Placental Shape Aberrations
Round or oval placentas are the predominant human placental form but many other shapes exist (Fig. 13.1). When the placenta is irregular, its shape is presumably determined by location, atrophy, and perhaps the manner of original implantation. Anomalies may develop from abnormal fetal genes expressed by the placenta, an abnormal maternal environment, or an abnormal fetal-maternal interaction. Interestingly, for each aberrant placental shape in humans, there is a counterpart in animals. Thus, the placenta membranacea is similar to the normally diffuse placenta in equines; the zonary placenta is typical of carnivores and so on.