Sequential analysis of spontaneous abortion. II. Collaborative study data show that gravidity determines a very substantial rise in risk.

Evidence from more than 14,000 reproductive histories in the Collaborative Perinatal Project of the National Institute of Neurological and Communicative Disorders and Stroke indicates that the risk of spontaneous abortion increases considerably with gravidity. Maternal age, memory errors, and sampling bias related to recurrence risk do not account for the trend. Clustering, or tendency for abortions to follow one another, occurs in histories which include live births.