We read with great interest the article from the ‘Worldwide Atosiban versus Beta-agonists Group’1. Since the presented results will probably have a great impact on the choice of pharmacologic substance in the treatment of preterm labour, we would like to shed some light on, as we believe, an important finding that has not been further discussed by the authors.
In Table 3, it is shown that among the twin pregnancies in the β-agonist group, 93.3% were still undelivered at 48 hours compared with 75.0% in the atosiban group (P= 0.003). The difference remained at seven days (76.7% vs 61.4%), albeit not significant. It would be of great interest to have information of the success rate in relation to gestational age at treatment for both the single and twin pregnancies. It may be that atosiban is less effective at earlier gestational ages. The results in favour of the β-agonist group among the twins might therefore be explained by a lower gestational age at treatment in these pregnancies. Myometrial sensitivity to oxytocin increases with gestational age due to an upregulation of the oxytocin receptor, which in fact has been demonstrated towards the end of pregnancy2. This relative lack of oxytocin receptors earlier in pregnancy might contribute to a possibly reduced efficacy of atosiban.
Since the increased perinatal morbidity and mortality in twin pregnancy as compared with single pregnancy is mainly due to an increased rate of prematurity, twin pregnancies should be of great concern in this context. We therefore find it important that the relationship between tocolytic efficacy of different agents and type of birth (single/twin) is clarified.