Women who take ecstasy in the early stages of pregnancy could be putting their baby at risk, say scientists.
Half a million young Britons regularly take ecstasy
Researchers in the US have found evidence to suggest the drug could cause behavioural problems or even brain damage in unborn children.
Their theory, published in the journal Neurotoxicity and Teratology, is based on tests on rats, which were given high doses of the drug.
However, it will add to concerns over the long-term effects of ecstasy.
An estimated three million Europeans have taken ecstasy at least once during their life.
The drug is popular with clubbers. In Britain, an estimated 500,000 people take ecstasy every weekend.
Surveys suggest that 15% of 16 to 24 year olds have tried the drug compared to just 1% of those over the age of 35.
There are no reliable figures on how many women take the drug while pregnant. Anecdotal evidence suggests many women stop taking it as soon as they find out they are expecting.
However, this opens up the possibility that some women may be putting their babies at risk without knowing it during the early stages of their pregnancy.
Dr Jack Lipton and colleagues at Rush-Presbyterian-St Luke's Medical Center in Chicago carried out tests on pregnant rats to try to find out what type of damage the drug may inflict.
They injected recently pregnant rats with the drug twice a day for six days.
They injected similar amounts of harmless saline in another group over the same period.
They carried out tests on all of the baby rats 21 days after they were born.
They found that those exposed to ecstasy in the womb had five times more dopamine neurone fibres in the frontal cortex than the other rats.
The frontal cortex is responsible for planning, impulse control and attention. High levels of dopamine have been associated with schizophrenia.
They also discovered higher than normal levels of these fibres in the striatum - an area of the brain associated with movement and reward.
In addition, the researchers said these rats had behavioural problems.
They placed ecstasy-exposed rats in a new environment. They found that they spent much longer exploring and took longer to familiarise themselves with it.
The researchers suggested this may be because they had learning or attention deficit problems or simply because they were hyperactive as a result of being exposed to ecstasy in the womb.
The researchers said their findings highlighted the need for doctors to monitor children who may have been exposed to ecstasy in the womb.
"Our findings show that exposing rats to ecstasy at a time of prenatal development that correlates with the first trimester in humans results in lasting changes in brain chemistry and behaviour," they said.
"This research warrants the continued monitoring of children exposed to this drug."
The UK charity DrugScope advised women who are planning to have children not to take ecstasy.
"Anyone who is pregnant or is about to become pregnant should not take ecstasy or any other drug," a spokeswoman said.
"Little is yet known about the effects of heavy, long term use of ecstasy but there are increasing concerns about the possibility of mental health problems, especially chronic depression and sustained memory loss."
Lesley King-Lewis, chief executive of Action on Addiction, said: "These findings are particularly disturbing. Women are often unaware they are pregnant in the early stages, and may continue to use.
"Research has shown that clubbers are more likely to take part in unsafe sex than other groups of people. This increases the likelihood of accidental pregnancy, and therefore intensifies the risk that the unborn baby will be exposed to ecstasy.
"Although research in rats should always be treated with caution when applying it to humans, this study highlights the need for better awareness of the dangers of ecstasy and of safe sex measures."