If you're planning to get pregnant, avoid conceiving in May – for your baby's sake.
It sounds like alarmist advice, but that's what scientists are warning based on a study of 1.5 million children who were found to suffer from a series of health problems if they were born in January or February.
They said that women who conceived in May were more likely to give birth to underweight, premature babies who carry a higher risk of health problems in later life.
And they added that the effect was independent of class, race, or factors such as smoking. It is most likely due to the risk of women who become pregnant in mid-spring being affected by seasonal flu in the winter months.
The study was based on analysing the health data of nearly 650,000 groups of siblings in New York whose mothers gave birth to two or more children.
By following children of the same mother, the scientists were able to eliminate other possible influences on premature births, such as social class or race, said Professor Janet Currie, of Princeton University.
"We found a sharp spike for premature births among babies conceived in May and we think the reason is due to the seasonal exposure of the pregnant mothers to flu," said Professor Currie.
"The funny take-home message from the study is that people should avoid getting pregnant in May but the more practical message is that pregnant women should consider getting a flu shot or avoid getting flu if at all possible."