Baby Boys More Fragile Than Baby Girls

More Baby Boys Conceived When Conditions Are Optimal

From the WebMD Archives

March 28, 2003 -- Males are the fragile sex -- and there's research to prove it. Now, a study from Italy shows that Mother Nature tries hard to compensate for the vulnerability of baby boys and fetuses.

In fact, more boys are conceived from September to November in Italy, the time of year when conditions for pregnancy and birth are optimal.

The study appears in the March 27 issue of Human Reproduction.

This is the first study designed specifically to focus on sex ratio -- number of baby boys to baby girls -- at conception rather than at birth. In analyzing records of more than 14,000 births, Italian researchers found that 535 males were conceived in the "best" months for conception, compared with 465 females -- while only 487 males were conceived in the "worst" months, compared to 513 females.

This pattern is linked with the length of daytime hours and environmental temperature. Therefore, it is different in different parts of the world, explains lead researcher Angelo Cagnacci, MD, an ob-gyn with the Policlinico of Modena, Italy, in a news release.

Optimal conditions for fertilization are daytime of 12 hours and an average temperature of 54 degrees Fahrenheit. In regions that are equidistant from the equator -- but located in different hemispheres -- maximum conception rates occur at opposite times, he points out.

His finding supports earlier theories regarding season of conception and a baby's sex, Cagnacci says. More boy babies and fetuses die when they are conceived in months not considered optimal.

"We could conclude from our data that if you want a boy in this region of the world [Italy], you would have a better chance if you tried to time conception between September and November," he says. "And, although the overall numbers of conceptions between March and May are lower than in the autumn, these are the months when you are more likely to conceive a girl."

Though humans are not seasonal breeders, it is still not clear how nature acts to favor one sex or another according to conditions, he says. "It is tempting to speculate that sex selection occurs in the early stage of pregnancy, probably already at the time of conception and/or embryo implantation."

Conception is a handicap race, and nature tries to give baby boys a head start by favoring them in optimal reproductive conditions, says Cagnacci.