Magnetic resonance imaging (MRI) of normal human placentae

Introduction The MRI appearances of the human placenta in the absence of maternal or fetal pathology have not been extensively studied, with only a few studies reporting findings in the uncomplicated pregnancy. The purpose of this study is to review the placental MRI appearances in low-risk pregnancies in a prospective study. Methods A prospective observational study of placental MRI in low-risk pregnancies was initially planned, however recruitment was terminated early due to the COVID19 pandemic. The protocol was subsequently modified to compare the placental appearances in the enrolled cohort with pregnancies having had MRI for non-placental pathologies. The data from the two groups were then pooled to assess the range of normal placental appearances. Results Eighty-three pregnancies were prospectively assessed with MRI at a median gestation of 29 weeks (range 14-39) from a mixed group of prospective cases (n = 28) and retrospectively recruited obstetric MRI (n = 55). Placental thickness in the third trimester ranged from 18 to 35 mm. T2 heterogeneity was seen in 75% (25/33) at second trimester and by the third trimester 50% (25/50) were moderately or markedly heterogenous. T2 dark bands (>6 mm) were seen in 9% (3/33) and 20% (10/50) of second and third trimester pregnancies, respectively. Undetectable myometrium or loss of the subplacental myometrial plane was present in 15% (5/33) of second and 38% (19/50) of third trimester placentae. Conclusion This qualitative study of normal placental MRI appearances expands the current knowledge base by confirming they vary, evolve with gestation, and can overlap with signs of placenta accreta spectrum.