SummaryIt is well established that, in general, pregnancies at advanced maternal ages or of advanced birth order are at greater risk of aborting spontaneously. It does not follow that each individual woman, as she proceeds through her reproductive life, becomes steadily more prone to abort. The present paper attempts to analyse evidence on how individual women's abortion probabilities vary during their reproductive lives. Statistical examination of the data suggests that:1. A spontaneous abortion in a given birth rank is more likely than a live birth in the same rank to be followed by a further pregnancy. Abortion-prone women have more pregnancies than other women.2. There is no significant overall tendency for spontaneous abortions to appear early or late in sibships even when account is taken of the reproductive compensation reported above.3. Pathological examination of spontaneously aborted fetuses suggests that more (perhaps many more) than 10% of spontaneous abortions (e.g. those associated with trisomy and maternal isoimmunization) are causally associated with advanced maternal age or birth order.4. The mean maternal age of spontaneously aborted fetuses which fail to grow in culture is higher than that of fetuses which are successfully cultured.One would infer:A. (from suggestions 1 and 2 above) that reproductive compensation (rather than a real birth order effect) is a major reason why abortion rates are higher at higher maternal ages and higher birth ranks;B. (from suggestions 2 and 3 above) that there must be another group of abortions, roughly equal in number, which are causally associated with low maternal age or early birth order.Little is known about this second group of reproductive casualties (those associated with low birth order or maternal age), but it is suggested that some have CNS malformations. I have made similar suggestions in regard to stillbirth and neonatal death.