Placental lesions associated with oligohydramnios in fetal growth restricted (FGR) pregnancies

Aim of the study was to investigate the association between placental pathology and oligohydramnios in pregnancies complicated by fetal growth restriction (FGR).

Placentas from 221 consecutive FGR pregnancies and 63 healthy controls were studied. Pathological lesions were described according to consensus nomenclature and standardized criteria; both elementary lesions and constellations of lesions (patterns) were considered. Statistics included analysis of linear trends and multinomial logistic regression.

Amniotic fluid index (AFI) was normal in 56 (25.3%) FGR pregnancies, whereas mild, moderate and severe oligohydramnios were diagnosed in 32 (14.5%), 44 (19.9%) and 89 (40.3%) subjects, respectively. In FGR pregnancies, after adjustment for potential confounders, membrane meconium staining (chi-square = 28.6, p < 0.001), chronic villous hypoxia pattern (chi-square = 18.8, p < 0.001) and fetal thrombotic vasculopathy pattern (chi-square = 9.2, p = 0.002) were positively and linearly correlated to AFI decrease. Odds ratios of meconium and chronic villous hypoxia were 9.2 (95% CI = 2.6-32.9) and 4.2 (95% CI = 1.3-13.6) in FGR pregnancies with normal AFI and 25.2 (95% CI = 6.9-91.8) and 9.7 (95% CI = 3-31.5) in those with severe oligohydramnios (p = 0.005 and p = 0.023 compared to normal AFI, respectively).

In FGR pregnancies, reduction of amniotic fluid volume is directly correlated to histological features of placental under-perfusion, meconium staining of membranes and fetal vascular damage. These findings support the clinical notion that in FGR pregnancies oligohydramnios is a risk factor of fetal hypoxia and possibly of increased adverse neonatal outcomes.