Why do pregnancies fail?

During the last 20 years, pregnancy rates have improved markedly in well-managed horse populations; mean overall per cycle fertility rates now exceed 65% for naturally mated Thoroughbreds and 45% for Warmbloods inseminated with frozen-thawed semen. However, early pregnancy loss (EPL) remains a significant cause of subsequent loss, with approximately 15% of pregnancies detected at Day 15 failing to survive to term. Moreover, the majority of EPL occurs before Day 42 of gestation, a period when pregnancy maintenance is dependent on progesterone produced by the primary corpus luteum (CL), and a number of essential developmental events take place, including maternal recognition of pregnancy, embryogenesis, endometrial cup formation and placental interdigitation (implantation). Despite its impact, the causes of EPL are poorly understood, which makes it difficult to design strategies for prevention. This is partly because EPL is often diagnosed retrospectively, i.e. the conceptus has already degenerated, when it is no longer possible to reliably establish the initiating event. Progress has also been hindered by a tendency to assume that the underlying problem is ‘progesterone insufficiency’, primarily because it can be rectified pharmacologically rather than because there is any real evidence that insufficient progesterone is a common cause of EPL. Our understanding of factors contributing to EPL is, however, improving and they can be broadly divided into: intrinsic abnormalities of the embryo, an inadequate maternal environment and ‘external’ factors. […]