If the drug proves effective in future trials, it could be far easier for patients than current treatments involving painful injections.

MS is a disabling disease which happens when the body's own immune system attacks the sheath which protects brain cells called neurons.

The myelin sheath acts as insulation for the electrical signals carried within, and once it is stripped away, some of those impulses can be lost.

Attacks can lead to sudden muscle weakness or even paralysis, and a variety of other neurological problems involving sight and speech.

I am excited by the prospect of finding an easily administered treatment for MS based on a naturally occurring phenomenon in pregnancy

Dr Rhonda Voskuhl

The trial, at the David Geffen School of Medicine at the University of California at Los Angeles, involved 12 women with various forms of the disease.

They were given doses of a hormone called oestriol, made in the foetal placenta.

It is a weak form of the human sex hormone oestrogen, and is found normally only in pregnant women.

The idea for the drug comes from the fact that while pregnant, many MS patients often experience an easing of their condition, suffering fewer relapses or a halt in deterioration during the nine-month period.

The women were given oestriol pills over a six-month period, then the treatment was stopped for six months and restarted for another four months.

Lesions descrease

Doctors used a scanner to look at the brain for areas damaged by MS attacks.

Six of the patients had relapsing-remitting MS - a type of disease characterised by brain inflammation and attacks that come and go.

Among these patients, there was a significant decrease in the number and size of these brain "lesions", and an improvement in cognitive test scores.

When the oestriol treatment was stopped, the lesions came back, only to disappear again once the drug was restarted.

No change

There was no significant improvement among the rest of the patients, who had secondary progressive MS - a more advanced form of the disease in which the disability is more lasting following attacks.

Dr Rhonda Voskuhl, an associate professor of neurology, said: "I am excited by the prospect of finding an easily administered treatment for MS based on a naturally occurring phenomenon in pregnancy.

"Finding an easily-administered oral treatment is important, in part, because patients are less likely to delay treatment if it involves a pill rather than weekly or daily shots."

Following the study, published in the journal the Annals of Neurology, it is likely that further, larger scale trials will be needed to test whether the treatment really works.