A trial where pregnant women were given an anti-impotence drug has been urgently stopped after 11 newborn babies died.
Women taking part in the Dutch study had been given the tablets to improve growth of their unborn children because they had poorly-developed placentas.
It appears the drug, which promotes blood flow, may have caused lethal damage to the babies' lungs.
Experts say a full investigation is needed to understand what happened.
There is no suggestion that there was any wrong-doing.
Earlier trials in the UK and Australia and New Zealand did not find any evidence of potential harm from the intervention. But they also found no benefit.
At that time, in 2010, researchers said the treatment should be used only in trials.
Foetal growth restriction caused by an underdeveloped placenta is a serious condition that currently has no treatment.
It can mean babies are born prematurely, with a very low birth weight and poor chances of survival.
A medication that could improve weight or prolong the time to delivery could have significant advantages for these very sick babies.
The latest Dutch study, which was due to run until 2020, was being carried out across 11 hospitals in the Netherlands, including the Amsterdam University Medical Centre.
In total, 93 women were given sildenafil (the non-brand name for Viagra) while the remaining 90 were given a dummy drug or placebo.
Twenty babies developed lung problems after birth - three in the placebo group and the rest in the treatment group.
Eleven in the sildenafil group died from lung complications.
The drug used was not made by Pfizer, which manufactures Viagra, and the safety of the use of sildenafil for erectile dysfunction is not being questioned.
Prof Zarcko Alfirevic, from the University of Liverpool, who led part of the UK research into sildenafil in pregnancy that found no benefit in terms of improving baby growth, said: "This finding in the Dutch study is unexpected.
"We need to be careful at this point to find out more.
"It needs a thorough investigation because the complications were not seen in the two other, similar trials that have already been done in the UK and Australia and New Zealand."