Among married women in the USA, 4% have experienced two clinical pregnancy losses and 3% three. The frequency of losses in human preimplantation embryos is very high. Of morphologically normal embryos, approximately 25-50% show chromosomal abnormalities, depending on maternal age. Luteal phase deficiency (LPD) has long been hypothesized, specifically due to inadequate progesterone secreted by the corpus luteum. Decreased conception rates and increased fetal losses are logically associated with overt hypothyroidism or hyperthyroidism. Only women with poorly controlled diabetes mellitus have increased risk for fetal loss. Leiomyomas plausibly could cause early pregnancy loss, but analogous to Müllerian fusion anomalies the coexistence of uterine leiomyomas and reproductive losses need not necessarily imply a causal relationship. An association between second- and third-trimester pregnancy loss and acquired thrombophilias is accepted, but the role thrombophilias play in first-trimester losses is less certain.