Time to pregnancy as a correlate of fecundity: differential persistence intrying to become pregnant as a source of bias

Background Subfecundity is a frequent and often serious problem and it is i mportant to identify its preventable determinants and to monitor fecundity over time. Since follow-up studies are difficult and expensive to conduct, time to pregnancy (TTP) in pregnant women is often used as a surrogate meas ure of fecundity. TTP data can be retrieved at low costs and they need no v alid population registry as a source for sampling. While TTP may serve as a valid surrogate measure in many situations, its validity rests upon a numb er of assumptions. We have analysed one of these overlooked assumptions, th e importance of persistence in trying to become pregnant. Methods By means of computer simulations we estimated bias caused by differ ences in persistence in pregnancy attempts. We investigated whether the ass umptions made in the simulation were realistic by using empirical data from a European study. Results The mean waiting time to pregnancy and other estimates of subfecund ity (or infertility) strongly depend upon the persistence of couples in pur suing a pregnancy. We show that even moderate changes in the planning behav iour considerably modify the waiting rime distribution. Empirical data conf irm that persistence in trying to become pregnant is age-related. Conclusions Persistence in pregnancy attempts affects outcome measures of s ubfecundity in studies based upon TTP in pregnant women. It is likely that the length of time during which couples keep trying to become pregnant is i nfluenced by a number of factors which would probably change over time or b e different between populations to be compared.