Probability of Spontaneous Abortion in Multiparae

Summary. Data on abortion incidence in reproductive histories of multiparae are re-analysed to obtain the probability of abortion at a given pregnancy. Division of the data statistically into abortion-prone and abortion-resistant components indicates (a) that there is a greater proportion of the former in the higher pregnancy groups; it is suggested that this may be partly responsible for the apparent increase in abortion incidence with parity and age; (b) that in the abortion-resistant component the probability of abortion at each pregnancy remains constant at about 0·12; (c) that in the abortion-prone component the probability of abortion diminishes from 0· 76 in the fourth pregnancy to 0·40 in the ninth. These findings are discussed.