The relationship between the sex of offspring and the time interval between coitus and ovulation/fertilization has been prospectively investigated in 33 pregnancies using the rise in luteinizing hormone in the early morning urine, the peak cervical mucus symptom, and the shift in basal body temperature as indicators for ovulation. Reference to the peak cervical mucus symptoms showed a significant association of conception of a male child with longer intervals (>2 days) between coitus and fertilization (P<0.03). However, while a similar trend for male conceptions was evident when the duration of sperm survival was determined by reference to each of the other two indicators of ovulation, the relationship was not significant. The results clearly refute the theory that intercourse close to ovulation favors male conceptions. While the findings are consistent with the contrary theory, it may be premature to conclude that a male child is more likely to be conceived if coitus takes place several days before ovulation.
France, J. T., Graham, F. M., Gosling, L., & Hair, P. I. (1984). A prospective study of the preselection of the sex of offspring by timing intercourse relative to ovulation. Fertility and Sterility, 41(6), 894–900. https://doi.org/10.1016/S0015-0282(16)47904-5