Pregnancy and child

Summer babies and risk of MS

“Babies born in the summer months have a higher risk of developing multiple sclerosis (MS) because their mothers do not get enough sun during pregnancy,” The Times reported.

This study in Australia investigated whether there is a link between the risk of developing MS and the month that people are born in. The researchers looked at associations between people having MS and the levels of sunlight their mothers were likely to have been exposed to in each trimester of their pregnancy.

The researchers found an association between lower ambient ultraviolet (UV) levels in the first trimester (first 12 weeks) of pregnancy and an increased risk of MS, indicating that women with babies that had been conceived in the autumn and winter months were more at risk.

There is an increasingly popular theory that MS is linked to sunlight exposure and levels of vitamin D, which is produced by the body in response to UV light. These findings appear to further support this idea. However, it is important to point out that vitamin D levels were not measured and vitamin D can be affected by several factors, including diet, lifestyle and skin type. Further research is needed in this area.

Where did the story come from?

The research was carried out by Dr Judith Stables and colleagues from Australian National University and the Royal Children’s Hospital, Melbourne. The study was funded by the Australian National University. The work was published in the peer-reviewed British Medical Journal.

What kind of research was this?

This retrospective cohort study investigated whether the month in which a person was born in Australia affected their risk of later developing multiple sclerosis (MS). MS becomes increasingly more prevalent the further from the equator a region lies. This has led to the theory that MS is linked to sunlight exposure and levels of vitamin D, which is produced by the body in response to UV light.

The researchers suggest that pregnant women are at particular risk of vitamin D deficiency, due to the physiological changes of pregnancy and because they spend less time outdoors. They say that this may affect the baby’s brain development, though there is no direct evidence to support this. The researchers looked at the month of birth and risk of MS in Australia, a country with a large seasonal and regional variation in ambient UV levels.

It is important to note that this type of study can only find associations between factors such as UV and disease. It cannot establish whether lack of sun exposure directly triggers the disease.

What did the research involve?

The researchers used data collected from a 1981 survey of MS prevalence in five Australian states. They identified the month of birth for all people with MS born between 1920 and 1950. They also identified their gender and the state in Australia in which they were born. All people with MS were interviewed and their condition verified through a medical exam, except in New South Wales where only 57% were interviewed owing to the large number of patients in this state.

A total of 1,524 people were born with MS in the five surveyed states between 1920 and 1950. As there was a small number of people with MS in each month, the researchers pooled the data into two-month groups. May-June was used as a reference period as this was the Australian winter when the ambient UV was at its lowest levels.

As a control reference group, the researchers used information from the 1981 census, including the month and location of birth for about 2.5 million people.

To estimate individuals’ exposure to UV, the researchers used the monthly averages of daily total ambient UV radiation in the capital city of each state, collected between 1996 and 2000.

What were the basic results?

The researchers found that the average total daily ambient UV radiation ranged from 1.6 erythemal dose units a day in Hobart, Tasmania, in July to 30.4 units a day in Perth, Western Australia, in January. An erythemal dose unit is a measure of the minimum amount of ultraviolet radiation exposure needed to induce erythema (skin redness) or sunburn.

As in previous studies, the incidence of MS was higher in women than men. Compared to that in New South Wales, the risk was lower for those born in Queensland in Northern Australia (risk ratio [RR] 0.59, 95% confidence interval [CI] 0.51 to 0.61) but higher for those born in Tasmania in Southern Australia (RR 2.70, 95% CI 2.06 to 3.51).

The risk of having MS was 1.23 to 1.34 times higher in people born in periods other than May-June. The highest risk was for people who had been born in the early summer months of November-December (RR 1.34, 95% CI 1.10 to 1.63). This pattern persisted after gender, age and region of birth were taken into account. When the risk ratio of May-June births was compared to November-December births for the different latitude regions, there was no difference in relative risk.

An analysis of stage of pregnancy and UV exposure showed there was an association between lower UV exposure and increased risk of MS in the first trimester (RR 0.72, 95% CI 0.62 to 0.84). However, there was no association between UV exposure levels and risk of MS in the later months of pregnancy.

How did the researchers interpret the results?

The researchers said that there is an “inverse association between low ultraviolet radiation in the first trimester and increased risk of multiple sclerosis in the offspring”. They also said that the “higher risk of multiple sclerosis for people born in November-December is consistent with these infants having experienced lower levels of ultraviolet radiation during the first trimester”.

The researchers say that as vitamin D receptors are found in cells that develop into the brain during early development of embryos, vitamin D may play a role in brain development.


These findings suggest there is a small increase in the risk of MS for babies born in the Australian early summer with a 34% increased risk relative to the risk in people who were born in May to June (Australian winter). This corresponds to the mothers having lower exposure to ambient UV radiation levels during the first trimester of their pregnancy.

The idea that MS is linked to inadequate exposure to sunlight is growing in popularity, and these findings appear to further support this theory. The study has several limitations that should be considered when interpreting the findings:

  • The researchers could not directly measure the vitamin D status of the mothers during pregnancy. Vitamin D levels are affected by exposure to UV, but also by vitamin D dietary intake, personal behaviour (such as how much time is spent outside) and the mother’s skin pigmentation. Additionally, the relationship between the mother and foetus’s vitamin D levels has not been determined in this study.
  • This study was relatively small and the number of individuals that were born in each bi-monthly period was not stated. There is, therefore, a greater risk that these associations are due to chance.
  • The values for the ambient UV levels were taken from averages of a period between 1996 and 2000, whereas the people with MS were born between 1920 and 1950. It is possible that UV exposure for mothers in their first trimester was different between these periods.

This study showed an association between low UV exposure in the first trimester and a small increased risk of MS. Further research would be needed to assess whether this is due to vitamin D levels and whether a mother’s exposure to the sun has any effect on her baby’s brain development that would result in increased susceptibility to MS. The causes of MS are not firmly established, but possibly involve a different susceptibility of individuals owing to their genetic make-up and exposure to environmental factors, such as viruses and vitamin D.

Vitamin D is made in the body from exposure to sunlight, but the hazards of excess exposure to UV light are well known. Everybody, including pregnant women, should take precaution if spending time in the sun and should always avoid sunburn.

NICE recommend

The National Institute for Health and Clinical Excellence (NICE) states: "There is a need for research into the effectiveness of routine vitamin D supplementation for pregnant and breastfeeding women. Although there is some evidence of benefit from vitamin D supplementation for pregnant women at risk of vitamin D deficiency, there is less evidence in the case of pregnant women currently regarded as being at low risk of deficiency. It is possible that there will be health gains resulting from vitamin D supplementation, but further evidence is required.

"As such, pregnant women may want to take a supplement of 10 micrograms of vitamin D every day.

"Women at greatest risk are advised to take this daily supplement. These include:

  • women of South Asian, African, Caribbean or Middle Eastern family origin
  • women who have limited exposure to sunlight, such as women who are predominantly housebound, or usually remain covered when outdoors
  • women who eat a diet particularly low in vitamin D, such as women who consume no oily fish, eggs, meat, vitamin D-fortified margarine or breakfast cereal
  • women with a pre-pregnancy body mass index above 30 kg/m2"

NHS Attribution