Validation of a telephone zygosity questionnaire in twins of known zygosity.

The aim of this study was to validate a zygosity questionnaire that can be administered over the telephone. Mothers of same-sexed twins of known zygosity and chronicity between 2 and 31 years of age were interviewed on a nine-item questionnaire. From the answers one unweighted and four weighted indices were computed. As single questions, the mother's opinion and the "two peas in a pod" question differentiated best between monozygotic and dizygotic twins. One independent well-trained observer assessed the zygosity based on the questionnaire and made the correct diagnosis in 96% of the cases. A weighted index of eight similarity questions yielded an accuracy of 98%. This study shows that the zygosity of same-sexed twins more than 2 years old and without gross physical malformation can reliably be determined by a telephone questionnaire with a high accuracy.