Stephen Stearns, teaching Yale's Evolution and Medicine course, explains why human placentas are unusually invasive compared to those of most mammals and what that invasiveness has to do with cancer risk. Drawing on the textbook Evolutionary Medicine, Stearns walks through how the placenta burrows into the uterine wall to secure nutrients for the fetus, a process that shares mechanisms with tumor invasion and metastasis. He connects this evolutionary tradeoff between maternal and fetal interests to broader questions about why certain reproductive traits leave humans vulnerable to disease. The segment is part of a longer lecture series applying evolutionary theory to medical problems, and this installment focuses specifically on reproduction as a case study in how natural selection can produce biological features that carry disease costs alongside reproductive benefits.