The association of placental to fetal ratio with pregnancy duration

Our objective was to study the association of placental size, fetal size, and placental size relative to fetal size (placental to fetal ratio) at gestational week 27 with time to spontaneous delivery.

We included 100 pregnancies in a follow-up study from gestational week 27 until spontaneous delivery. Placental and fetal volume (in cm³) were measured at gestational week 27 by magnetic resonance imaging (MRI), and the association of placental to fetal ratio (placental volume/fetal volume) with delivery after spontaneous onset of labor was estimated as hazard ratios (HR) by applying Cox regression models. Pregnancies with deliveries after planned cesarean section or induction of labor provided follow-up time until these events. An HR lower than 1.0 indicates decreased risk of spontaneous delivery.

Mean placental volume was 532 cm³ (SD 136 cm³) at gestational week 27, and fetal volume was 961 cm³ (SD 112 cm³). This yielded a mean placental to fetal ratio of 0.55 (SD 0.12). The HR of spontaneous delivery decreased with increasing placental to fetal ratio (HR 0.013 (95% CI: 0.001-0.121), Wald statistic 14.704 (p < 0.001)), indicating a longer duration of pregnancy with a higher placental to fetal ratio at gestational week 27. The HR of spontaneous delivery also decreased with increasing placental size, but the association was less prominent than the HR associated with placental to fetal ratio (HR 0.997 [95% CI: 0.995-0.999], Wald statistic 7.638 [p = 0.006]). We estimated no association with fetal size (HR 1.001 [95% CI 0.999-1.003], Wald statistic 1.728 [p = 0.189]).

Our findings suggest that the placental to fetal ratio at gestational week 27 may be an indicator of the remaining duration of pregnancy until the onset of spontaneous labor.