Viagra is now being tested on pregnant women as Pfizer tries to expand the $1.6bn (£800m, E1.2bn) market for the blockbuster drug

Viagra, which originally underwent trials as a treatment for angina, was tested as a remedy for high blood pressure in pregnant women, as well as those suffering from pre-eclampsia, a serious condition associated with premature deliveries of babies.

Pfizer could file for approval for use of the drug as a treatment for pregnancy-related conditions in late 2008, which would have the potential of adding hundreds of millions of dollars to sales.

It would be the latest proposal in a line of alternative uses for Viagra.

The worldwide patent for Pfizer's Viagra expires between 2011 and 2013, and Pfizer want to maximize sales while it has the chance.

Researchers at Pfize and elsewhere are experimenting with use of the drug in treating strokes, pelvic pain and heart failure.

The latest trials are being conducted at the Rambam Healthcare Campus in Israel on women in weeks 24 to 33 of pregnancy suffering from high blood pressure or pre-eclampsia.

Doctors believe that Viagra will widen the blood vessels that deliver vital nutrition between the mother and the foetus. This would make babies less likely to be born early or underdeveloped.

High blood pressure, also known as hypertension, is the most common health problem associated with pregnancy, and causes complications in 2%-3% of pregnancies.

Pre-eclampsia occurs in approximately 5% of all pregnancies, and is twice as likely in first-time mothers. Together, these disorders kill 76,000 women and newborns each year. The cause of pre-eclampsia has yet to be proved conclusively, but there is a link between the condition and high blood pressure.

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