This chapter reviews evidence that failure of physiological transformation of the spiral arteries is observed in preterm labor, preterm premature rupture of membranes (PROM), and abruptio placentae. The first study to examine the morphology of the placental bed in idiopathic, sporadic, and recurrent spontaneous abortion was reported by T. Y. Khong and his colleagues. Preterm PROM accounts for one-third of all preterm births, and is often a leading cause of spontaneous preterm labor. Pregnancy requires vasculogenesis and angiogenesis in the fetal compartment and angiogenesis in the maternal compartment. The current taxonomy of disease in obstetrics is based on the clinical presentation of the mother and/or fetus, and not on the mechanism of disease responsible for the clinical manifestations. Evolutionary pressures derived from the potential conflictual relationship between the fetus and mother is likely to play a role.