ANN ARBOR— In California, New York, Michigan and most other states, how hot it gets in August will influence how many babies are born next April, according to a University of Michigan study that documents the long-suspected link between fertility and the weather.

An increase of 10 degrees Fahrenheit in summer temperatures reduces spring births by up to 6 percent, the study shows.

It is not clear whether the decline in conceptions during hot weather is because people don’t feel like having sex in hot weather, or because the reproductive physiology of men and/or women is somehow affected by heat, says economist David Lam, director of the U-M Population Studies Center and first author of the working paper.

Although spring is the peak season of birth for the young of most species around the globe, it isn’t the most likely birth date for human babies in the United States.

“In this country, about 20 percent more births occur in September than in April or May,” Lam says. ” And that pattern is more pronounced in the South, where the summers are extremely hot, than it is elsewhere in the nation.

Comparing monthly birth data with monthly temperature data for a 15- to 20-year period, Lam and Boston University colleague Jeffrey Miron found that above-average temperatures in August are associated with below-normal April births in many of the states they examined, including California, Georgia, Kansas, Louisiana, Massachusetts, Michigan, Minnesota, New York, Texas and Virginia.

Since air conditioning has become more widely available, Lam notes, this pattern has been changing. ” In the 1950s, warm places like Louisiana reported 40 percent more births in September than in March, April or May,” Lam notes. ” Now there are only about 20 percent more babies born in fall than in spring. ”

Lam and Miron found that between 1941 and 1967 in Georgia, for example, an increase in mean August temperatures from 77 degrees Fahrenheit to 82 degrees Fahrenheit was associated with a 10 percent drop in conceptions. But from 1968 to 1988, the contraceptive effect of high August temperatures had virtually disappeared.

Contrary to popular belief, Lam and Miron found no evidence that below-average temperatures in winter were followed by higher-than-average August and September births, however.

“Even in places like Minnesota, where mean January temperatures range from below zero degrees Fahrenheit to just over 20 degrees, there is no evidence that extreme cold has an effect on conceptions. ”

The research was funded by the National Institute for Child Health and Human Development.

National Institute for Child Health and Human Development