Researchers say the finding has implications for a greater understanding of the causes of MS and the potential use of hormone therapy to delay symptoms.

It is estimated that more than 2.5 million people worldwide have the condition, with women four times more likely to than men.

The study, which used a global database of more than 70,000 MS patients and was run by Monash University in Australia, looked at whether pregnancy can delay the onset of MS, which is very frequently diagnosed in women of childbearing years.

Dr Vilija Jokubaitis, from the university's department of neuroscience, studied more than 3600 women attending four MS clinics in the Czech Republic and Australia, all of whom were enrolled in MSBase - a database of more than 70,000 people with MS in 35 countries.

The study published in the JAMA Neurology journal found women who have been pregnant were diagnosed with their first MS symptoms on average 3.3 years later than women who had never been pregnant.

A similar delay was also observed in women who had carried a baby to term - with onset delayed by an average of 3.4 years.

Dr Jokubaitis suggests pregnancy could reduce the abnormal overactivity of the immune system that causes MS, potentially long term.

"At present, we don't know exactly how pregnancy slows the development of MS, but we believe that it has to do with alterations made to a woman's DNA," she said.

"We are now seeking funding opportunities to explore this exciting possibility."

The study suggests an association between previous pregnancies and childbirths and timing of clinically isolated syndrome onset.

However having more pregnancies or childbirths did not appear to be associated with a later onset.

The researchers said further studies are needed to help explain the mechanisms behind the associations between pregnancy and onset of multiple sclerosis.

"MS affects over 130,000 of us in the UK, with women 'of reproductive age' almost three times more likely to be diagnosed," said Dr Emma Gray, assistant director of research at the MS Society.

"We know many women living with MS experience fewer symptoms while they are pregnant.

"This new study builds further evidence for pregnancy's protective influence, by showing that having been pregnant is associated with later onset of clinically-isolated syndrome, which can be the first sign of MS.

"The reasons for pregnancy's positive effect are not fully understood but it's thought the answer lies in our immune system - which misbehaves in MS but is dampened during pregnancy.

"We want to slow, stop and eventually prevent MS, and these findings could help us with that goal."

Australian Associated Press