Tocolytics

by Professor Peter Husslein

The incidence of preterm birth has remained unchanged for the last few decades. This is due, in part, to the complex aetiology of preterm labour, and the limited ability of tocolytic agents to prolong pregnancy as a result of limited efficacy and poor safety profiles. 

Conventional tocolytic agents have been able to demonstrate a prolongation of pregnancy only for short periods of time, up to a maximum of 48 hours, and their use is further restricted by poor safety profiles. The principal objective is to delay pre-term labour for 48 hours, which allows maternal transfer to a perinatal centre with a delivery room adjacent to a neonatal intensive care unit (NICU) and the time to administer a full course of corticosteroids to promote fetal lung maturation. Such measures could improve survival rates in babies at the very cusp of viability, i.e. at very early gestational ages (<28 weeks). This is supported by evidence which suggests that, between 23 and 26 gestational weeks, for every day that delivery is delayed survival is improved by 3%. Furthermore, a course of antenatal steroids has been shown to reduce morbidity and is recommended for virtually all pregnancies complicated by pre-term birth.

The recent introduction of the oxytocin antagonist, atosiban, represents a new generation of uterine specific tocolytics, which are associated with more favourable safety profiles.

The introduction of such a safer tocolytic agent offers the potential to change the current approach to the management of preterm labour. This includes a prolonged period of treatment at earlier or later gestational ages and possibly an extended use to women with contraindications who would normally have been excluded from treatment, e.g. preterm premature rupture of the membranes.