Birth Intervals: Structure and Change.

The average birth interval may be analysed into several additive time components: gestation, postpartum amenorrhoea, anovulatory cycles, time required for conception after ovulation is established (ovulatory exposure), and time added by pregnancy wastage. Dissecting the birth interval in this fashion helps to clarify several points regarding fertility in the absence of family limitation. Spontaneous pregnancy wastage may add three months or a little less to the average birth interval of Western women in their twenties. The average birth interval increases with advancing age and it is possible, though not certain, that an increased frequency of foetal loss is the primary factor and declining fecundability a more secondary factor. Among societies practising little or no birth control for which there are statistics, mean birth intervals vary from two years to somewhat less than three years. The principal source of this variation appears to be differential length of postpartum amenorrhoea. Length of ovulatory exposure may vary within a narrower range, perhaps 4-7 months among mothers in their twenties, although confident generalization is impossible until more data are available from non-Western groups. The `ovulatory ratio', or ratio of ovulatory exposure to total birth interval, is basic to an under- standing of the relationships of contraception and induced abortion to pregnancy rates. The higher this ratio, the more nearly equivalent an induced abortion is to averting a live birth since the spans of childbearing time absorbed by the two events become more nearly equal. While ovulatory ratios increase moderately with more pregnancy wastage or lactation, they may be greatly increased by effective contraception. For that reason induced abortion has much greater impact when used as a supplement to contraception than when relied upon alone. Contraception, because it lengthens only the period of ovulatory exposure and not the other segments of the birth interval, must be disproportionately effective in order to register any specified percentage increase in the total birth interval. The lower the ovulatory ratio, the greater is this disproportion. [ABSTRACT FROM AUTHOR]