Seasonal distribution of abortions.

At least one pregnancy in eight terminates in an abortion recognized as such and recalled by women when asked for a pregnancy history (McDonald, 1958; Warburton and Fraser, 1964). Some women have a greater tendency to abort than others (War- burton and Fraser, 1964) and a few abort repeatedly (Eastman and Hellman, 1961). Gross fetal mal- formation undoubtedly accounts for some spon- taneous abortions, and recently chromosomal abnormalities, especially trisomy, have been demon- strated in a substantial proportion (Singh and Carr, 1967; Dhadial, Machin, and Tait, 1970). The asso- ciation of fetal trisomy with advancing maternal age may partly explain the higher risk of abortion observed in older women (Warburton and Fraser, 1964).