It seems like every couple of years there are some new baby rules. Don’t lay them down on their stomachs. Don’t lay them down on their backs. Do yoga while pregnant. Don’t do yoga while pregnant. Breast feed. Don’t breast feed. In light of a new study, the latest piece of baby advice you might hear from your doctor may be “don’t conceive in the spring or summer.”

A report published in the April 2009 issue of the medical journal Acta Pædiatrica shows, for the first time, a correlation between babies conceived in April, May, June, or July, and birth defects. Those conceived in the spring and summer months have a higher overall rate of birth defects such as spina bifida, cleft lip, clubfoot and Down’s syndrome. In fact, half of the 22 categories of birth defects had a statistically significant correlation to time of conception. The study compiled data from the 30.1 million births that occurred in the U.S. between 1996 and 2002. That’s not just a lot of Gerber’s, but a serious amount of informative material.

In searching for a reason behind the numbers, researchers found that during those same spring and summer months there are increased levels of pesticides in surface water across the country. Levels of nitrates, atrazine, and the particularly nasty herbicide atrazine (which is in fact banned in Europe but legal in the U.S.), are at their peak during warm months. Many of these pesticides are known to be harmful to embryos. As of now, there is no proof the two occurrences are directly related, but scientists have reason to believe there is a cause-and-effect relationship occurring between high pesticide levels in the water and a higher rate of birth defects in babies conceived during the same time.

Common knowledge tends to blame birth defects (which occur in 3 out of every 100 newborns) on a mother’s irresponsible behavior: drinking, smoking, or being too old, for example. It’s no longer quite so simple, though. Mothers in the study who did not report any of those characteristics still had a higher rate of birth defects among their children born in April through July. The good news here is that if a causal relationship is found between pesticides and birth defects, then a lot can be done to reverse or ameliorate the situation.