Almost 70% of women surveyed about a powerful epilepsy drug have not received new safety warnings about the dangers of taking it during pregnancy, the BBC has been told exclusively.
Sodium valproate, known as Epilim, carries a 10% risk of physical abnormalities in unborn babies.
About 20,000 children have been harmed by valproate medicines in the UK since the 1970s.
The medicines regulator said the drug had been kept under constant review.
Babies exposed to the drug in the womb have a 40% risk of developing autism, low IQ and learning disabilities.
The survey of 2,000 women and girls with epilepsy under 50 was commissioned by three charities - Epilepsy Society, Epilepsy Action and Young Epilepsy.
Of the 475 currently taking the drug, 68% said they had not received the new warnings known as the Valproate Toolkit.
The toolkit was launched in February 2016 and included printed warnings for patients in GPs' surgeries, hospitals and pharmacies, containing up-to-date information on the risks for neuro-developmental disorders in children of women taking the drug.
The toolkit was created after the European Medicines Agency told all national watchdogs to improve information for patients.
The survey also showed that one in six women taking sodium valproate did not know the drug could "negatively affect the development and/or physical health of children born to women taking this medication".
Twenty-one per cent said they had not had a discussion initiated by a healthcare professional about the risks of taking the drug during pregnancy.
Sodium Valproate has been prescribed since the 1970s but it was only last year that warnings were put on packets, as part of the new warning system.
Many mothers of some of the 20,000 children harmed by the drug in the UK since that time insist they were never told about the harm it could do.
Natasha Mason's son Alfie, who is three years old, has only just learned to walk. He has been severely harmed by the drug.
Natasha said: "I wasn't aware it could cause any problems to the unborn child.
"It wasn't until I went to see a paediatrician with my son when he was 10 months old, that it began to click…
"He mentioned foetal valproate syndrome and that was when I started to panic."
The Epilepsy Society is now calling on the Health Secretary Jeremy Hunt to make immediate changes to prevent GPs issuing repeat prescriptions to women for more than 12 months without face-to-face consultations with them.
It also wants routine checks at family planning clinics to include discussions about the risks of sodium valproate in pregnancy.
Its chief executive Clare Pelham, said the survey showed that the number of women who had never had a conversation with their doctor about the risks of the drug was "stubbornly high, hovering around 20%".
She added: "Thousands of babies have been born with disabilities whose mothers were totally unaware of the risks - and will continue to be, if nothing is done.
"Politicians don't very often get the chance to do something as important as this for mothers and babies; and even less often is it so simple and so inexpensive."
The results of the survey will be presented next week to the European Medicines Agency, which will examine whether current warnings are adequate.
The Medicines & Healthcare Products Regulatory Agency (MHRA) in the UK says valproate had been, "kept under constant review and as new data have become available, the warnings have been updated".
The MHRA said: "The results of the survey are important in helping us understand the effectiveness of the measures taken to date in the UK. We want to encourage all women to have access to the valproate toolkit materials that we made available in February 2016.
"We constantly monitor the safe use of valproate and support this latest review by the European Medicines Agency on the use in pregnancy and women of childbearing age."
Its advice is that patients should not stop taking the medicine without consulting their doctor.
Prof Helen Stokes-Lampard, chair of the Royal College of GPs, said: "Any incidences in which valproate has been prescribed with pregnant women are a cause for concern.
"We call on all agencies to redouble efforts to ensure that the most up to date warnings are widely distributed to all healthcare professionals and patients - and the RCGP will continue to play our part in ensuring we achieve this.
"Patients should never hesitate to raise any concerns they have about valproate, or any other medication, with their GP or other health professional."
The Association of British Neurologists said it was working closely with the MHRA to ensure that any young woman who is taking sodium valproate is fully aware of - and reminded of - the risks of becoming pregnant while taking this treatment.