In our study, duration of the total labour, the first labour and the second labour stage was significantly shorter in those in whom labour was induced at 40–40+ 6 weeks than in those who were induced at 41–41+ 6 weeks, yet the two groups showed similar peri- and postnatal outcomes, including Apgar scores and rates of induction failure, caesarean section, complications and NICU admission.
It is well known that the risk of placental dysfunction and meconium-stained amniotic fluid increases after 41 weeks of gestation, as well as the risk of foetal distress and even foetal death 13–14. Therefore, induction of labour after 41 weeks in low-risk pregnancies has become a global obstetric consensus to reduce perinatal mortality and neonatal meconium aspiration syndrome2,6−7. In recent years, a number of systematic reviews have shown that full-term (39–40+ 6 weeks) labour induction does not increase the risk of caesarean section or adverse maternal or foetal outcomes, regardless of cervical conditions 5,15−16. In fact, those reviews suggest that full-term induction can actually reduce the risk of caesarean section and NICU admission. A large study conducted in the UK found that the risk of stillbirths was 0.86–1.08 per 1,000 cases at 40–41 weeks of gestation, while it increased to 1.2–1.27 per 1000 cases at 41–42 weeks 17. Another study showed that, compared with those who gave birth at 39–41 weeks, those who delivered at 41–42 weeks were at significantly higher risk of postpartum anaemia, meconium aspiration, neonatal Apgar score < 5 at 1 min, and caesarean section due to foetal distress 18. Therefore, since maternal-foetal risk increases after 41 weeks of gestation, and the induction of labour after full term does not increase the rate of caesarean section or of adverse maternal or foetal events in low-risk pregnancies without complications, many obstetricians wonder whether to wait until 41 weeks or to induce labour earlier.
Our results suggest that earlier induction is not associated with significantly higher risk of adverse maternal or foetal events, but is associated with shorter labour, which may bring several advantages. Labour at later gestational stages can be complicated due to higher foetal weight, skull deformation, and amniotic fluid reduction. Prolonged labour can consume the physical strength of the mother, aggravate her anxiety and that of her family members, increase the rate of unindicated caesarean sections and increase risk of uterine atony, foetal distress and postpartum haemorrhage. Therefore, our results suggest that shortening labour is likely to bring several advantages without additional risks. For determining the choice of induction at 40–40+ 6weeks rather than 41–41+ 6weeks in the low risk women,the factors include nulliparous women receiving standard prenatal care, singleton pregnancy, no obstetrics complications, no medical and surgical complications.
Limitations of the study include the fact as follows: First, our hospital, as a large tertiary-care medical center in western China, admission and treatment of difficult and serious diseases in the surrounding area, and high-risk pregnancy accounts for nearly 80%. Therefore, the number of pregnant women with low-risk is relatively small, which causes the number of cases finally included in this study relatively small. Second, there was no statistically significant difference in neonatal birth weight between the two groups. Analysis the reason may be that all the cases included in the study were pregnant women who received regular prenatal care in our hospital, the weight gain of them during pregnancy was reasonable and the fetal growth was normal. Third, we found that the proportion of female newborns was significantly higher at 41–41+ 6 weeks than at 40–40+ 6 weeks; we looked at the literature and there was no research on this problem. This result should be confirmed in further studies, and the reasons should be explored. Last and most important, this is a retrospective cohort study, all of these factors above may have influenced the results to some extent. To control the bias and confounding factors, logistic regression analysis and multiple linear regression analysis were used to adjust the potential confounding biases.