Birth order, maternal age and birth interval in epidemiology

Foetal wastage and congenital malformations are not so closely associated with birth order, maternal age and birth interval as is suggested by data pooled across sibships of different sizes. The point could only have been established by data on large numbers of individual sibships. Maternal age, parity and birth interval are convenient but confusing variables to work with. However if they are to be used, it is essential to recognize that if the condition being studied may be associated with fertility of the parents, one needs to ascertain numbers of completed sibships containing affected cases. Incidence rates by age, parity and birth interval imply nothing by themselves.