Sex ratio of births resulting from artificial insemination.

A survey of available data has shown that the sex ratio of births is significantly affected both by the practice of artificial insemination (AI) and by the cryostorage of semen. Comparison of the sex ratio of all infants born in England, Scotland and Wales between 1969 and 1978 (51.5% males) with that of British births after AI showed that the prevalence rate of male births after AI with either fresh or cryostored semen was lower than that for the general population: fresh semen (47.8%); cryostored semen (42.9%). When data from European countries and the US were combined, AI with fresh semen significantly increased the prevalence rate of male infants to 57.7%, whereas AI with cryostored semen significantly decreased the rate to 49.7%. The mechanism/s whereby the cryostorage of spermatozoa affects the secondary sex ratio remains to be determined.