Pregnant women undergoing drug-induced labour are at greater risk of a rare, but potentially fatal syndrome, a study published in the Lancet has suggested.
The condition, where amniotic-fluid leaks into the blood, was almost twice as common in such women.
The scientists said doctors and women should be aware of the small risk if they choose to have an induced birth.
But a UK expert said inducing a birth could avoid the risks of an emergency Caesarean section.
Older women
An amniotic-fluid embolism arises where tears cause amniotic fluid to escape into the mother's circulatory system.
Among the 180 cases of the condition, called amniotic-fluid embolism, studied by researchers from McGill University in Montreal, 24 were fatal.
The study also showed that in every 100,000 women induced, there were up to five embolism cases with up to two of those resulting in death.
They also recorded a higher rate of amniotic-fluid embolisms among women with diabetes, pre-eclampsia, older women and with caesarean, vacuum and forceps deliveries.
"If you don't induce, someone might need an emergency Caesarean which holds a far greater risk to the mother"
They said that with induction rates in the US approaching 20%, the practice could be causing amniotic-fluid embolism in 30 to 40 women a year in the US.
The researchers, led by Dr Michael Kreamer, said: "Although the small absolute risk of amniotic-fluid embolism is unlikely to affect the decision to induce labour in the presence of compelling clinical indications, women and physicians should be aware of this risk if the decision is elective."
Writing in the Lancet, James Moore, from the department of critical medicine at Pittsburgh University, said cases of non-fatal amniotic fluid embolisms used in the research may have been misdiagnosed.
He called for a standard clinical definition of the syndrome to be developed.
Professor of obstetrics, James Walker, said the study seemed to confirm what was already known.
Speaking on behalf of the Royal College of Obstetrics and Gynaecology, he added that the scale of the research made it difficult to identify what other factors beyond a drug-induced labour may have brought on the embolism.
"Amniotic-fluid embolism is an extremely rare event," he said
"We have to remember if you don't induce, someone might need an emergency Caesarean which holds a far greater risk to the mother than an amniotic-fluid embolism."
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