Tests for the diagnosis of early pregnancy have been available since 1974. However, no studies have systematically verified the accuracy of routine clinical laboratories in measuring human chorionic gonadotropin (hCG) prior to the time that pregnancy is clinically evident. We have conducted such a study in association with the NICHD-funded Diabetes in Early Pregnancy Study (DIEP). The purpose of this study was to elucidate the etiology of malformations in pregnancies complicated by diabetes mellitus, which probably occurs within the first few weeks of pregnancy, and therefore uniformity of pregnancy testing was necessary among the five centers to find an association of a teratogen at the time of organogenesis. We confirmed that routine clinical laboratories, in fact, could measure accurately hCG at the time of the missed menses; however, detection was not necessarily possible prior to that time. We conclude that in order to assure accurate diagnosis of early pregnancy, tests should ordinarily be delayed until time of the missed menses. When the test is used at this time, it is a reliable tool for early pregnancy testing and thus can be used to resolve questions relating to early pregnancy pathophysiology.