Comparison of placental pathology in preterm, late-preterm, near-term, and term births

The purpose of this study was to determine whether placental pathologic condition supports the recent suggestion of subcategorizing preterm and term births into smaller gestational age subgroups with different perinatal mortality and morbidity rates.

Twenty-seven clinical and 43 placental phenotypes were retrospectively compared in 4617 third-trimester births: 1332 preterm pregnancies (28-33 weeks' gestation), 1066 late preterm pregnancies (34-36 weeks' gestation), 940 near-term pregnancies (37-38 weeks' gestation), and 1279 term pregnancies (≥39 weeks' gestation).

Acute inflammatory pattern of placental injury was seen mostly at both gestational sides of the third trimester; the clinical conditions linked to in utero hypoxia (preeclampsia, diabetes mellitus, fetal growth restriction) and their placental associations (atherosis, membrane chorionic microcysts, chorangiosis, intervillous thrombi) were associated statistically significantly with mid third trimester. Acute fetal distress (abnormal fetal heart tracing and clinical and histologic meconium) were increasing with gestational age and were statistically significantly most common in full-term pregnancies.

Differences in placental pathologic condition among the 4 subgroups of third-trimester pregnancy not only challenge the use of an arbitrary cutoff point of 37 weeks' gestation that separates the preterm birth and term birth but also further support separation of late preterm births from preterm births and term births from near-term births. Based on placental pathologic condition, chronic uteroplacental malperfusion is the dominating etiopathogenetic factor in the mid third trimester (late preterm and near-term births), and acute fetal distress is the factor in full-term births. This obscures relative frequencies of perinatal death and management modalities in the third trimester.