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Editorial: On rhythm and sex ratio.


 Comments are made on Rodrigo Guerrero's article on the association of the type and time of insemination within the menstrual cycle with the human sex ratio at birth. Much of the data used in the study involved couples who were practicing some form of the rhythm method in an attempt to prevent pregnancy. The shift in the basal body temperature was used as a reference point. A rough correction for the difference in relative coital frequency applied to Guerrero's data indicates that 100 normally fertile women not using contraceptives but abstaining from intercourse for 11 days, from Day 7 before the day of the rise in basal body temperatures to Day 3 afterward, would have between 4 and 5 live births a year. The only safe practice of rhythm involves abstaining from intercourse until 4 or 5 days after a sustained rise in basal body temperature has occurred. The findings indicate that couples wishing to double the chances of having a boy rather than a girl should not practice intercourse after Day 4 or 5 before the temperature shift; however, they might have to wait 5 or more years before a live birth occurs. The data suggest that spermatozoa carrying X and Y chromosomes respond differently to at least some differences in environmental conditions. Methods of preselecting the sex of offspring may be developed which might contribute to lowering rates of population growth in countries where people are willing to limit the number of their offspring if they can be sure of having the desired number of male children. The variations in sex ratios of live births which occur during or just after a war may be accounted for at least partly on the basis of Guerrero's findings. During the war years intercourse occurs less, and the coital days are distributed randomly throughout women's menstrual cycles. There is greater time between ovulation and the last insemination before ovulation and higher sex ratios would be expected.
