Impaired development of the early placenta has long been implicated in aberrations of placental shape at term. It has become possible to construct a hypothesis linking placental shape and complications of pregnancy based on oxidative and endoplasmic reticulum stress. The onset of maternal circulation is interlinked with remodelling of the chorion frondosum into the definitive placenta, and oxidative stress plays a key role in this process. This chapter proposes that a spectrum of defects may occur, ranging from variations at the physiological end in clinically 'normal' placentas that may relate to programming, to frank pathological changes associated with severe complications of pregnancy. It discusses spiral artery conversion, the onset of the maternal arterial circulation to the human placenta, abnormal villus regression, abnormal maternal blood flow, and the implications for developmental programming. Conversion of the spiral arteries is associated with invasion of the endometrium and the arterial walls by extravillous trophoblast.