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The sex ratio of monoamniotic twin pairs.

Data on more than 300 monoamniotic twin pairs have been reviewed. The sec ratios (proportions of males) and of malformed monoamniotic twins seem to be lower than that of all monozygotic twins as a whole. The sex ratio of acardiac monsters (which are all members of monochorionic monozygotic twins) seems to be lower than that of monozygotic twins as a whole. Taken in conjunction, these two pieces of evidence suggest that monochorionic monozygotic twins have a lower sex ratio than dichorionic monozygotic twins. It is argued that this is evidence for the hypothesis that the sex of a zygote is associated with the time within the menstrual cycle that it is formed.