 17 percent (20/120), rates similar to those found in previous data collections.

The same dataset also shows that, in 2004, 79 cycles of PGD were preformed for "social sexing," most of which were in couples requesting a male embryo. However, social sex selection remains controversial and the debate about its application continues. Sex selection for non-medical reasons is still prohibited in India, Europe and Australia and patients having any type of PGD are not permitted to choose embryos on the basis of gender.