Sweden has cancelled a major study of women whose pregnancy continued beyond 40 weeks after six babies died.
The research was halted a year ago after five stillbirths and one early death in the babies of women allowed to continue their pregnancies into week 43.
“Our belief is that it would not have been ethically correct to proceed” with the study, the researchers concluded.
There is no international consensus on how to manage healthy pregnancies lasting more than 40 weeks, although it is generally accepted that there is an increased risk of adverse effects for mother and baby beyond 41 weeks.
But because the risks are small, research into late-term pregnancies requires large numbers of women in order to achieve statistical significance. Led by Gothenburg’s Sahlgrenska university hospital, the Swedish post-term induction study (Swepis) set out to survey 10,000 women at 14 hospitals.
Women in their 40th week of pregnancy were invited to join the study and divided randomly into two groups, with labour induced at the beginning of either week 42 or week 43, unless it occurred spontaneously.
When abruptly halted in October 2018, the study had involved only a quarter of the target number of expectant mothers. But the six deaths were already judged to indicate a significantly increased risk for women induced at the start of week 43. No infants died in the group whose pregnancies were ended a week earlier.
Although concern about the findings was first reported by Swedish television in the summer, researchers have declined to make the results public, or to speak to the media, until their work is published in a medical journal. But details are contained in a doctoral thesis by one of the researchers, recently made available on Gothenburg University’s website.
The immediate consequences of the study “may be a change of the clinical guidelines to recommend induction of labour no later than at 41+0 gestational weeks”, its author concludes.
Sahlgrenska hospital announced on Thursday that it would change its pregnancy management policies based on the results of Swepis trial.
“We have awaited the scientific analysis showing that it is really true that there is a greater risk of waiting two weeks beyond term,” the head of childbirth operations at the hospital told Swedish television.
“Now we plan, as soon as we possibly can, to offer induction in week 41 to all women who go over term.”
Another of the hospitals involved in the study has already changed its policy after two infant deaths there. Other Swedish hospitals say they will follow suit.
Sweden’s national charity supporting parents who have lost an infant called for an immediate change to hospital policies across the country.
“Since they terminated the study due to ethical reasons, it is highly unethical not to go public with those results,” said Malin Asp, chair of Spädbarnsfonden, the Swedish Infant Death Foundation. “There is potential to save babies’ lives.”
Sara Kenyon, professor in evidence-based maternity care at Birmingham University, said the death of six babies in the later pregnancy group represented an important clinical and statistical difference between the two groups of women. The outcome of the study “is important for practice”, she said.
In the UK, the NHS offers to induce all women who have not gone into labour by 42 weeks. As many as one in five women give birth after 41 weeks in the absence of medical intervention.
Professor Robert M Silver, co-author of US research last year suggesting that it is appropriate to offer women the option to be induced at the beginning of week 40, told the Guardian that the Swedish study appears to be “fully in line” with available research.
“If you continue a healthy pregnancy after week 40 you still have a low risk of complications,” he said. “But the more babies that are delivered, the fewer that are stillborn. Earlier induction also reduces the risk of caesarean section.”
Jan Jaap Erwich, professor of obstetrics at Groningen university, in the Netherlands, said the Swepis research could be the final attempt to determine the exact effects of late-term pregnancy: “It is now likely we will never know the precise risks, because nobody will do a study with so many women again.”