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Researchers, publishing in PLOS, report that there is an increased risk of still birth in longer term pregnancies.
Dr James Doidge, Senior Research Associate, UCL Great Ormond Street Institute of Child Health, said:
“Big datasets can give us the power to identify very small differences in risk, such as the increase in risk that this study observes between 37 and 40 weeks of gestation. But big datasets don’t allow us to understand the causes underlying those risks, in particular whether inducing labour before 40 weeks would lead to similar reductions in risk as when labour occurs naturally before 40 weeks. It could be just that pregnancies with higher risk of stillbirth tend to last longer and therefore make up a larger amount of the pregnancies as gestational age increases. This is certainly part of the story, as this study shows that the differences in risk between longer and shorter pregnancies become even smaller when results are restricted to studies of ‘low risk’ pregnancies without congenital malformations.
“Despite this ‘sensitivity analysis’, it is also possible that the remaining risk of stillbirth would not be affected by inducing labour early, or that any reduction could be offset by increases in other risks. Whether inducing labour actually reduces the risk of stillbirth is something that would be best examined by comparing outcomes between pregnancies that were induced and similar pregnancies that were not, rather than by comparing babies born at different ages. It is also worth remembering that very small increases in risk of one outcome can easily be offset by reductions in risk of other outcomes, such as the decrease in risk of neonatal death (i.e. death soon after birth) that was observed in this study between 37 and 38 weeks (which was greater than the increased risk of stillbirth during the same window). There are many other short- and long-term outcomes to consider too, and evidence to support better outcomes among ‘late term’ pregnancies (39–41 weeks) compared to ‘early term’ pregnancies (37–38 weeks). (1,2) In short, this study at best provides reason for further research on the benefits and harms of inducing labour during the early term period, but is insufficient grounds for changing practice or decisions about inducing labour at this stage.”
Dr Joy Leahy, Statistical Ambassador, Royal Statistical Society, said:
“This study demonstrates that for babies who are carried to term, the likelihood of being stillborn increases the longer the baby remains in the womb. However, it does not distinguish between babies who were induced and those who weren’t, which makes it difficult to use this study for decision making. As a statistician, who also happens to be in my third trimester of pregnancy, it still doesn’t answer the question of whether I should proactively seek to be induced when I reach 37 weeks, or whether I should simply be hoping that my baby comes naturally before 42 weeks.
“This study helpfully sets out the numbers needed to harm for one additional stillbirth when pregnancy is continued to the next week. For example, according to the results of the study, 1,449 pregnancies would need to be continued from 40 to 41 weeks, in order for one extra stillbirth to occur. Given that stillbirth is relatively rare in seemingly healthy pregnancies, there may be other benefits for the baby to stay in the womb for an additional week. Again, as a statistician, who also happens to be in my third trimester of pregnancy, I would still want to weigh up any benefit of reducing stillbirth against other potential benefits that may occur from letting my baby come in his own time.”
Dr Alexander Heazell, Senior Clinical Lecturer in Obstetrics, University of Manchester, said:
“It is an important piece of work which confirms current knowledge that the risk of stillbirth is increased in pregnancies which continue after 41 weeks of pregnancy and at all stages of pregnancy in women of Black ethnic origin.
“This association is biologically plausible as the placenta ages as pregnancy continues so by the end of pregnancy it is less able to deliver the oxygen and nutrients needed to sustain a baby.
“The authors acknowledge that the risk of continuing a pregnancy is low, equivalent to an additional stillbirth for every 1,449 pregnancies which continue beyond 40 weeks. This equates to approximately 120 stillbirths per year in the UK or 4% of the total.
“Women should be encouraged to use these figures to have open discussions with their healthcare providers to inform their choices about their maternity care.”
Prof Christoph Lees, Professor of Obstetrics and Trustee of the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)”, said:
“This large and important systematic review and meta-analysis based on data from 1990-2017 suggests that the stillbirth risk is slightly increased at 40 weeks, though the absolute magnitude of the risk remains very small. Hence waiting until 41 weeks before offering induction of labour may not offer complete reassurance. Importantly, neonatal death rates do not change between 38-41 weeks so in balancing risks and benefits a slightly earlier induction does not appear to translate to a higher risk of death once baby is born.
“As the authors themselves comment, though the data were drawn from ‘low risk’ cohorts, obstetricians have in the time period in which the studies were conducted become better at identifying causes of stillbirth, for example fetal growth restriction. How applicable these results are with the more recent improved fetal surveillance techniques and lower threshold for induction of labour following NICE and other international guidance cannot be directly inferred. The authors had planned a sensitivity analysis with respect to time period of the different studies – this information is not available and would be very useful in addressing the specific point about the effect of changes in practice on stillbirth in the 27 years over which these data were collected.
“Whether it is feasible or necessary ever to test the central hypothesis of this study is uncertain as given the very low frequency of stillbirth, it would require a very large number of women to be recruited to a prospective randomised study.”
* ‘Risks of stillbirths and neonatal deaths with advancing gestation at term: a systematic review and meta-analysis of cohort studies of 15 million pregnancies’ by Muglu et al. was published in PLOS Medicine at 19:00 UK time on Tuesday 2nd July.
DOI: 10.1371/journal.pmed.1002838
Declared interests
Dr James Doidge: “Nothing to declare.”
Dr Joy Leahy: “I have no conflicts of interest to declare.”
None others received.