Frequency and clinical significance of placental histological lesions in an unselected population at or near term

Associations between specific placental histological abnormalities and obstetric outcomes are reported. However, most data are based either on high-risk cases or relate to case-control studies selected from those with abnormal placental histology findings, with the unavoidable biases that these approaches entail. This study reports the frequency of the several common, objective and predefined histological abnormalities of the placenta as identified by pathologists blinded to all clinical information. A total 1,153 women were recruited from an unselected population delivering at 34-43 weeks. Histological findings in common obstetric outcome groups were compared to those of the unselected population, and odds ratios and predictive values were calculated. Normal histological findings were present in 72.1% of pregnancies with normal outcomes and in 79.1%, 66.6%, 80%, and 74.8% of pregnancies affected by pre-eclampsia (PET), pregnancy-induced hypertension (PIH), gestational diabetes (GDM), and small for gestational age (SGA), respectively. Chronic placental underperfusion was seen more frequently in PIH (odds ratio (OR) 2) and SGA (OR 1.4), while villitis of unknown aetiology was observed more commonly in cases with PIH (OR 3.2). Fetal thrombotic vasculopathy was twice as common in cases with GDM whilst massive perivillous fibrin deposition was much more frequent in those with PET (OR 20.2) and SGA (OR 8.9). Chorangiomata were 13 times more common in pregnancies with PET. However, in all cases, positive predictive values were low, with the majority of cases with histological abnormalities being associated with normal outcome. At term, specific placental histological lesions are significantly more common in complicated pregnancies, but the clinical significance of such lesions in a specific case remains uncertain, since the majority will be identified from clinically uncomplicated normal pregnancies.