VALIDATION OF A TELEPHONE ZYGOSITY QUESTIONNAIRE IN TWINS OF KNOWN ZYGOSITY

The aim of this study was to validate a zygosity questionnaire that ca n be administered over the telephone. Mothers of same-sexed twins of k nown zygosity and chorionicity between 2 and 31 years of age were inte rviewed on a nine-item questionnaire. From the answers one unweighted and four weighted indices were computed. As single questions, the moth er's opinion and the ''two peas in a pod'' question differentiated bes t between monozygotic and dizygotic twins. One independent well-traine d observer assessed the zygosity based on the questionnaire and made t he correct diagnosis in 96% of the cases. A weighted index of eight si milarity questions yielded an accuracy of 98%. This study shows that t he zygosity of same-sexed twins more than 2 years old and without gros s physical malformation can reliably be determined by a telephone ques tionnaire with a high accuracy.