Introduction Reproduction is extraordinarily inefficient in humans. Only a distinct minority of conceptions result in liveborns. High pregnancy loss begins before implantation and continues with decreasing frequency throughout gestation. Among married women in the USA, 4% have experienced two clinical pregnancy losses and 3% three. This high frequency of early pregnancy loss has a corollary – subfertility. Couples may experience repeated early losses and never realize pregnancy had been achieved. Instead, their problem is considered to be “infertility”. Yet, the etiologies of early loss and “infertility” are similar and should be considered a clinical continuum. Here we shall focus on the most common causes. More details are available elsewhere where additional references are provided. Frequency and timing of pregnancy losses, Embryos implant 6 days after conception. Physical signs are not generally appreciated until 5–6 weeks after the last menstrual period. Fewer than half of preimplantation embryos persist, as witnessed by ART success rates that rarely exceed 30–40% of cycles. Even after implantation, judged preclinically by the biochemical presence of β-hCG, approximately 30% of pregnancies are lost. Following clinical recognition, 10–12% are lost. Most clinical pregnancy losses occur prior to 8 weeks. Before widespread availability of ultrasound, embryonic demise was often not appreciated until 9–12 weeks' gestation, at which time there was bleeding and passage of tissue (products of conception). With widespread availability of ultrasound, it was shown that fetal demise actually occurs weeks before the time overt clinical signs are manifested.
