The status of the hypothesis that the human sex ratio at birth is associated with the cycle day of conception

William H. James

The Galton Laboratory, University College London, 4 Stephenson Way, London NW1 2HE, UK

Dear Sir,

Data have been published (Gray et al., 1998Go) suggesting that the sexes of human offspring are not related to their cycle day of conception. These authors acknowledged that with their data, they had only around 80% power to detect (at the 5% level of significance) a difference of 20% in the sex ratios (proportions male) of conceptions on the most fertile days and on other days. But it is possible that if there were a difference, it would be less than this. So it seemed worth pooling these data with all the further recent data known to me. They are given in Table IGo.


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Table I.
 
For the pooled data, {chi}2 = 9.9, P < 0.002. So direct data support the hypothesis that cycle day of conception is (weakly) associated with sex ratio of offspring. Conceptions on the most fertile days have an overall sex ratio of .492, while those on other days have a sex ratio of .536.

However this interpretation is itself dependent on the quality of the data. Some notes on this are appended.

In conclusion, it is fair to draw attention to all the earlier data (cited in James, 1971) and the present data in Table IGo; and to the data on other mammals (e.g. as cited in James, 1996Go). To disregard this accumulation of data sets, one would need to be persuaded that the bulk of them are affected by bias. It may be acknowledged that the data in Table IGo are characterized by sampling error; but that is unlikely to be responsible for the present result. Critics must offer plausible grounds for the existence of bias in these data. Otherwise it would seem reasonable to suppose that there is an element of truth in the hypothesis.

I am grateful to Dr M.E. Bernstein for drawing my attention to the work of Koller and Degenhardt (1983).

Notes on the Data

The meta-analysis of Gray (1991) was most heavily dependent on the previous studies (Guerrero, 1974Go; Harlap, 1979Go). In such studies it is required to identify (i) the date of fruitful coitus and (ii) the date of ovulation. A number of doubts (in regard to both measures) were expressed in the New England Journal of Medicine following these two publications. However, though both measures may be subject to random error, no persuasive argument suggested that bias was responsible for the result

I noted (James, 1971Go) that, earlier in this century, a number of authors in Continental Europe published data suggesting an association between cycle day of insemination and the sex of offspring. These papers elicited criticisms which seem unlikely now to be resolved. But it is noteworthy that the U-shaped regression (of sex ratio on cycle day of insemination) suggested by these papers is similar to that suggested here.

Koller and Degenhardt (1983) only published data on cycle day of insemination by mean cycle length. So their data set is less efficient than others at identifying the day of ovulation. Indeed it may be acknowledged that: (i) different techniques were used to identify ovulation day in the different studies and; (ii) slightly different criteria were used to define `most fertile' days in the different studies. In large studies, days –2 to 0 (where `0' is the estimated ovulation day) are obviously more fertile than other days. In smaller studies, the decision is more arbitrary. But examination of the raw data reveals that the present result is not critically dependent on such definition.

The new hormonal techniques of identifying ovulation should eventually help establish whether the hypothesis is true. But so far these techniques have not been used in sufficient number in such studies to give an unequivocal answer.

The hypothesis has existed since the time of Empedocles (c 490–430 B.C.). So researchers may share the sense of community generated by one of the oldest unsolved scientific problems.

References

France, J.T., Graham, F.M., Gosling, L. and Hair, P.I. (1984) A prospective study of the preselection of the sex of offspring by timing intercourse relative to ovulation. Fertil. Steril., 41, 894–900.[ISI][Medline]

France, J.T., Graham, F.M., Gosling, L. et al. (1992) Characteristics of natural conceptual cycles in a prospective, study of sex preselection: fertility awareness symptoms, hormone levels, sperm survival and pregnancy outcome. Int. J. Fertil., 37, 244–255.[ISI][Medline]

Gray, R.H. (1991) Natural family planning and sex selection : fact or fiction? Am. J. Obstet. Gynecol., 165, 1982–1984.[ISI][Medline]

Gray, R.H., Simpson, J.L., Bitto, A.C. et al. (1998) Sex ratio associated with timing of insemination and length of the follicular phase in planned and unplanned pregnancies during use of natural family planning. Hum. Reprod., 13, 1397–1400.[Abstract]

Guerrero, R. (1974) Association of the type and time of insemination within the menstrual cycle and the human sex ratio at birth. N. Engl. J. Med., 291, 1056–1059.[ISI][Medline]

Harlap, S. (1979) Gender of infants conceived on different days of the menstrual cycle. N. Engl. J. Med., 300, 1445–1448.[Abstract]

James, W.H. (1971) Cycle day of insemination, coital rate and sex ratio. Lancet, i, 112–114.

James, W.H. (1996) Evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels at the time of conception. J. Theor. Biol., 180, 271–286.[ISI][Medline]

Spira, A., Ducot, B., Guihard-Moscato, M.-L. et al. (1993) Conception probability and pregnancy outcome in relation to age, cycle regularity and timing of intercourse. In Gray, R., Leridon, H. and Spira, A. (eds), Biomedical and Demographic Determinants of Reproduction. Clarendon Press, Oxford, UK, pp. 271–284.

von Koller, S. and Degenhardt, K.H. (1983) Risikofaktoren der Schwangerschaft. Springer Verlag New York, USA.

Wilcox, A.J., Weinberg, C.R. and Baird, D.D. (1995) Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy and sex of the baby. N. Engl. J. Med., 333, 1517–1521.[Abstract/Free Full Text]





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