Framing postpartum hemorrhage as a consequence of human placental biology: an evolutionary and comparative perspective

Abstract

Postpartum hemorrhage (PPH), the leading cause of maternal mortality worldwide, is responsible for 35 percent of maternal deaths. Proximately, PPH results from the failure of the placenta to separate from the uterine wall properly, most often because of impairment of uterine muscle contraction. Despite its prevalence and its well-described clinical manifestations, the ultimate causes of PPH are not known and have not been investigated through an evolutionary lens. We argue that vulnerability to PPH stems from the intensely invasive nature of human placentation. The human placenta causes uterine vessels to undergo transformation to provide the developing fetus with a high plane of maternal resources; the degree of this transformation in humans is extensive. We argue that the particularly invasive nature of the human placenta increases the possibility of increased blood loss at parturition. We review evidence suggesting PPH and other placental disorders represent an evolutionarily novel condition in hominins.

Keywords: comparative placentation; evolutionary medicine; implantation; postpartum hemorrhage; trophoblast.

Figure 1

The pre–implantation stage of the human embryo. The implanting blastocyst is a hollow ball of cells that contains an internal mass of cells that will give rise to the embryo proper surrounded by an outer shell of cells called trophoblasts. These cells will form part of the placenta. Later migrations of extraembryonic mesoderm from the developing embryo will complete the placental structures. Together, the trophoblast and embryonic mesoderm form the placental villi. (Modified from Rutherford 2009)

Figure 2

Darkly staining invasive extravillous cytotrophoblasts (ECT, some indicated by white arrows) surrounding maternal arterioles (outlined in black hashed circles). Arteriole on left is completely surrounded by ECT and further along the conversion process than the arteriole on the right. (Micrograph used with permission from Harvey Kliman, MD, PhD, Yale University)

Figure 3

Localization of differentiated human trophoblast cells. Human placenta, section through chorionic villus (embryonic mesoderm core covered in trophoblast) and underlying endometrium. Dark-staining cells are various trophoblast phenotypes: SCT = syncytiotrophoblast; VCT = villous cytotrophoblast; ACT = anchoring cytotrophoblast; ECT = extravillous invasive cytotrophoblast. (Micrograph used with permission from Harvey Kliman, MD, PhD, Yale University)