Epidemiology of Recurrent Pregnancy Loss

Spontaneous abortion (SAB) is defined as the expulsion of a fetus before 20 weeks of gestation or weighing less than 500 g.’ Losses after 20 weeks are described as stillbirths or premature births and, generally, have different etiologies than losses earlier in gestation. The vast majority of SABs occur before 12 weeks of gestation. Those occurring at less than 8 weeks often do not contain an embryo and are sometimes referred to as “blighted ova. Recurrent abortion (RAB) is frequently defined as the occurrence of three or more consecutive SABs, although some of the case series of RAB have included women with two consecutive losses. The rationale for distinguishing RAB is based on the assumption that RAB may have an etiology distinct from that of a “sporadic” SAB. In considering the topic of RAB from an epidemiologic standpoint, a critical issue is whether only studies of women with RAB should be included or whether studies of SAB are also relevant. A key premise of this article is that risk factors for RAB must be understood in the context of risk factors for SAB. Thus, we believe that both types of studies are relevant; and we will address the following questions. First, what types of epidemiologic studies are used to address SAB or RAB? Second, what is the probability of SAB in any given pregnancy and how is that risk affected by the outcome of the previous pregnancy? Third, what are the risk factors for SAB in general and for RAB in particular? Finally, on the basis of our review, how can epidemiologic studies of the causes and prevention of RAB be improved?