Influence of parity on fetal mortality in prolonged pregnancy

In England an estimated 50,000 inductions of labour at or beyond 41 weeks' gestation are conducted each year. However, the published evidence on the effect of parity on stillbirth in prolonged pregnancy is limited, and has produced conflicting data. The aim of this study is to evaluate the influence of parity on fetal mortality in prolonged pregnancies.

Retrospective analysis of 145,695 singleton births with known parity and no malformation noted at birth to residents in the former North-East Thames Region, UK. The parity and gestation specific stillbirth risks and relative risks per 1000 ongoing pregnancies were calculated in relation to parity between 37 and 45 weeks.

Before 41 weeks the stillbirth risk rose gradually but did not differ by parity. By 41 weeks there was a substantial increase in the stillbirth risk in nulliparous women but not in parous women. The pattern of rise is such that the stillbirth risk is 2.9 times higher (95% CI 1.06-8.19) in nulliparous women at >42 weeks' gestation.

Being parous appears to have a protective effect on fetal mortality in prolonged pregnancy. These findings question the need for routine induction of labour at 41 weeks in parous women.