Fig. 1: Changes in terminal villi over gestation.

In the early 3rd-trimester (1, 3), syncytiotrophoblast (ST) nuclei are evenly spaced. Capillaries (C) are distant from maternal blood, which bathes the villi. The stroma consists of loose extracellular matrix proteins with frequent macrophages and fibroblasts (brown and pink stars). At term (2, 4), the villi are smaller. Syncytiotrophoblast nuclei are gathered into knots (K), thinning the vasculosyncytial membrane. Capillaries are directly beneath the syncytiotrophoblast layer. Stroma is denser with lower cellularity.