Placental contractions in uncomplicated pregnancies

In 2020 we first described placental contractions; we have now undertaken a study to characterise these events and seek features that might automatically distinguish them from uterine contractions. We recruited 36 healthy pregnant women to undergo magnetic resonance imaging (MRI) between 29 and 42 weeks of pregnancy in a single-centre, prospective, observational study. Participants had fetal ultrasound to confirm normal growth. Dynamic MRI was acquired for between 15 and 32 minutes using respiratory triggered, multi-slice, single shot, gradient echo, echo planar imaging covering the whole uterus. All participants had a live birth of a healthy baby weighing over the 10th centile for gestational age and none developed any associated conditions of placental dysfunction, e.g., pre-eclampsia, or severe maternal or fetal villous malperfusion on placental histopathology. Any visible contractions were recorded for all participants who completed their MRI scan. Placental contractions occurred in at least 60% of our healthy pregnant population with a median frequency of approximately 2 per hour, and a median duration of 2.4 minutes. Contractions involving a decrease in placental volume of >10% were classified as either placental or uterine by visual observation. Placental contractions occurred more frequently than uterine contractions (p = 0.0061), were associated with a larger increase in the surface area of the uterine wall not covered by the placenta (p = 0.0015), placental sphericity (p < 0.0001) and longer duration (p = 0.0151). All contractions led to an increase in the MRI parameter R2* in the placenta. There was large variation both between participants and between contractions from the same individual, in terms of time course and contraction features, with no apparent change across the gestational age range studied, although the largest fractional volume changes were detected at earlier gestations. We found that the change in sphericity could provide a marker for separating placental from uterine contractions causing >10% volume decrease.