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.
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."