Children born in May have an increased risk of going on to develop multiple sclerosis, research has suggested.
Being exposed to too little sunlight during pregnancy could be a factor
The analysis of MS rates among over 42,000 people born in the northern hemisphere showed a significantly lower risk for those born in November.
The effect was most evident in Scotland, where the prevalence of MS was the highest.
The study, published online by the British Medical Journal, was carried out by Oxford University researchers.
The team suggest that complex interactions between genes and the environment before or shortly after birth may help to explain the links they found.
They analysed data on the birth month, medical and family histories of 17,874 Canadian patients and 11,502 British patients with MS.
They were compared with a matched group of people from the general population and unaffected brothers and sisters of those with MS
In Canada, significantly fewer people with MS were born in November compared with the general population or sibling groups.
And in Britain, fewer people with MS had been born in November and significantly more had been born in May.
The number born in December was also significantly lower.
The researchers also looked at data from Denmark and Sweden, which again showed a May peak and a November fall.
Overall, it was found that around 10% more people born in May and 10% fewer people born in November went on to develop MS, compared to rates seen in the other months of the year.
The researchers, led by Professor George Ebers of the department of clinical neurology at the Radcliffe Infirmary at the University of Oxford, said the explanation for the difference remained unclear.
But writing in the BMJ, they said: "The risk factors for the effects of the timing of birth must vary seasonally and probably interact with the development of the central nervous system or immune systems, or both."
The team said other studies had suggested exposure to the sun or seasonal variations in a mother's vitamin D levels during pregnancy may have an impact on brain development.
Mothers of babies born in May would be exposed to less sunlight because they are pregnant during the winter.
Professor Ebers added: "MS is an adult-onset disease. So you wouldn't think that what happened right back at the beginning would be a risk factor.
"But this study does show that the month in which people are born appears to have an effect."
And he said people who had MS in their family may use the information to decide when they would try to conceive.
Mike O'Donovan, chief executive of the MS Society, said, "This is an interesting study which adds weight to the accepted view that something related to the environment - including around the time of birth - may influence the development of MS.
"It is also worth noting it does not negate the likelihood of environmental factors operating at other times as well. It is another piece in a very complex jigsaw of research."
It affects around 85,000 people in the UK and is most often diagnosed in
people between the ages of 20 and 40. Women are almost twice as likely to develop MS as men.