Twins in the sibships and parental sibships of women with Turner's syndrome.

The twin birth frequency in the sibships of 45 females with Turner's syndrome was 3.7% for the 21 females with 45,X and 5.7% for the 24 with other karyotypes. The 5.7% is significantly higher than the expected mean frequency of 1.4% in the Danish population from 1900 to 1974. The twin birth frequency in the sibships of the mothers of the 24 females with karyotypes other than 45 X was 4.7% which is also significantly higher than expected. The twin birth frequency in the sibships of the 21 mothers of 45,X females did not deviate significantly from the expected frequency, and nor did that of the father's sibships. It is concluded that these findings indicate that there might be an association between the increased risk of X chromosome aberration in the progeny due to mitotic non-disjunction, isochromosome or ring chromosome formation on one hand, and twin formation on the other hand. Such X chromosome aberration and twin formation might thus have some common aetiological factors, and some of the possibilities are discussed.