Spontaneous abortion rates, gravidity and neural tube defects

Kline and her co-workers [20, 21] suggest that spontaneous abortions show no overall gravidity effect. However, spontaneous abortions seem to show a negative gravidity effect in some sibships (those containing an infant affected by anencephaly or spina bifida) [22]. It is inferred that there is a heterogeneity among spontaneous abortion sibships, and that there are other sibships in which spontaneous abortions show a positive gravidity effect (perhaps as a result of e.g. iso-immunization).