Concordance rates in twins for anencephaly.

New estimates are offered of the concordance rates in twins for anencephaly. In MZ pairs, the percentage which are concordant is about 7. This is significantly higher than (about three or four times) the value expected on the assumption that concordance is accounted for by raised recurrence rates within sibships. In DZ pairs, the data are less numerous and the percentage concordant (about 2 to 5) does not so far throw much suspicion on this hypothesis in regard to them.