Induction of labour confers benefits in prolonged pregnancy

In 1954 Clifford described a syndrome found in infants born after the expected date of delivery which in many respects resembled intrauterine growth retardation in pregnancy : thus there was often thick meconium staining of the amniotic fluid and signs of fetal distress in labour in postmature infants. Hasseljo and Anberg (1962) and Lanman (1968) showed that there was an increased risk of intrapartum death associated with prolonged pregnancy; studies from Scandinavia confirmed that prolonged pregnancy was associated with an increased risk of perinatal death (Bergsjer 1985). An observational study from Dublin examined the risks of postmaturity in which the outcomes of 56 503 pregnancies delivered between 37 and 42 weeks were compared with 6301 pregnancies delivered after 42 weeks (Crowley 1989). In the postmature women intrapartum stillbirth was four times and neonatal death three times as common as in the women delivered at term, and early neonatal seizures were ten times as common. Crowley also compared the outcomes of labour in 247 women delivered after 42 weeks with 247 matched controls delivered between 37 and 42 weeks : meconium stained amniotic fluid occurred twice as often in the postmature women and the need for fetal blood sampling was four times as common (Crowley 1989). Minchom et al. (1987) have confirmed the increased risk of neonatal seizures in prolonged pregnancy. Thus prolonged pregnancy is associated not only with an increased incidence of meconium staining of the amniotic fluid, but also with an increased risk of intrapartum fetal hypoxia which may result in fetal acidosis, neonatal seizures, and intrapartum stillbirth or neonatal death. Obstetricians’ perceptions of this increased risk resulted in many maternity units adopting a policy of induction of labour at term to avoid the hazards of postmaturity, a policy which often led to about 40% of women undergoing induction of labour at or soon after term. But this was an excessive response, for in the Dublin study even after 42 weeks gestation only 1.7 per 1000 pregnancies resulted in intrapartum stillbirth and only 1-6 per 1000 pregnancies resulted in neonatal death. Another difficulty in managing postmaturity is confusion over the definition, and different terms are used to mean the same thing : post-term pregnancy, prolonged pregnancy, postdates pregnancy and postmaturity. The World Health Organisation and FIG0 have defined prolonged pregnancy as 42 completed weeks or more; but sometimes 41 weeks is used as the definition and occasionally 43 weeks. There are problems in estimating the incidence of prolonged pregnancy, not just because of differing definitions but also because of incomplete recording of pregnancies, differences between hospital and population surveys, differing policies for induction of labour, and varying proportions of women with uncertain dates (Bakketeig & Bergsjer 1989). Between 4 % and 14% (average 10%) of women are prepared to reach 42 weeks gestation, and 2 % to 7% (average 4%) to reach 43 weeks gestation, depending on the population studied (Crowley 1989). Nonetheless, a policy of routine induction of labour at 42 weeks will affect a substantial number of women in any population. Is such a policy justified? There have been doubts expressed about the value of induction of labour in prolonged pregnancy, mainly that it may result in more operative intervention without necessarily preventing fetal hypoxia and perinatal death from asphyxia. Furthermore, there is a perception among obstetricians that women do not want induction of labour which may stem from the outcry in the lay press in the 1970s against induction. Thus in many maternity units induction rates have been falling. This controversy has been partly resolved by the results of sixteen randomised trials, a meta-analysis of which provides clear answers to many of the questions concerning induction. Seventeen trials are listed in the Oxford Database of Perinatal Trials (Crowley 1991a, b), but one of these (Martin 1978) is of such poor quality, with 30% of the trial subjects being excluded from the analysis, that its results should be excluded from any overview. Indeed, any meta-analysis of induction of labour for prolonged pregnancy is difficult. One of the main problems is the variation in the definition of prolonged pregnancy, such that the conservative arm of one trial may correspond to the active arm of another. Thus, in the trial of Cole et al. (1975) the active policy was induction at 39 to 40 weeks and the conservative one induction at 41 weeks; in the trial of Cardozo et al. (1986) the active policy was induction at 41 to 42 weeks, the conservative policy being fetal monitoring until the onset of labour. Another problem has been the various methods used for labour induction, and the confinement of some trials to women with an unfavourable cervix. Despite these difficulties, separate meta-analyses of induction of labour at 41 -plus weeks and at 40 weeks are justified. Eleven of the trials investigated a policy of induction of labour at 41-plus weeks gestation compared with a conservative policy of surveillance (Henry 1969; Katz et al. 1983; Suikkari et al. 1983; Cardozo et al. 1986;