The existence of high rates of induced abortion in the population may distort currently employed measures of the rate of spontaneous abortion. Since the frequency of induced abortion varies greatly between and within populations, an appropriate correction is needed to restore the comparability of spontaneous abortion rates. A simplified estimate of the "true" rate of spontaneous abortion is proposed. This statistic yields a sufficient approximation for most purposes.
PIP: High rates of induced abortion in a population may distort currently employed measures of the rate of spontaneous abortion. 2 different ratios have been used to represent the spontaneous abortion rate in cross-sectional studies of populations with high rates of induced abortion. The numerator in both is the number of spontaneous abortions, while the denominator of 1 is the number of spontaneous abortions plus the number of births after 28 weeks. The denominator of the other adds the number of induced abortions to this sum. Variations in the frequency and patterns of induced abortions influence both measures. Inflation of the 1st ratio will be greatest in a population where women undergo induced abortion of late gestations but abort spontaneously at early gestations, while the 2nd ratio will be most seriously diminished when women elect induced abortion at early gestations but abort spontaneously at a late stage. The simplified estimate of the "true" rate (STR) of spontaneous abortion is a simple and convenient statistic that offers a good estimate. The numerator of the STR is the number of spontaneous abortions while the denominator is the sum of the number of spontaneous abortions, the number of live and stillbirths after 28 weeks, and 1/2 the number of induced abortions. The simplified estimate generally yields a result close to the "true" spontaneous abortion rate, because induced abortions are concentrated during the same period of gestation as spontaneous abortions; as a result, on average induced abortions represent pregnancies that survived approximately 1/2 the risk for spontaneous abortion that a live birth survived. The simplified equation can be modified if discrepancies are found on applying the full procedure for estimating the true rate.