Between 10 and 15 women were still taking part up in the trial until last Thursday and now face a terrifying wait to learn whether their unborn children have been affected.
"The chance of a disease of the blood vessels of the lungs appears to be greater and the chance of death after birth seems to have increased".
Researchers recommend at this stage that no one takes the drug to aid the development of unborn babies.
Increasing placental blood flow could spur fetal growth, which is why a growing number of doctors around the world are prescribing sildenafil to pregnant women whose fetuses suffer from impaired growth.
Of the group, 9 babies that died soon after birth were from mothers in the placenta group and did not show any signs of lung problems.More news: Karius Hints at Liverpool Exit, Klopp Targets Napoli Match for Alisson Debut
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In the roughly equal control group, just three babies had lung problems and none died.
He revealed the results had also been shared with Canadian researchers working on similar trials. It's feared that the drug caused blood pressure to build up in the lungs, meaning the babies received a diminishing amount of oxygen.
Nineteen babies born to the women treated with the drug died, 11 of them due to the lung disorder. At the time the earlier studies were published in 2010, researchers said the treatment should only be used in trials, BBC News reports. Growth restriction is diagnosed using ultrasound, but now the only medical treatment is closely monitoring the pregnancy and inducing birth when the risk of stillbirth is deemed high. This is in line with the conclusion by the authors of the United Kingdom study, which recommended that "clinicians worldwide should stop prescribing sildenafil for this indication outside of research studies with explicit participants' consent". Spokeswoman Dervila Keane wrote in an email Tuesday that the research is "an investigator initiated study and Pfizer have no involvement in the trial".
The research had been borne out in rats, and so the next step was humans: Some 93 pregnant Dutch women whose placentas weren't operating optimally were given sildenafil, the drug better known as Viagra.
In a statement, Amsterdam UMC said that it believed the trial had been conducted properly, but will launch an external investigation to find the cause of deaths. The Dutch researchers have advised women in the United Kingdom trial to heed the results of their study. The relaxation results in an increase in blood flow which has been used to treat not only erectile dysfunction but also reduce the severity of mountain sickness symptoms and in treating pulmonary hypertension.