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Women who drink during pregnancy may be damaging the future fertility of their sons, research suggests.

In a study of almost 350 young men, sperm levels were a third lower in those whose mothers had drunk more than four drinks a week during pregnancy compared with teetotallers.

The Danish researchers told a fertility conference these men may have a harder time getting their partner pregnant.

UK experts said alcohol may not be the issue, but a marker for other factors.

Current advice is to avoid alcohol during pregnancy, but those who do so are advised to have no more than one or two units of alcohol once or twice a week.

The study included men, now aged between 18 and 21, whose mothers had taken part in a large study on lifestyle while they were pregnant with them.

Researchers told the European Society of Human Reproduction and Embryology conference that they split the men into four groups - those whose mothers drank nothing, those who had one to one and a half drinks a week, two to four drinks a week, or more than four drinks a week.

One drink was classed as a beer, small glass of wine or one measure of spirits.

Four drinks in the study is equivalent to around six UK units.

When they looked at sperm counts in the men's semen samples, they found those with the highest alcohol exposure in the womb had average concentrations of 25 million per millilitre compared with 40 million/ml in those whose mothers drank no alcohol.

After adjusting for factors which might influence sperm, such as smoking and medical history, they calculated that average sperm concentration was 32% lower in the highest alcohol group than the abstinence group.

Fertility effects

The World Health Organization says that a normal range of sperm is 20 to 40 million/ml.

It is known that lower concentrations of sperm - even within the normal range - may mean it takes longer to conceive.

Study leader Dr Cecilia Ramlau-Hansen, from Aarhus University Hospital, said that if the link is proven in further studies, it may explain why semen quality seems to have fallen in recent decades.

"If exposure to alcohol in foetal life causes poor semen quality in adult life, we would expect that populations with many pregnant women drinking, possibly heavily, in pregnancy would have lower fertility in comparison with populations where pregnant women do not drink."

Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said the study was very interesting and supported the theory that male fertility was influenced at an early stage by factors in the womb.

But he pointed out that the low sperm levels seen did not equate to infertility.

"I don't think we can be certain that alcohol is necessarily the bad thing here - it could be a surrogate marker for something else - but clearly there is some kind of relationship.

"It needs following up but it might help us understand factors which affect testicle development in the womb."

Pregnant women in the UK should be told to routinely take vitamin D supplements, researchers say.

The team at University College London Institute of Child Health says official bodies currently offer conflicting advice.

Writing in the British Journal of Nutrition, they say there is a "strong case" for a daily dose of vitamin D in pregnancy.

But one leading expert said more evidence was needed.

The Department of Health advises pregnant women to ensure they receive a certain level of vitamin D - 10 micrograms per day. The researchers say this in effect endorses use of supplements, because diet and the sun provide too little.

But the National Institute of health and Clinical Excellence (NICE) said in its guidance that it did not support supplements.

It has instead said that women should simply "be informed" about the importance of having adequate vitamin D levels during their pregnancy and while breastfeeding - adding that some women may choose to do this via supplements.

Additionally, pregnant women on low incomes are entitled to a range of nutrients - including vitamin D - as part of the Healthy Start Scheme.

'Unacceptably high'

The paper said the UK was the only one of 31 countries examined which did not recommend that women of reproductive age took a vitamin D supplement, and that it also fails to endorse a daily supplement for expectant mothers.

Dr Elina Hypponen, co-author of the paper, said: "The incidence of vitamin D deficiency in pregnant women in Britain is unacceptably high, especially during winter and spring.

"This is compounded by a lack of exposure to sunlight and the limitations of an average diet to meet the optimal need.

"In the most severe cases, maternal vitamin D deficiency can be life threatening to a newborn.

"We believe that the routine provision of a daily supplement throughout pregnancy would significantly decrease the number of mothers who are clearly vitamin D deficient, reducing related serious risks to their babies."

Dr Hypponen said past evidence showed a proactive approach to supplements coincided with a much lower incidence of deficiency linked diseases such as infantile hypocalcaemia [also known as William's syndrome, which affects development], and rickets [which weakens bones].

One in four

The authors say women from some ethnic minorities are recognised to be at greater risk because their darker skin means they do not absorb as much sunlight.

But they add that the problem is also common among white women, even when they live in southern England.

Women are more likely to be vitamin D deficient than men, and a previous study found one in four pregnant mothers has been found to be vitamin D deficient during winter and spring

The Royal College of Obstetricians and Gynaecologists recommends only women at risk of vitamin D deficiency should take a supplement.

Patrick O'Brien, a spokesman for the college, said: "There is gradually accumulating evidence that universal vitamin D supplementation in the UK might be beneficial for the whole population.

"But more research is needed on the balance of risks and benefits in women at low risk of vitamin D deficiency, and on the correct dosage to use."