A clinical trial in the Netherlands testing the effectiveness of a generic version of Viagra given to pregnant women whose fetuses had growth problems has been halted after 11 babies died. The researchers say it appears the drug, which stimulates blood flow, may have caused damage to the babies' lungs, ultimately leading to their death.

"Based on these findings, the study stopped immediately," according to a statement from Amsterdam University Medical Center. "All participants were approached personally and almost everyone was informed and know by now whether they have taken the drug or the placebo. All concerned women are accompanied as well as possible by the doctors involved in the study."

The research, conducted in 10 hospitals throughout the Netherlands, involved 183 pregnant women whose babies had a severe growth limitation early in pregnancy. There is no known therapy to help these babies grow, and their prognosis was considered poor. But researchers believed that sildenafil, which is sold under the brand name Viagra, might help by stimulating blood flow in the placenta.

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Previous clinical trials in the U.K., Australia, and New Zealand did not find any evidence that the intervention was harmful. However, they also did not document any benefits. At the time the earlier studies were published in 2010, researchers said the treatment should only be used in trials, BBC News reports.

In the Dutch study, 93 women had been given sildenafil and 90 got a placebo drug. Of the sildenafil group, 19 babies died. Eleven of these deaths were due to a possible lung disease, a form of high blood pressure in the lungs. Six additional babies in the group had this lung disease but did not die.

In the placebo group nine babies died, none from the lung disease. Three babies in the placebo group did have the lung disease, but did not die of it.

In its statement, Amsterdam UMC says that the "adverse effects that have now been found were unknown to date. The researchers expect that the use of sildenafil for this application will stop worldwide."

According to The Guardian, gynecologist Wessel Ganzevoor, who led the research, told Dutch daily newspaper De Volkskrant, "We wanted to show that this is an effective way to promote the growth of the baby. But the opposite happened. I am shocked. The last thing you want is to harm patients."

The researchers say they will continue to analyze the data and closely monitor the children in the study.

Professor Zarko Alfirevic from the University of Liverpool, who led part of the U.K. research into sildenafil in pregnancy, said that the findings in the Dutch study were "unexpected."

"We need to be careful at this point to find out more," he told the BBC. "It needs a thorough investigation because the complications were not seen in the two other, similar trials that have already been done in the U.K. and Australia and New Zealand."