decidua basalis

During implantation, the endometrium undergoes decidualization under progesterone influence, differentiating into three regions: decidua basalis (beneath the implanted embryo), decidua capsularis (overlying the embryo, facing the uterine lumen), and decidua parietalis (the remainder of the uterine lining). The decidua basalis is clinically and physiologically the most important because it becomes the maternal portion of the placenta, fusing with the chorion frondosum (fetal placental component) to form the definitive placenta. It contains spiral arteries that are remodeled by extravillous trophoblast invasion, converting them into high-flow, low-resistance vessels essential for adequate uteroplacental perfusion. Inadequate trophoblastic invasion and spiral artery remodeling in the decidua basalis is central to the pathogenesis of preeclampsia and intrauterine growth restriction (IUGR). After delivery, separation occurs at the boundary between decidua basalis and the myometrium (the zona basalis is retained for future endometrial regeneration), and retained decidua basalis or abnormal attachment (as in placenta accreta spectrum) can cause postpartum hemorrhage. Histologically, decidual cells are large, polygonal cells with abundant cytoplasm and glycogen, and the region is infiltrated by uterine natural killer (NK) cells that help mediate maternal immune tolerance to the semi-allogeneic fetus while permitting trophoblast invasion. Understanding decidua basalis is essential for grasping placental development, the pathophysiology of preeclampsia, and complications of placental separation.