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Further concepts on regulators of the sex ratio in human offspring. Non-optimal maturation of oocytes and the sex ratio.

Introduction In a prospective analysis on women who had discontinued contraception to start pregnancy, Weinberg et al. (1995) uncovered new data on the notion of why some conceptions result in boys, while others result in girls. The length of the follicular phase (from the onset of the menses to ovulation) was found to be related to the sex of the baby in 133 pregnancies that survived to delivery: a short follicular phase tended to produce boys, while a long phase tended to produce girls. These results were not in line with those presented by others, particularly not with those on an orthodox Jewish population (Harlap, 1979) and with the U-shaped regression of producing a male vis-a-vis ovulation: the relative risk of a male conception is lower in the middle of the fertile interval, in contrast with at the beginning and end, as proposed by Guerrero (1974). The results of a study on the sex ratios among polygamous and monogamous marriages (0.47 versus 0.53 respectively) in each of seven ethnic groups in Kenya (Whiting, 1993) do not correspond totally with this evidence. Female conceptions in polygamous relations would be increased if the decision about the timing of intercourse was determined by the woman and was therefore influenced by the (maximum) strength of her sexual desire at the time of ovulation. In monogamous marriages, the timing of intercourse would be determined by the husband, independent of the fertile days and would heighten the probability of producing a son. The relationship of male conceptions with ovulation is also substantiated by the consistent excess of female births after conceptions during the most fertile days and male ones after failure of the rhythm method (Shiono et al, 1982; Gray, 1991). The study by Weinberg et al (1995) was well-designed, although the women enrolled were highly motivated to comply with the protocol of daily control and therefore highly selected. In addition, we were not informed about either the kind of contraception used, or the waiting time to conception after discontinuation. After stopping the pill, the follicular phase is known to be prolonged in the first (and subsequent) menstrual cycles (Larsson-Cohn, 1969; Homesley and Goss, 1970). Shiono et al. (1982) found a small excess of male conceptions in women who conceived within 2 months, while Rothman and Liess (1976) did not. The relatively high twin rate (six pairs of same-sex twins) in this population is remarkable and is also a reason for caution. Higher rates of twins have been related to cessation of oral contraceptives (Rothman, 1977; Metneki and Czeizel, 1980; Allen, 1981) and higher rates of monozygotic twins have been related to delayed ovulation (Bomsel-Helmreich and Papiernik-Berkhauer, 1976). For all these reasons, it can be questioned whether the results reported by Weinberg et al (1995) apply to a 'naturally conceiving population'. James (1995) compared the results of Weinberg et al (1995) to the more traditional concept. To him, it seemed unlikely that both hypotheses could be true independently; direct and indirect evidence in animals and humans would favour the latter. In addition, he assumed a causal connection between a high coital rate and a high male sex ratio, which would not cohere with dependence on the length of the follicular phase. His concept is based on, among others, the well-established male excess during and just after wars and a decrease across the first year of marriage. A causal relation to higher coital rates during the first year of marriage and during or after wars (in which the home leaves of soldiers are not accommodated to their wife's mid-cycle), should be questioned. As stress has been found to affect the gonadotrophin secretion (Rivier and Rivest, 1991), these high male sex ratios can be connected with the emotional state of these women and with the inherent reproductive hormone imbalance and thus, more inappropriate maturation of the oocyte. This possibility is supported by the high male sex ratios found about 4 months before the wedding date (pregnant brides) and, again coinciding with this date, ratios that are higher than during the first year of marriage (Renkonen, 1970; Manila, 1980). Before the contraceptive era, social stress was inherent to illegitimate rapports. These high male sex ratios also contradict the other claim of James (1990), that high levels of parental gonadotrophins would favour the production of females.