New Delhi, March 30: Female foetuses are at a slightly higher risk than male foetuses of dying in the womb from natural causes, according to a new study that scientists say could help explain the dominance of boys at birth documented for over three centuries but which has remained largely unexplained.

A team of scientists that conducted the largest and most comprehensive study yet to explore how the human sex ratio changes from conception through pregnancy to birth has found that male and female embryos start out with an equal chance of being conceived.

The study which examined sex ratios of three-day and six-day-old embryos conceived through assisted reproduction technology procedures to estimate the sex ratio at conception found it to be 0.502, or 502 male embryos conceived for every 498 female embryos conceived.

But the total female mortality through the nine months of pregnancy exceeds the total male mortality.

"We really can't explain the mechanism for this yet," said David Steinsaltz, associate professor of statistics at Oxford University, a team member told The Telegraph .

Female embryos shut down one of their pairs of X chromosomes in every cell through a complicated sequence of biological events. "There may be a link between this X inactivation and miscarriage but it is still pure speculation to suggest that this has an effect on the sex ratio," Steinsaltz said.

A 17th century British demographer, John Graunt, was among the first to document a net excess of male births in a publication in 1662. Since then, numerous studies have suggested that the primary sex ratio (male-to-female) at birth is 0.56.

"The equal sex ratio at conception is what one would expect given our understanding of how sperm and eggs are formed," said Steven Orzack, a senior research scientist who specialises in population studies and genetics at the Fresh Pond Research Institute in Cambridge, Massachusetts, who led the study.

The study also analysed and pooled the results of earlier research, including records of foetal deaths and live births, to detect a slight increase in the total mortality of female foetuses through the course of human pregnancies.

The researchers also observed that the sex ratio among abnormal embryos is biased towards males, while the sex ratio among normal embryos is female-biased - in other words, male embryos are at a slightly higher risk of having chromosomal abnormalities than female embryos.

In their sample of embryos, the researchers estimated that the sex ratio among abnormal embryos is 0.508, male biased - 508 abnormal male and 492 abnormal female embryos. And the sex ratio among normal embryos is 0.492, female-biased.

Although the study has established an equal sex ratio at conception in humans, the reason why it changes during the trajectory of pregnancies is unclear. "We're raising more questions than we're answering," Steinsaltz said.