a1 MRC Mammalian Development Unit, University College, London
Foss (1982) writes:… ‘Guerrero (1974) in a large series showed no relationship of the child's sex to the cycle day of insemination…’. It seems to me that this is ambiguous. If the first day of menstrual bleeding is taken as the time origin, then Foss is correct. But if the biologically more relevant estimated day of ovulation is taken as the time origin, he is not. Using the latter time origin, Guerrero found that for natural inseminations the variance of cycle day of male conceptions was significantly greater than that of female conceptions. In other words, boys are more likely to be conceived at the beginning and end of the fertile period, and girls in the middle. Harlap (1979) has reported findings which seem to confirm those of Guerrero. It has been suggested (James, 1980a, b) that these and other findings would be explained if maternal gonadotrophin levels at the time of conception partially controlled the sex of the zygote.