Placental size at gestational week 36: Comparisons between ongoing pregnancies and deliveries

We aimed to compare placental size and placental size relative to fetal size (ratio) in ongoing pregnancies examined by magnetic resonance imaging (MRI) at gestational week 36 with placental size among all deliveries at gestational week 36 during the same time period.

Ongoing unselected singleton pregnancies (n = 89) were examined by MRI at median gestational week 36+5 days during 2017-2018, and placental and fetal volumes (cm³ ) were calculated. The placental size and ratio in ongoing pregnancies were compared with placental size and ratio among all deliveries in Norway at gestational week 36 (median gestational week 36+4 days) during 2016-2019 (n = 5582). For comparison of size, we converted volume (cm³ ) in ongoing pregnancies into grams as: cm³  × 1.05 (density of placental and fetal tissue).

In ongoing pregnancies, median placental size was 873  (interquartile range [IQR] 265) grams and median size of all delivered placentas was 613 (IQR 290) grams. Placental size was smaller among the delivered placentas independent of delivery mode: 760 (IQR 387) grams among elective cesarean deliveries (n = 465) and 590 (IQR 189) grams among vaginal deliveries after spontaneous onset of labor (n = 2478). Median ratio in ongoing pregnancies was higher than among deliveries: 0.31 (IQR 0.08) vs 0.21 (IQR 0.08). The ratio was higher in ongoing pregnancies independent of delivery mode: 0.24 (IQR 0.17) among elective cesarean deliveries vs 0.21 (IQR 0.05) among vaginal deliveries after spontaneous onset of labor.

The placenta is larger in ongoing pregnancies than among deliveries. This finding suggests that placental size decreases during labor and delivery, possibly by transfer of blood to the fetus. Our finding also suggests that reference values of placental size based on delivered placentas are not valid for ongoing pregnancies.