Relation between placental site and length of gestation

Two theories related to uterine action have been brought together to see whether the site of placental implantation influences the onset of labour: (1) that uterine contractions begin asymmetrically from a "pacemaker" in one uterine horn, and (2) that progesterone from the placenta blocks myometrial contractility primarily at the site of implantation. Case records were examined of 182 patients who had placental locations performed and who had a spontaneous onset of labour. Where the placenta was implanted in the right upper quadrant of the uterus labour occurred on average four days sooner than when it was implanted in the left upper quandrant: the difference was statistically significant.