We propose to use a general mixing distribution in modeling the heterogeneity of the fecundability of couples. We introduce a sequence of parameters called canonical moments, which is in one to one correspondence with the moments, to characterize the mixing distribution. By using the bootstrap method, we can estimate the standard errors of our estimates. Our method modifies the usual moment estimates so that the resulting mixing distribution is always supported on [0, 1]. Moreover, the downward bias of the moment estimate of the number of support points would be reduced. Our approach can be used for censored data. The application of our technique in finding the sterile subpopulation is also discussed. The theory is illustrated with several data examples and simulations.