Couples should try to conceive at Christmas for the best chance of a healthy baby, a study suggests, writes Andrew Gregory in Baltimore.
US experts discovered women who conceived in December were more likely to give birth to a healthy baby than women who conceived in any other month.
The month to avoid conceiving in is “toxic” June, according to the research presented at the world’s biggest fertility conference in Baltimore.
The reason for seasonal variation is not clear.
But speaking at the annual American Society for Reproductive Medicine meeting, scientists said they believed high levels of pesticides sprayed on crops during the summer may play a part in the “June effect”.
And they think higher levels of light and Vitamin D in the summer and autumn - in the final stage of pregnancies conceived in December - could help give newborns a better chance.
Vitamin D is vital for maintaining healthy bones and is thought to help protect against certain illnesses.
A further suggestion is that success rates may follow ancient evolutionary patterns which encourage babies to be born when times are good, like during the harvest.
Indiana University experts studied 270,000 pregnancies between 2004 and 2009 and found differences in the percentage of healthy babies born at different times of the year.
The highest chance of having a healthy baby was conceiving in December with three extra babies surviving per every 200 pregnancies, compared with in the summer months.
Dr Paul Winchester, of Indiana University, said: “There are a lot of things we are finding that are seasonal and very disturbing. We have seen significant seasonal differences in reproduction. Valentine’s Day is one of the least likely times to conceive a baby, whereas Christmas seems a very positive time.
“June is a toxic month. The June effect was something that we saw develop at a very early stage. White mothers have lowest survival rates in June and significantly shorter pregnancies, with premature babies.
“If you want to avoid a birth defect or a premature birth then it might be worth avoiding June. Other studies have shown that spina bifida incidence and sudden infant death peaks in June.”
British fertility expert Dr Stuart Lavery, of the Department of Reproductive Medicine at Hammersmith Hospital , said there was growing evidence that pregnancy timing has an impact on the health of babies.
He explained: “There are a few different theories, female reproduction may be sensitive to hours of light exposure. There are many examples in nature and it fits in with a Darwinian model of when the offspring are born to have the best chance of survival.
“Also, air pollution varies at different times of year - this may affect air quality in IVF labs and impact on results.”