The baby's sex was studied in 3658 births to Jewish women who observed the orthodox ritual of sexual separation each month and who resumed intercourse within two days of ovulation. The day of resuming intercourse relative to ovulation was estimated from the characteristics of each woman's menstrual cycle and the number of days of sexual abstinence observed after the last menstruation. The proportion of male babies was significantly higher (65.5±3.9 per cent, mean ± S.D.) in the offspring of women who resumed intercourse two days after ovulation. This proportion tended to be lower on or near the day of ovulation than on the previous one or two days. These results, which were consistent in different demographic subgroups, demonstrate that insemination on different days of the menstrual cycle does lead to variations in sex ratio. Couples should be cautioned against attempting to conceive a boy by delaying intercourse until after ovulation, until further research has established whether delayed fertilization causes birth defects. (N Engl J Med 300:1445–1448, 1979) GUERRERO1 has reported that there may be a variation in the proportion of male babies conceived at different times during the menstrual cycle. A high proportion of male babies was observed in infants conceived several days before ovulation (as measured by a change in basal body temperature); the proportion fell to a minimum on ovulation day, and then climbed steeply during the next two to three days. Despite the consistency of the results in studies reviewed by Guerrero, his observations have been regarded with skepticism. One objection has been that many of the women studied were suffering from subfertility. Some. © 1979, Massachusetts Medical Society. All rights reserved.
