Abstract
ObjectiveOver half of twin pregnancies in US and UK deliver prematurely but the reasons for this are unclear. The contractility of myometrium from twin pregnancies has not been directly investigated. The objective of this research was to determine if there are differences in the contractile activity and response to oxytocin, between myometrium from singleton and twin pregnancies, across a range of gestational ages. Furthermore, we wished to determine if contractile activity correlates with increasing level of stretch, using neonatal birth weights as a marker of uterine stretch.MethodsThis was an in vitro, laboratory based study of myometrial contractility in women pregnant with one or two babies, using biopsies obtained from non-labouring women undergoing Caesarean section. Spontaneous, oxytocin-stimulated and depolarization induced contractile activity was compared.ResultsDirect measurements of myometrial contractility under controlled conditions show that the frequency of contractions and responses to oxytocin are significantly increased in twins compared to singletons. The duration of contraction however was significantly reduced. We find that contractile activity correlates with increasing levels of stretch, using neonatal birth weights as a surrogate for uterine stretch, with response to oxytocin being significantly positively correlated with birth weight.ConclusionsWe have found significant differences in contractile properties between myometrium from singleton and twin pregnancies and that increasing uterine stretch can alter the contractile properties of myometrium. We discuss the implication of these findings to preterm delivery and future studies.
Highlights
Twin pregnancies are known to occur spontaneously in one in every eighty pregnancies
We find that contractile activity correlates with increasing levels of stretch, using neonatal birth weights as a surrogate for uterine stretch, with response to oxytocin being significantly positively correlated with birth weight
There was a higher rate of primiparity in women with twin pregnancy compared to singleton pregnancy
Summary
Twin pregnancies are known to occur spontaneously in one in every eighty pregnancies. Premature activation of contractility underlies spontaneous preterm delivery [1] Despite this and the very high rate of preterm labour, there is limited research into how contractile activity in twin pregnancy differs from singletons [2]. Home monitoring studies suggest that twin pregnancies [3,4], and triplets [4] have a higher frequency of contractions, after 35 weeks. How well the increased frequency of contractions reported in twin pregnancies correlates or predicts preterm labour is unclear: Some studies show that an elevated rate of contractions in twin pregnancies does increase the risk of preterm labour [5], and uterine monitoring studies show that twin pregnancies that go on to deliver preterm, have a contraction frequency that is significantly greater than twins that deliver at term [6]. Other studies concluded that the increased frequency was not predictive of preterm labour [4,7]
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