Inducing labor after 41 weeks appears to be safer in terms of perinatal survival

Reviewed by Kate Anderton, B.Sc. (Editor)Nov 21 2019

Inducing labor after 41 instead of 42 full weeks' pregnancy appears to be safer in terms of perinatal survival, new Swedish research shows. The current study is expected to provide a key piece of evidence for upcoming decisions in maternity care.

In Sweden, the risk of a baby dying before, during or shortly after birth ("perinatal death") is generally very low. However, a progressive rise in risk from a low level is known to take place after the 40th week, for as long as the pregnancy continues.

The purpose of the current study, published in The BMJ, was to investigate these risks and compare outcomes of induction after 41 and 42 full gestational weeks. To date, there have been some doubts regarding the best means of protecting mother and baby alike.

The study comprised 2,760 women admitted to 14 maternity hospitals in Sweden in the years 2016-2018. None had an underlying disease, they were all expecting one baby only (a "singleton birth"), and their pregnancies had lasted for 41 full weeks at the time of inclusion in the study.

Half of the women in the study (1,381 individuals) were randomly assigned to receive induction at 41 full weeks. In this group 86% underwent induction, while labor started without assistance among the others.

In the second group (1,379 women), induction was planned to take place at 42 full weeks. This is routine management at most birth centers in Sweden for pregnancies not involving complications. In practice, 33% of this group of women needed induction to start labor.

Regarding the women's state of health after childbirth, there was no difference between the groups. The proportions of cesareans and instrumental deliveries (using a suction cup, and/or forceps) were also approximately equal. According to the criterion that combines perinatal morbidity and death, just over 30 babies were affected in each group.

On the other hand, specifically in terms of the deaths, a significant difference was found. In the group where labor was induced after 41 full gestational weeks, there were no deaths. In the other group, in which spontaneous labor was waited for ("expectant management") and induction carried out at 42 weeks, six cases of perinatal death were registered: five of the babies were stillborn and one died immediately after birth.

Because of these deaths, the study was discontinued early. In the event, the number of women included was therefore 2,760 instead of the planned 10,000. Sweden's birth centers were informed of the outcome and, after external review of the material, the study -- headed by Sahlgrenska Academy at the University of Gothenburg, and Sahlgrenska University Hospital -- is now being published.

Ulla-Britt Wennerholm, senior clinical physician and associate professor of obstetrics and gynecology at Sahlgrenska Academy, University of Gothenburg, is one of the two lead authors.

"The study shows that there's a difference in the number of deaths when induction is done at 41 and 42 weeks. All the same, we should be a bit cautious in interpreting the results, since we had to break of the study early. The outcome might have been slightly different in a larger study, but the pattern would probably have been the same," she says.

Henrik Hagberg, senior clinical physician and professor of obstetrics and gynecology at Sahlgrenska Academy, University of Gothenburg, is one of the senior authors.

If you put together all the data from our own and previous studies, you can see that mortality is lower if labor is induced at 41 weeks than if we wait and start labor at 42 weeks. The state of current knowledge is now being reviewed, and it's not unreasonable to expect women to be offered induction at 41 weeks.

What we've also shown in this study is that there don't seem to be any medical disadvantages of induction at 41 weeks instead of 42. Many people predicted that it would increase the risk of cesarean and instrumental deliveries, but neither rate increased.

Henrik Hagberg at Sahlgrenska Academy, University of Gothenburg