Placental contractions in uncomplicated pregnancies

In 2020 we first described placental contractions, and we have now undertaken a study to characterise them and seek features that might automatically separate 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 and 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 contractions features, with no apparent change across the gestational age range studied, although the largest fractional volume changes were detected at early gestation.

💡 Research Summary

This paper reports a prospective, single‑center observational study that used dynamic magnetic resonance imaging (MRI) to characterize placental contractions (PCs) in uncomplicated pregnancies and to identify imaging features that distinguish them from uterine contractions (UCs). Thirty‑six healthy women with singleton pregnancies between 29 and 42 weeks gestation were recruited. All participants underwent a fetal ultrasound confirming normal growth (≥10th percentile) and had no maternal or fetal conditions associated with placental dysfunction. Dynamic MRI was performed on a 3 T Philips Ingenia scanner using a respiratory‑triggered, multi‑slice, single‑shot gradient‑echo echo‑planar imaging (EPI) sequence. Acquisition times ranged from 15 to 32 minutes, with a repetition time of approximately 15 seconds, voxel size 2.4 × 2.4 × 6 mm³, and 32 slices covering the whole uterus.

Images were automatically segmented with a convolutional neural network (nnU‑Net) trained on 169 manually annotated volumes. Dice coefficients were 0.813 ± 0.056 for placenta and 0.954 ± 0.016 for the non‑placental uterine wall, indicating high segmentation accuracy. Contraction events were identified by two experienced observers who inspected both the cine series and the time‑course plots of volume and area changes. A contraction was classified as a PC when the placental volume decreased by more than 10 % relative to a locally defined baseline; otherwise it was labeled a UC. Consensus labeling was achieved for the start and end times of each event.

PCs were observed in at least 60 % of the cohort (22 of 36 participants). The median frequency was approximately two events per hour, and the median duration was 2.4 minutes. PCs occurred significantly more often than UCs (p = 0.0061). When a PC occurred, the surface area of the uterine wall not covered by placenta increased by a median of 12 % (p = 0.0015), placental sphericity decreased markedly (p < 0.0001), and the duration was longer (p = 0.0151). All contraction events were accompanied by an increase in the MRI parameter R2* within the placenta, with an average rise of about 8 %, reflecting a transient reduction in oxygenated blood or a change in tissue microstructure.

No significant correlation was found between gestational age, maternal BMI, or other clinical variables and the frequency or characteristics of PCs, although the largest fractional volume reductions were detected in the earlier part of the gestational window (29–32 weeks). Substantial inter‑individual and intra‑individual variability was noted in both frequency and intensity of contractions, suggesting dynamic changes in placental hemodynamics across pregnancies.

The study demonstrates that placental contractions are a common physiological phenomenon in healthy pregnancies and can be quantitatively distinguished from uterine contractions using objective imaging metrics: (1) a ≥10 % drop in placental volume, (2) a concurrent increase in non‑placental uterine surface area, (3) a measurable decrease in placental sphericity, and (4) an increase in placental R2*. These markers provide a foundation for developing automated algorithms that could classify contraction events in future large‑scale MRI datasets.

Limitations include the single‑center design, modest sample size, reliance on visual expert labeling for ground truth, and the temporal resolution imposed by respiratory triggering (9–15 s), which may miss very brief events. Moreover, the cohort consisted exclusively of uncomplicated pregnancies, so the relevance of PCs in pathological conditions such as pre‑eclampsia, fetal growth restriction, or stillbirth remains to be investigated.

Future work should involve multi‑center, larger cohorts, inclusion of pregnancies with known placental pathology, and the development of machine‑learning models that can automatically detect and classify PCs versus UCs. Such tools could eventually be integrated into clinical MRI protocols to provide early, non‑invasive biomarkers of placental health and to identify pregnancies at risk for adverse outcomes.