The finding raises fears that the drugs used to treat the condition could affect fertility, although it could also be due to social factors or fear of a baby being born with defects.

The research, published in The Lancet medical journal, are contained in the first study to look at the fertility rates of women with epilepsy.

It also looked at the prevalence of epilepsy and found that incidence of the condition was growing among older age-groups and decreasing among children.

It was based on a database containing details of 3.6 million patients in the UK.

Hormonal effects

Epilepsy is is a brain disorder characterised by repeatedly occurring seizures.

The drugs used to treat it, antiepileptic agents, have been known to affect the hormone system in ways that could lead to conditions that in turn result in infertility.

However, although the study firmly establishes that women with the disease are less likely to have children, it cannot say why.

Professor Simon Shorvon, of the Institute of Neurology in London, took part in the research. He said: "There are, however, a number of possible explanations.

Drug treatments may be to blame

"They could include social factors such as the stigma induced by epilepsy and that the marriage rates among people with epilepsy are slightly lower.

"It may reflect a reluctance of women who have epilepsy and are on treatment to have children because of the risks of the treatment to the unborn foetus.

"It is also possible that the treatment itself, or indeed epilepsy itself, results in lower fertility."

He said that the third possibility was alarming and further research would be needed to establish whether or not it was related to the differences in birth rates.

The studies that showed antiepileptics had an effect on hormones suggested that the drugs may affect fertility, he said.

Birth defects

However, it was important not to focus on the possibility of drugs being responsible as the other possibilities offered compelling suggestions as to why women with epilepsy might choose not to have children, Professor Shorvon added.

For example, there are a number of known risks to children born to women with epilepsy.

These affect up to 10% of pregnancies in women with the condition and include:

• Stillbirths
• Miscarriages
• Malformations
• Restricted growth in the womb
• Development of congenital conditions such as spina bifida.

Dr Lisa Barie Schwarz, of New York University Medical centre in the US, considered the role of drugs in a commentary on the study.