Males and females are conceived at nearly equal rates, according to a new study.

Researchers examined 30 million records of embryos, fetuses and babies to determine their gender ratio at each stage of development. They found that males and females are conceived at nearly identical rates, but females are more likely to die in the first trimester of pregnancy, while males perish more often during the last three months of pregnancy. Female embryos die at an overall greater rate than males, according to the new study. Most medical professionals and biologists believed the opposite to be true.

Raw data for the study dates back as far as the 1930s. At that time, some parents would abort a pregnancy if it was learned the fetus was a female. This could have some effect on the study, but the practice was rare enough that it should have little effect on the conclusions.

Some critics of the report suggest that because the investigation used such a vast database, the study would have included many mothers who were not healthy. However, researchers who wrote the study contend there is no reason to believe the health of the mother would have an effect on the rate at which each gender was conceived.

"Our dataset is the largest ever assembled to estimate the sex ratio at conception and is the first, to our knowledge, to include data from three- to-six-day-old embryos, induced abortions, chorionic villus sampling, amniocentesis and fetal deaths and live births," researchers wrote in an article announcing their study.

Future research will examine the causes behind the correlation between gender and death rates at different stages of pregnancy.

"We'd like to pursue research that would help us understand why it appears that more females are dying than males [during the first trimester]," said Steven Orzack of the Fresh Pond Research Institute.

About 105 boys for every 100 girls are born, but average lifespans of males are shorter than for females. As the newborns begin to age, females survive more often then males. The ratio of the sexes evens out over time, then women become more common than men. Far more females than males survive to extreme old age.

Gender ratio in human babies has been a subject of interest to biologists, gynecologists, statisticians and other researchers for much of the history of science. However, this work is the most in-depth investigation ever undertaken of gender makeup from conception to birth.

Analysis of gender ratios at birth and survivability of fetuses was detailed in the journal Proceedings of the National Academy of Sciences (PNAS).

Photo: Nathan Bittinger | Flickr