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.
PIP: Spontaneous abortion can be caused by chromosomal, endocrine, or immunologic factors as well as teratogens. Increased risk of abortion had been associated with gravidity. This paper presents data on 14,000 reproductive histories collected by the Collaborative Perinatal Project in the National Institute of Neurological and Communicative Disorders and Stroke (NCPP) during the years 1959 and 1966. Majority of the women were young: less than 10% of gravidity 1 or 2 registrants were over 30; at parity 2, mean maternal age was 26.7 for whites and 25.1 for blacks. Calculation of overall risks for reach gravidity within each history size revealed a rise in risk with gravidity for each history length for both blacks and whites. Analysis of risks for spontaneous abortion within 5-year maternal age groups showed that among whites, there was no significant linear correlation of abortion risk on age in any category. Among blacks, maternal age had significant linear regression in 3 or 4 categories. However, the average age of women in this series was so young that the rather large differences observed in risk for successive gravidities cannot possibly be attributed to effects of maternal age. It was concluded that risk of spontaneous abortion significantly increase with gravidity and that maternal age, memory errors, and sampling bias related to recurrence risk do not affect this trend.