Mothers-to-be who conceive in the spring or summer are more likely to get pre-eclampsia 'because they get less vitamin D at a crucial time of their pregnancy'

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Pregnant women who conceive in the spring or summer have today been warned they are at higher risk of pre-eclampsia. 

A study of 50,000 women in Denmark found those who became pregnant between June and August were more likely to get high blood pressure disorders.

Pre-eclampsia, a complication that strikes up to six per cent of pregnancies, usually begins after 20 weeks. 

At this point, women who conceived in the spring or summer are be in the depths of winter – when they get less vitamin D because of a lack of sunshine.

Danish researchers therefore believe the lack of vitamin D triggers changes in the placenta which may lead to pre-eclampsia. 

However, an expert slammed the Aarhus University study for providing 'absolutely no proof' of the vitamin D claim. 

Pregnant women who conceived in the spring or summer have been warned they may be at higher risk of pre-eclampsia and other high blood pressure disorders 

The research involved 50,665 women who were part of the Aarhus Birth Cohort (ABC), a registration of births at university's hospital. 

Some 8.5 per cent were diagnosed with a hypertensive disorder of pregnancy - either pre-eclampsia or gestational hypertension.

Pre-eclampsia affects up to six per cent of pregnancies in the UK and eight per cent in the US.  

It is characterised by high blood pressure, which puts extra strain on the blood vessels and heart, and extra protein in urine (proteinuria). 

The exact cause is unknown. However, it’s thought a problem with the placenta causes pre-eclampsia, according to the NHS. 

Gestational hypertension, which affects around six per cent of pregnancies, is blood pressure above 140/90mmHg without proteinuria. Ideal blood pressure is no more than 120/80mmHg.

Women with a hypertensive disorder typically reported to be non-smokers, drink less alcohol and have a higher BMI. 

An increased risk was found for women who conceived during the spring and early summer, with the effects peaking during midsummer.

The risk subsequently decreased steadily during the autumn to reach a low by winter. 

Women who conceived in June were 17 per cent more likely on average to have pre-eclampsia. The second part of their pregnancy would have been during the winter months before birth in March.

Those who conceived in August, giving birth in May, were on average 35 per cent more likely to get gestational hypertension. 

Adjustment for pre-pregnancy BMI, the mother's age, ethnicity, smoking habits or whether she had IVF did not change the results. 

Seasonal variations in vitamin D levels may help to explain the findings, PhD student Christine Rohn Thomsen and colleagues said.

From September to March every year, vitamin D is harder to get because there is less sunlight.

Ms Thomsen said: 'Our results are of great interest, as vitamin D may have caused the observed seasonal variation in the hypertensive disorders. 

'It has long been assumed that vitamin D affects the pathogenesis of hypertensive disorders of pregnancy and our results support this hypothesis.'

Other studies have shown a link between vitamin D in plasma - components of the blood - and risk of pre-eclampsia.

The researchers believe vitamin D has an anti-inflammatory role in the placenta - which is believed to play a role in the development of pre-eclampsia. 

They added that vitamin D regulates calcium levels, and calcium is linked with high blood pressure conditions. 

However, Dr Richard Weller, a reader in dermatology at the University of Edinburgh, disagrees with the researchers assumptions. 

Dr Weller, who has done extensive research into UV exposure and its health benefits, cautioned the study did not look at women's vitamin D levels.

He told MailOnline: 'All it [the study] shows is seasonal variations.

'You cannot say from this that vitamin D prevents pregnancy hypertension, there is absolutely nothing to prove it. The whole vitamin D thing is wholly overplayed.'

The body creates vitamin D from direct sunlight on the skin. But sunlight causes a variety of other processes in the body that were not considered by the Danish researchers.

Dr Weller said: 'Vitamin D is one of the means that sunlight is good for your health. But fixating on it means forgetting about the other mechanisms. 

'There is more and more research coming out showing that sunlight has benefits, such as reducing heart disease and blood pressure. 

'I've done a lot of research showing sunlight reduces stuff called nitric oxide in the skin. This reduces blood pressure.

'But trials show vitamin D has absolutely no effect on blood pressure.' 

The study was published in Acta Obstetricia et Gynecologica Scandinavica journal.