This study sought to determine the time interval from fetal defecation to membrane staining and subsequent meconium uptake by amnionic macrophages. Pieces of external membranes were fastened to Teflon rings to create a well. The amnionic surface was exposed to meconium and amniotic fluid for 1 to 48 hours and analyzed microscopically for the presence of free meconium and meconium-laden amnionic macrophages. In each of the experiments, prior to the 12-hour time point, few meconium-laden macrophages were present in the membrane layers. A significant rise in the number of meconium-laden macrophages and a concomitant increase in staining intensity was noted in the membranes at 24 and 48 hours. Contrary to previous reports, our in vitro model of meconium incorporation into placental membranes demonstrated that significant numbers of meconium-laden macrophages were only observed after 24 and 48 hours.