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Am J Obstet Gynecol. 1991 Dec;165(6 Pt 2):1982-4.

Natural family planning and sex selection: fact or fiction?

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  • 1Department of Population Dynamics, School of Hygiene and Public Health, Johns Hopkins University, Baltimore, MD 21205.


Determining sex of offspring by timing of intercourse has been a subject of intense interest for the lay public and professional community. This metaaanalysis of couples practicing natural family planning provides an opportunity to evaluate sex ratios in relation to the timing of conception with several parameters, including basal body temperature shift and peak mucus as markers of ovulation. Data from six studies show a statistically significant lower proportion of male births among conceptions that occur during the most fertile time of the cycle.


This study disproves the suggestion by Shettles and Billings that the selection of male offspring by intercourse around the time of ovulation is possible. Shettles states that smaller Y bearing sperm may have more motility in the mid cycle cervical mucus, and thus more male offspring. In 22 cases of midcycle conceptions, 86% (19) were males, while of 19 distant from ovulation conceptions, 84% were females. Though there is a statistically significant relative deficit of male births with conceptions during the most fertile time of the menstrual cycle, the authors consider this to be insufficient to allow sex selection by timing of intercourse. The author discussed the results from 4 studies of day of conception and mucus peak to estimate the % of male births and the relative risk of a male birth among conceptions during the most fertile time. Using the data from all of the studies, the proportion of male births and relative risk of a male birth was reduced during the most fertile time. The relative risk was .90 at a 95% confidence interval of .77-.95. There is a small but statistically significant difference amounting to a deficit in male births with conceptions in the most fertile period. The deficit of male births is associated primarily with conception 2-0 days before ovulation. The excess of male births occur 3 or 4 days before ovulation or 2-3 days after. One explanation offered for the deficit of male births midcylce is that levels of gonadotropin may favor X bearing sperm. Support for this is given in another study of in vivo induction of ovulation and the sex ratio where 46% of 2608 births were males. In vitro fertilization studies have reported a sex ratio of 51.7% male births. Thus hormonal factors are an important determinant of the primary sex ratio. In spite of the empirical findings, natural family planning programs continue to promote natural methods to help couples determine their child's sex by timing of intercourse.

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