Timing induction of labour at 41 or 42 weeks? A closer look at time frames of comparison: A review

Background: Postterm pregnancy is associated with increased perinatal risk. The WHO defines postterm pregnancy as a pregnancy at or beyond 42 weeks + 0 days, though currently labour is induced at 41 weeks in many settings. Guidelines on timing of labour induction are frequently based on the Cochrane systematic review 'Induction of labour for improving birth outcomes for women at or beyond term' in which is concluded that a policy of induction of labour is associated with fewer adverse perinatal outcome and fewer Caesarean sections. However, the included trials differed regarding the timing of induction, ranging from 39 to beyond 42 weeks while the upper limit of expectant management exceeded a gestational age of 42 weeks in most studies. Objective: to evaluate perinatal mortality, meconium aspiration syndrome and Caesarean section rate of trials comparing a policy of elective induction of labour and expectant management according to timeframes of comparison with a focus on studies within the 41-42 weeks' timeframe. Design: Review. Methods: The systematic review of Cochrane was used as a starting point for assessing relevant trials and a search was performed for additional recent trials. We evaluated incidence and causes of perinatal mortality, incidence of meconium aspiration syndrome and Caesarean section according to three time frames of comparison. We pooled estimates and heterogeneity was tested. The quality of the included trials was assessed using the Quality Assessment Tool for Quantative Studies (EPHPP). Findings: In total 22 trials were included which had all different timeframes of comparison. Only one trial compared induction of labour at 41 weeks + 0-2 days with induction at 42 weeks + 0 days, three other trials compared induction of labour at 41 weeks + 0-6 days with induction at 42 weeks + 0-6 days. In 18 trials the comparison was outside the 41-42 weeks' timeframe: in six trials induction was planned ≤ 40 weeks and in another 12 trials expectant management was beyond 43 weeks. The incidence of potentially gestational age associated perinatal mortality between 41 and 42 weeks was 0/2.444 [0%] (induction) versus 4/2.452 [0.16%] (expectant management), NNT 613; 95%CI 613 - infinite. Two trials in the timeframe of comparison 41-42 weeks were available for evaluation of meconium aspiration syndrome (6/554 (induction) versus 14/554 (expectant management), RR 0.44; 95%CI 0.17-1.16). Three trials in the timeframe 41-42 weeks could be evaluated for Caesarean section, with different inclusion criteria regarding Bishop score. There was no significant difference in the Caesarean section rate 93/629 (induction) versus 106/629 (expectant management), RR 0.88; 95%CI 0.68-1.13. Conclusion: Evidence is lacking for the recommendation to induce labour at 41 weeks instead of 42 weeks for the improvement of perinatal outcome. More studies comparing both timeframes with an adequate sample size are needed to establish the optimal timing of induction of labour in late-term pregnancies.