Investigation of the twinning rate in families with the fragile X syndrome.

An excess of twins in families with the Martin-Bell or fra(X) syndrome was noted previously in one family study [Fryns, 1986]. We tried to confirm this observation in a second large sample of families from a different population. We calculated the number of twin births among the total number of live births of known obligate carriers found in fra(X) families ascertained in New South Wales, Australia. We only included births of known sex and excluded triplets. There were 5 male pairs, 3 female pairs and 9 unlike sex pairs of twins born among 752 live births. Thus the twining rate was 1/44 per live birth. We compared this rate to that found in two different types of individuals: 1) the rate of 1/96 which was obtained from the 1985 vital statistics for New South Wales, and 2) the rate 1/75 obtained from a sample of live births of obligate carriers with hemophilia A. The increase in twinning among heterozygotes with the fra(X) was highly significant when compared to the census data (p less than 0.001). However, it was not significantly different from that in the hemophilia data (p less than 0.05) which were collected in the same way as in the fra(X) families.