Interval between pregnancies and risk of spontaneous abortion.

Abstract

To the Editor: In a recent study, Zhu et al. concluded that “the optimal interpregnancy interval for preventing adverse perinatal outcomes is 18 to 23 months.”1 We speculated whether this is true for the risk of spontaneous abortion. To evaluate the risk of spontaneous abortion in relation to the interval between pregnancies, we analyzed all pregnancies in Denmark during the period 1988 to 1992 using Danish National Registries including all live births, stillbirths, hospitalized spontaneous abortions, induced abortions and information on previous pregnancies. The use of these registries has previously been described in detail. 2,3 We found 181,483 pregnancies to be preceded by a live birth. In these women we observed a high risk of spontaneous abortion when the interpregnancy interval was less than 6 months or more than 5 years (Table 1).The pattern was the same when stratifying by parity, history of spontaneous abortion, and maternal age. Table 1: Adjusted Relative Risk for Spontaneous Abortion*A potential limitation of the study by Zhu et al. was that they did not know whether the present pregnancy was preceded by a pregnancy with an adverse outcome. This issue might be important, as indicated in our findings from an additional analysis of the 31,594 pregnancies in which the previous pregnancy was a spontaneous abortion. In such pregnancies the lowest risk of spontaneous abortion was observed in women that became pregnant less than 6 months after the end of the previous pregnancy (Table 1). Our findings indicate that fecundity may be of importance in the interpretation of such data. After a live birth the optimal choice of interval between delivery and the next conception might very well be about 2 years and the most fertile couples (with the lowest risk of spontaneous abortion) will achieve that to a greater extent. After a spontaneous abortion, many couples might try to conceive again shortly after, and the lowest risk of spontaneous abortion is therefore in the group with a short time interval since latest pregnancy. In conclusion, parous women with about 2 years since the previous live birth have the lowest risk of several adverse pregnancy outcomes including spontaneous abortion. It is an open question, however, whether a woman can reduce her risk of adverse outcomes by having her births with 2-year intervals or whether the lower risk simply reflects the fact that this interpregnancy interval is obtained more frequently by couples with a high fecundity and thereby a lower risk of adverse perinatal outcomes. Jan Wohlfahrt Anne-Marie Nybo Andersen Mads Melbye