Zygosity Misclassification of Recent Young Twins by Maternal Reports in Japan

The problem of zygosity misclassification of twins has been paid little attention in Japan. By analyzing maternal reports collected using an internet questionnaire, the author found that around 30% of monozygotic twins were misclassified as dizygotic twins, mainly by obstetricians. On the other hand, the percentage of dizygotic twin pairs misclassified as monozygotic twins was lower. Misclassification was more frequently observed in cases of spontaneous pregnancy compared to iatrogenic pregnancy. The percentage of monozygotic twins misclassified as dizygotic is in very good accordance with that of monozygotic twins having two placentas. The reported number of placentas was very strongly associated with the reported zygosity. Thus, many monozygotic twins in Japan were still misclassified as dizygotic by obstetricians, presumably based solely on the placental findings.