That’s the question being asked by researchers, after a large study linked rises in birth defects to levels of agricultural chemicals in the water supply.

Researchers at the universities of Indiana and Cincinnati knew that agricultural chemicals, such as pesticides and herbicides, are used more often in the spring and summer months. They checked federal records of water quality monitoring, and found levels of agrichemicals peaked from April through July.

They then looked at records of all US births—more than 30 million—during the same years. They found babies conceived between April and July were more likely to have birth defects than those conceived during other months.

It sounds like a case of cause and effect—but the researchers urge caution. They say the seasonal rise in birth defects could be down to other factors. This study shows a link, but it can’t show that birth defects are caused by chemicals in the water. In the meantime, it’s important to keep the seasonal rise in perspective. On average, there were 48 more birth defects for every 100,000 babies conceived during the higher risk spring and summer months. That’s not a huge increase in risk.