Induced abortion and the chromosomal characteristics of subsequent miscarriages (spontaneous abortions)

Data from a case-control study of miscarriages (spontaneous abortions) were used to test whether single and multiple induced abortions are associated with miscarriage in subsequent euploid (chromosomally normal) pregnancies. Cases and controls were identified in three New York City hospitals between April 1974 and November 1982. It was hypothesized that, if induced abortion increased the risk of subsequent miscarriage, an association would be observed with euploid but not aneuploid (chromosomally abnormal) miscarriage. The frequencies of single and multiple induced abortions among euploid cases and aneuploid cases were compared with those among controls. Among both private and public patients, the proportion of women reporting a single induced abortion was similar in euploid cases and in aneuploid cases compared to controls. Among public patients only, the proportions reporting multiple induced abortions were also similar in euploid cases and in aneuploid cases compared to controls (odds ratios = 0.9 and 1.0, respectively). In contrast, among private patients, the proportion reporting multiple induced abortions was raised among euploid cases (odds ratio = 2.2, 95% confidence interval = 1.3-3.7), although not among aneuploid cases. This association was strongest when the first induced abortion was carried out before 1973, at a young age, or with a procedure other than suction curettage. In public patients, the associations with miscarriage did not vary with characteristics of the first induced abortion, but multiple induced abortion histories when the first two induced abortions occurred before 1973 were in excess among euploid cases compared to controls. These results suggest that, for both private and public patients, neither single nor multiple induced abortions as now performed are likely to increase the risk of miscarriage in subsequent pregnancies. They also suggest a mechanical origin for some miscarriages.