Four major patterns of placental injury: a stepwise guide for understanding and implementing the 2016 Amsterdam consensus.

The Amsterdam classification system defines four major patterns of placental injury, maternal vascular malperfusion, fetal vascular malperfusion, acute chorioamnionitis, and villitis of unknown etiology, and lists the histologic findings that characterize each. However, there continues to be uncertainty regarding specific definitions, histologic mimics, grading and staging, and what combination of findings is required to diagnose each pattern of injury in a reproducible fashion. The purpose of this review is to clarify some of these issues by suggesting a stepwise approach to more fully realize the potential of this new classification system. In our view, the critical steps for correctly identifying and communicating each pattern of injury are (1) familiarity with the underlying pathophysiology and known clinical associations, (2) incorporation of important gross findings, (3) learning to recognize underlying architectural alterations and defining features at low power, (4) using higher magnification to narrow the differential diagnosis and assess severity (grading) and duration (staging), and (5) adopting a template for generating standardized placental reports that succinctly provide useful information for patient care and research applications.