Birth intervals: Structure and change

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Abstract
The average birth interval may be analysed into several additive components: gestation, post-partum amenorrhoea, anovulatory cycles, time required for conception after ovulation is established, and time added by pregnancy wastage. Dissecting the birth interval in this fashion helps to clarify several points regarding fertility in the absence or near absence of family limitation. In addition the ratio of time required for conception after ovulation is established, relative to the total birth interval, termed the ‘ovulatory ratio’, has considerable bearing on the effects registered by contraception and induced abortion. 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. Ovulatory ratios are 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. A higher initial ovulatory ratio also means a larger percentage increase in birth interval length from a given effectiveness of contraception.