Data are offered on the sex ratio of 81 'new' pairs of monoamniotic (MA) twins. When pooled with data on previously collected MA pairs, the total sample has a sex ratio that is highly significantly lower than that estimated for all monozygotic (MZ) twin pairs. This adds support to the notion that the later an MZ pair is formed, the more likely it is to be female. This in turn supports the hypothesis that anomalous X-inactivation is involved in the formation of some female MZ twin pairs.