Placental influences on the rate of labour progression: a pilot study.

Regional variations in intrapartum myometrial contractility were assessed using ultrasound. In 10 women with normal labours, the myometrial thickness increased in all areas during contractions, whilst in 10 women with slowly progressing labour, the retro-placental myometrium thinned during contractions. Locally acting placental tocolytics may be a cause of dysfunctional labour.