Christmas is the best time to try for a baby, new research suggests
Scientists who looked at more than 270,000 pregnancies find the highest chance of having a healthy baby is in December
Women hoping to fall pregnant should avoid "toxic June", scientists have suggested, after showing that babies do less well if they are conceived in the summer month.
Researchers at Indiana University looked at data from more than 270,000 pregnancies between 2004 and 2009 and found marked differences in the percentage of healthy babies born at different times of the year.
The highest chance of having a healthy baby was in December with three extra babies surviving per every 200 pregnancies compared with in the summer months.
Scientists believe that high levels of pesticides sprayed on crops and differing levels of light and vitamin d could be responsible for the differences.
It is also thought that success rates may follow ancient evolutionary patterns which encourage babies to be born when times are good, such as during the harvest.
There was a notable slump in healthy babies born around Valentine's Day which is the due date for the June babies. The overall birth rate also slumped in February when winter disease peaks among adults.
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.
"It has been my suspicion that this not accidental but may have some biological basis. Vitamin D levels and pesticides might be relevant factors.
"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.
"It's and interesting finding for fertility clinics who want to stay open all year around."
Research published earlier this month by Cambridge University found that babies born in the summer months were likely to be stronger and taller.
It is though t that during spring and summer pregnant mothers absorb more of health-boosting Vitamin D from sunshine in the second and third trimesters, giving their offspring a head-start.
Vitamin D is important for maintaining healthy bones and is thought to protect against cancer, type 1 diabetes and multiple sclerosis.
Recent studies have also shown that around one quarter of genes are seasonal and so time of conception may have a huge impact on health in later life.
Dr Stuart Lavery, of Imperial College, said evidence was growing that timing of pregnancy had a big impact on the health of babies.
"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 best chance of survival e.g. not born in midwinter
"Also air pollution varies at different times of year - this may affect air quality in IVF labs and impact on results."
Previous studies have found that people born in November are far less likely to develop Multiple Sclerosis than those born in May, which has again been linked to vitamin D.
Research has also shown that children grow more quickly in spring and summer, and more slowly in autumn and winter.
Dr Winchester believes that chemicals in the environment could be causing a huge fertility crisis in the west which may be bigger than global warming.
Studying US birth data going back to the 1900s he found that there used to be a birth peak in February but that trend has reversed in line with the wide spread use of pesticides.
"When I looked back through the data I discovered something shocking," he said. "The current pattern peaks in September but in 1900 there was a second peak in February and March.
"It seems as if the introduction of pesticides has eliminated that band of children. I think this is actually worse than global warming."
The research was presented at the annual meeting of the American Society for Reproductive Medicine in Baltimore.