Figure 1. Failed spiral artery remodeling.

During pregnancy the spiral arteries that underlie the placenta, which in non-pregnant women are typical, small muscular arteries containing smooth muscle and an inner elastic lamina, undergo physiological remodeling. This includes terminal dilatation, the loss of the internal elastic lamina and the loss of smooth muscle. This change extends into the inner third of the myometrium resulting in the loss of a condensation of vascular smooth muscle near the myometrial decidual junction. This “sphincter” is proposed to be responsible for terminating blood flow at the time of menses. In preeclampsia this process is not complete. The terminal dilatation is not as extensive and the removal of smooth muscle is not complete and does not extend beyond the decidua leaving the functional vascular sphincter intact (Reprinted from Parham P. NK cells and trophoblasts: Partners in pregnancy. J Exp Med. 2004; 200(8): 951-5).⁵¹