The higher levels of stretch present in the uterus of multiple pregnancy mothers is implicated in the higher rates of preterm labour they exhibit. The relaxant effect of progesterone in multiple pregnancies is reduced in comparison to singletons by unknown mechanisms. Our aims were therefore to compare in vitro the response of myometrium from singletons and twin pregnancies to progesterone and examine the effects of stretch on the response to progesterone (10 µM).
Methods Informed consent term biopsies were taken and contractility measured. After control measurements, in some tissues stretch was applied to 100% of the maximal response to 40 mM K+. Time and vehicle (EtOH) controls were also performed.
Results Progesterone exposure produced a reduction in force amplitude significantly greater in singleton (16) tissue than twins (10) at 10 and 100 µM. Twin data (n=5) also indicates that the effects of 10 µM progesterone were attenuated in response to mechanical stretch compared to non-stretched controls – an effect that preliminary data suggests is more pronounced in twins than singletons.
Conclusions These data show significant differences in multiple pregnancies in myometrial response to progesterone and suggest that stretch can reduce the tocolytic ability of progesterone. The reduced efficacy of progesterone in twins, may be due to accelerated switching in progesterone isoforms in preparation for labour in multiple pregnancies, and is consistent with clinical findings, but leaves open the question of whether higher progesterone doses would be efficacious.