Women with epilepsy should not be given a drug known to increase the risk of birth defects, experts have said.
Epileptics experience brain seizures
A government advisory committee said women who might become pregnant should be given other epilepsy drugs instead.
Studies have shown women taking sodium valproate were around twice as likely to have babies with birth defects as women generally.
Women already taking the drug, also known as Epilim, are advised to talk to their doctor before they stop.
Epilepsy is the tendency to have recurrent seizures. It affects one in every 133 people in the UK.
The Committee on Safety of Medicines examined the use of sodium valproate after mothers taking the drug who had given birth to children with birth defects began a legal challenge.
A group of around 20 parents of children with birth defects has won legal aid in a fight to win compensation from health authorities, whom they accuse of negligence.
If their challenge is successful, it is believed that many other parents may also make claims.
In its latest report, the CSM said women who might become pregnant should not start taking Epilim, which has been linked to a higher risk of a child being born with birth defects such as spina bifida or heart disease, without specialist advice from a neurologist.
Newer drugs are available, but some women may still choose to take sodium valproate if it is the only way they can control their seizures.
If women do take the drug while they are pregnant, the CSM says it should be prescribed at the lowest possible dose, and should not be given in conjunction with other epilepsy medications.
It also advises that women who are planning to become pregnant should take folic acid, which has been shown to reduce the risk of birth defects in babies born to women at high risk.
A spokesman for the CSM said "Sodium valproate remains the drug of choice in patients with certain types of epilepsy such as generalised epilepsy.
"However, women of childbearing potential should not be started on sodium
valproate without specialist neurological advice, and women who are likely to get pregnant should receive specialist advice because of the potential risk of defects to the unborn baby."
He added: "Women of childbearing age should be informed of the risks and benefits of continuing anti-epileptic treatment throughout pregnancy."
But he stressed: "It is essential that patients taking anti-epileptics do not suddenly discontinue use of the drug. Any changes must take place under medical supervision."
A spokeswoman for the charity Epilepsy Action told BBC News Online it welcomed the CSM's guidance.
She said: "Women with epilepsy should be given pre-conception counselling, and should be given specialist advice about the treatment they are on. They shouldn't take sodium valproate without specialist advice."
"Women who are already taking the drug should not stop without talking to their doctor first."
She added: "Women need to be informed about these risks, and doctors may decide to try them on newer drugs.
"But uncontrolled seizures can be devastating, and if nothing else is going to stop someone's seizures, women may decide with their doctor that they should carry on taking sodium valproate."