A new study has found that babies born in sunnier months have an increased risk of developing multiple sclerosis (MS) later on in life.
The study, carried out in Australia, found that women who fail to get enough sunshine when pregnant could increase the chances of their children developing MS.
Sunshine is a vital source of vitamin D, the researchers said.
The team of scientists said a baby's nervous system and immune system could be affected during development by how much natural sunlight its mother is exposed during pregnancy.
Currently, around 100,000 people in the UK suffer from MS.
The disease is caused by damage to myelin, which acts as a protection shield around the nerve fibres of the central nervous system.
The latest study from Australia noted that exposure to sunlight in the first trimester of pregnancy and the early part of the second trimester played a key role.
For babies whose mothers did not get enough sun, the chance of developing MS was 32% higher if they were born in early summer months than if they were born in early winter.
The researchers said: "Our results show a trough in multiple sclerosis in people born in May-June, when a protective effect is evident, compared with a peak in those born in November-December, thus mirroring the northern hemisphere pattern of a peak associated with May."
The study also found exposure to sunlight during childhood and early adulthood was important for youngsters.
The experts, from the Royal Children's Hospital in Melbourne and the Australian National University, called for further studies on the benefits of vitamin D supplements in pregnancy.
"I am not sure about this at all! I have MS but was born in October. I was exposed to sunlight as much as any one is! I was born in Scotland but so was my sister and she does not have MS!" - Helen Campbell, Brighton
"How controlled was this research? Was maternal seasonal diet also considered?" - F Hayward, UK