Economists, sociologists and demographers must often attempt to answer important questions with data not well suited to the problem at hand. One example that crops up frequently in socio-economic-demographic literature is the use of samples of women, whose ages span the entire fecund period, to study the effects of couples’ characteristics on “completed” fertility, or on the demand for children. In this case, the usual procedure is to control either for age or for duration of marriage, and to assume that issues concerned with timing and spacing of children can be ignored. Under this assumption, differences in the level of the stock of children among families at any point in time (any age) bear a one-to-one correspondence to differences in completed fertility observed at the end of the fertile period. In this paper, we explore some of the pitfalls researchers may encounter when using data with this characteristic, especially in the absence of exact knowledge of the functional form of the relationship between age or duration of marriage and other variables thought to affect actual fertility.