The post-partum non-susceptible period: Development and application of model schedules

Summary The duration of the post-partum non-susceptible period is a major determinant of birth intervals in populations with relatively low levels of contraception. Since it depends largely on the length of breast-feeding, it may change rapidly with socio-economic development. Its study is, however, seriously hampered by the limited and defective nature of most data that can be obtained. In the absence of intensive, large-scale prospective studies, sizeable reporting errors and/or sampling fluctuations are almost inevitable. A system of model schedules that summarise the underlying regularities found in empirical schedules is proposed. With a logit transformation, observed schedules of breast-feeding can be related to a standard breast-feeding schedule by just two parameters. Similarly, observed schedules of post-partum amenorrhoea can be related to a standard amenorrhoea schedule. The two systems provide a means of obtaining relatively robust estimates of the duration of lactation or amenorrhoea from partial and flawed data. Several applications of the models for estimation problems, for simulation purposes and for the analysis of the relationships between breast-feeding, amenorrhoea and post-partum abstinence are discussed.