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Am J Obstet Gynecol. 1995 May;172(5):1567-72.

Timing of conception and the risk of spontaneous abortion among pregnancies occurring during the use of natural family planning.

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Department of Population Dynamics, School of Hygiene and Public Health, Johns Hopkins University, Baltimore, MD 21205, USA.



Our purpose was to ascertain the effects of timing of conception on the risk of spontaneous abortion.


To assess these effects, women who conceived while using natural family planning were identified in five centers worldwide between 1987 and 1993. Timing of conception was determined from 868 natural family planning charts that recorded day of intercourse and indices of ovulation (cervical mucus peak obtained according to the ovulation method and/or basal body temperature). Conceptions on days - 1 or 0 with respect to the natural family planning estimated day of ovulation were considered to be "optimally timed," and all other conceptions were considered as "non-optimally timed." The rate of spontaneous abortions per 100 pregnancies was examined in relation to timing of conception, ages, reproductive history, and other covariates with bivariate and multivariate statistical methods.


There were 88 spontaneous abortions among 868 pregnancies (10.1%). The spontaneous abortion rate was similar for 361 optimally timed conceptions (9.1%) and 507 non-optimally timed conceptions (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 conceptions (22.6%) as compared with optimally timed conceptions (7.3%). This association was not observed among 697 women with no history of pregnancy loss. The adjusted relative risk of spontaneous abortion among women with non-optimally timed conceptions and a history of pregnancy loss was 2.35 (95% confidence intervals 1.42 to 3.89). The excess risk of spontaneous abortion was observed with both preovulatory and postovulatory conceptions.


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 with preovulatory or postovulatory delayed conceptions.


The results of a large, multicenter study suggest that users of natural family planning are not at increased risk of spontaneous abortion, even when conceptions are not optimally timed. Between 1987-93, data on timing of conception and spontaneous abortion were collected from the charts of 868 natural family planning clients at five centers in the US, Chile, Colombia (two sites), and Italy. This issue was investigated due to concerns that unplanned pregnancies arising from failures of the natural family planning method could be associated with aged gametes and adverse pregnancy outcomes. The day of peak mucus was used as the marker for estimating ovulation. The number of days from the most probable conception intercourse to probable day of ovulation provided an estimate of the time the gametes remained in the reproductive tract before fertilization; less than 48 hours was considered optimal. There were 88 spontaneous abortions (10.1%) and eight stillbirths (0.92%) in this series. Of the 361 conceptions judged to have occurred during the optimal time frame, 33 (9.1%) were spontaneously aborted. The miscarriage rate among the 507 conceptions that occurred at nonoptimal times was 10.9% (55). The difference was not statistically different. Among the 171 women with a previous pregnancy loss, an optimally timed conception in the index pregnancy was associated with a low rate of miscarriage (7.3%). There was, however, an excess rate of spontaneous abortion (22.6%) in women with a prior miscarriage and a pre- or post-ovulatory conception in the recent pregnancy. Although the biologic reasons for this finding are unclear, it is recommended that women with a prior spontaneous abortion use the ovulation method mucus peak or the basal body temperature rise to conceive during the optimal time.

[Indexed for MEDLINE]

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