Conceived in the steaming heat of summer, a baby is slightly more likely to be a boy. Conceived in winter, the odds favor a girl.
That's the conclusion of researchers who studied birth dates and weather patterns over 50 years, reports Correspondent Dr. Michael Breen of CBS News station WBBM-TV in Chicago.
The research, published in New Scientist magazine, supports scientific suspicions that the ratio of male and female births might vary with the seasons.
"Sex ratio seems to correlate with temperature about one month before conception," the magazine said.
The research was conducted by Alexander Lerchl, of the University of Muenster in Germany.
Lerchl compared monthly temperatures in Germany from 1946 to 1995 with German birth records covering the same period to see if temperatures influenced the sex of children.
He found that more boys were born after hot summers and girls followed cold spells. Even small fluctuations in temperature had an impact on the sex ratio.
"Temperature seems to play its part when the father-to-be's sperm starts maturing. Lerchl speculates that hot spells may damage sperm carrying an X chromosome more than sperm carrying a Y, so more boys are conceived," the magazine said.
Lerchl said his findings do not mean people living in hot climates have more sons than daughters because people adapt their clothing to local weather and skin temperature does not vary too much from different locations.
Still, he believes global warming could further increase the ratio of males to females.
The magazine said the other possibility to explain the results is that people have sex more often in balmy weather, which increases a woman's chance of having a boy because sperm carrying the Y male chromosome are faster swimmers and more likely to fertilize an egg.
"The message to the general audience should not be that immediately, 'Let's get our husbands in the cold showers' or 'Let's put them in the hot jacuzzi,'" says Dr. Edmond Confino, Northwestern Memorial Hospital, who was not involved in the study.
While it may be a fascinating link, it's not quite foolproof. One mother, Nina Griffith, told Dr. Breen her daughter Katie was not conceived in December, but in the hottest month of the year Â– August.