This chapter reviews the epidemiological evidence in support of a pathological link and discusses the implications for clinical management. Several epidemiological studies have investigated pregnancy outcome in singleton pregnancy after in vitro fertilisation/intracytoplasmic sperm injection (IVF/ICSI). The lack of consensus diagnostic criteria and the heterogeneous nature of contemporary infertility investigations make it difficult to define the risk of adverse pregnancy outcome in relation to specific reproductive disorders, such as endometriosis or polycystic ovary syndrome (PCOS). In humans, the formation of a functional placenta requires invasion of fetal trophoblast not only into the maternal decidua and inner myometrium (interstitial invasion) but also into the maternal spiral arteries. Progesterone support improves pregnancy outcome after IVF treatment and two recent randomised trials have reported that progesterone also markedly reduces the incidence of preterm labour in at-risk women.