Birth ranks of spontaneous abortions in sibships of children affected by anencephaly or spina bifida.

Data on the birth ranks of miscarriages within sibships in which a case of anencephaly or spina bifida (ASB) had occurred were analysed by the Haldane-Smith statistical test. This showed that miscarriages in ASB sibships tend to have a negative birth order effect--that is, they occur in earlier rather than later pregnancies within the sibship. The strongest source of bias in this analysis--the fact that mothers tend to forget early spontaneous abortions--worked against this finding, so the negative birth order effect is probably genuine. Because ASB itself shows a negative birth order effect and because spontaneous abortions in normal sibships do not show such an effect, the additional spontaneous abortions in ASB sibships are probably of fetuses affected by ASB.