TIMING OF CONCEPTION AND THE RISK OF SPONTANEOUS-ABORTION AMONG PREGNANCIES OCCURRING DURING THE USE OF NATURAL FAMILY-PLANNING

OBJECTIVE: Our purpose was to ascertain the effects of timing of conce ption on the risk of spontaneous abortion. STUDY DESIGN: To assess the se effects, women who conceived while using natural family planning we re identified in five centers worldwide between 1987 and 1993. Timing of conception was determined from 868 natural family planning charts t hat recorded day of intercourse and indices of ovulation (cervical muc us peak obtained according to the ovulation method and/or basal body t emperature). Conceptions on days -1 or 0 with respect to the natural f amily planning estimated day of ovulation were considered to be ''opti mally timed,'' and all other conceptions were considered as ''non-opti mally timed.'' The rate of spontaneous abortions per 100 pregnancies w as examined in relation to timing of conception, ages, reproductive hi story, and other covariates with bivariate and multivariate statistica l methods. RESULTS: There were 88 spontaneous abortions among 868 preg nancies (10.1%). The spontaneous abortion rate was similar for 361 opt imally timed conceptions (9.1%) and 507 non-optimally timed conception s (10.9%). However, among 171 women who had experienced a spontaneous abortion in a prior pregnancy, the rate of spontaneous abortion in the index pregnancy was significantly higher with non-optimally timed con ceptions (22.6%) as compared with optimally timed conceptions (7.3%). This association was not observed among 697 women with no history of p regnancy loss. The adjusted relative risk of spontaneous abortion amon g women with non-optimally timed conceptions and a history of pregnanc y loss was 2.35 (95% confidence intervals 1.42 to 3.89). The excess ri sk of spontaneous abortion was observed with both preovulatory and pos tovulatory conceptions. CONCLUSIONS: Overall, there is no excess risk of spontaneous abortion among the pregnancies conceived during natural family planning use. However, among women with a history of pregnancy loss, there is an increased risk of spontaneous abortion associated w ith preovulatory or postovulatory delayed conceptions.