Women with epilepsy should be given more information and warnings that a common anti-seizure medication could harm their unborn child if they become pregnant, experts have said.
Charities are calling on the Health Secretary to take action to prevent "heartbreaking" cases of children born with "an avoidable disability" as a result of taking the drug sodium valproate while pregnant.
They say women should get a face-to-face appointment once a year to ensure they are aware of the risks.
One in 10 babies born to mothers who are taking the drug are born with birth defects such as spina bifida.
And between 30% and 40% can experience delays in early development such as walking and talking, have poor intellectual abilities and language skills and memory problems, according to the Epilepsy Society.
But over a third of women who have epilepsy were unaware of the risks, according to a poll of 2,000 carried out for the UK's three leading epilepsy charities - Epilepsy Society, Young Epilepsy and Epilepsy Action.
The survey also found 18% of the 515 women who were taking sodium valproate who completed the survey were not aware of its potential affects on unborn children.
Clare Pelham, chief executive of the Epilepsy Society, will present the survey's findings to the European Medicines Agency's (EMA) public hearing into medicines containing valproate.
She says a change in regulations is "crucial" to ensure women get an in-person appointment before their prescriptions of the drug are renewed.
At present, the NHS guidance recommends a 12-monthly review for epilepsy, but the guidelines are not mandatory.
The epilepsy society said the issue of children born in the UK with birth defects linked to valproate stretches back over 40 years.
Some estimates suggest that up to 7,000 children may have been affected.
The Epilepsy Society said that between January and June last year, around 28,000 women aged 14 to 45 across the UK received a prescription for valproate.
Data shows prescribing of valproate in approximately 0.06% of pregnancies - which equates to around 450 children born a year, the charity added.
Health officials have taken a number of steps to highlight the potential risks surrounding the use of sodium valproate - which is prescribed under brand names including Epilim, Episenta and Epival.