A study of 2620 pregnancies ending in spontaneous abortion revealed a CNS defect in 3.6% of embryos and fetuses, and 3% of all complete conceptuses. The type of malformation observed varied with the gestational age at expulsion, encephaloceles being predominant in earlier specimens, while more typical anencephalus and spina bifida were more common among later abortions. Chromosome abnormalities were found in 40% of abortuses with CNS defects, but were almost entirely confined to those which were still at the embryonic stage of development. 53% of the latter were chromosomally abnormal, which is the same as the proportion found among embryos without a CNS malformation. Using published life-tables of recognized pregnancies it was estimated that the prevalence of anencephalus, spina bifida, or related malformation (other than hydrocephalus), without a chromosome anomaly, is 5.3 per thousand conceptuses at the beginning of the eighth week of gestation. By comparing this with the prevalence in total births, it was further estimated that only 24% of these are born alive, with 54% aborting spontaneously and 22% being stillborn.
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