Fig 1.
A schematic illustrating placental structure.
Both arteries and veins contain smooth muscle in their vessel walls, but large stem villi (over 300µm) also contain myofibroblasts (smooth muscle cells) in the stroma surrounding the vessels (termed the Perivascular Contractile Sheath or Extravascular Contractile System). Stromal myofibroblasts are arranged parallel to the longitudinal axis of the stem villi and thus have the potential to induce longitudinal contraction (Demir 1997) (depicted by yellow dashed arrows), which could force blood out of the intervillous spaces via the maternal veins.
Fig 2.
Uterine and placental changes during an example placental contraction.
The top row shows axial MRI images at selected times points before, during and after a placental contraction (indicated by vertical lines on the graph below) with segmentations shown below. Changes in placental and non-placental volumes, wall areas and placental R2* (all measured across the whole volume of the uterus, not just the single slice shown) are plotted underneath. The legend indicates the colours used for the lines in the plots and the regions indicated in the segmentation.
Fig 3.
The morphological changes used to visually categorise placental and uterine contractions.
Placental contractions are generally associated with an apparent shortening of the placental bed in the 2D images, a thinning of the wall not covered with the placenta, and a change in placental shape particularly at the periphery of the placenta. Uterine contractions are generally associated with little change in placental shape except some thinning, with thickening of the uterine wall that can be either (a) local (red arrows) or (b) uniform. The legend colours match Fig 2 but regions are not shaded to improve visualization, so the pink line indicates the amniotic surface of the placenta. The plots show the changes in volumes, areas and signal with the example contraction highlighted. Both time courses show additional contractions with <10% placental volume change: LHS from 14.7-16.6 mins and 17.8–20.3 mins, RHS from 10.8–13.2 mins.
Table 1.
Participant demographics.
Table 2.
Ultrasound assessment results to demonstrate fetal wellbeing.
Table 3.
Labour and birth outcomes.
Fig 4.
Number of contractions detected per hour (calculated from number detected divided by observation time).
Median and interquartile values are marked. a: The left-hand bar shows this for all contractions; the right-hand bar shows it only for contractions with a placental volume change exceeding 10%. b: For placental and uterine contractions separately (with a placental volume change > 10% in both cases).
Fig 5.
Characteristics of contractions with volume change >10%, separately for placental and uterine contractions. a) duration, b) placental volume change, c) change in R2* during a contraction, d) non-placental bed uterine wall area change and e) change in placental sphericity.
Median and interquartile range are shown for each group. Results were analysed using the Mann-Whitney U test. Uncorrected p-values are reported, with a significance threshold set at p = 0.01, which was subsequently adjusted to p = 0.002 following Bonferroni correction.
Fig 6.
Variation in contraction features with gestational age for all contractions identified visually (including those with volume changes of less than 10%): a) rate of any contraction, b) duration, c) change in placental volume and d) change in R2* of all contractions across gestational age.
Contraction type is shown by marker style (with contractions affected by motion indicated) and contractions from the same individual scan session are joined.