Objective. To evaluate the relation between duration of placental exposure to recently passed meconium in vivo and histological evidence of meconium uptake by macrophages. Design. Retrospective cohort. Setting. University hospital. Population. A total of 44 term deliveries of singleton infants with moderate or thick meconium had placental examination and documented timing of meconium appearance after membrane rupture. Methods. Placentas were examined to assess the extent of meconium uptake by macrophages based on location in the amniochorionic membranes, chorionic plate and umbilical cord, and the intensity of uptake, based on the number of macrophages per field. An arbitrary score of severity of uptake was also created by multiplying the intensity of meconium uptake (number of meconium-laden macrophages) by the extent in the three placental areas. Twenty cases of singleton term pregnancy with clear amniotic fluid throughout labor and at delivery were included as negative controls. Main Outcome Measures. Relation between interval of meconium exposure in vivo and uptake by macrophages. Results. The median interval from meconium appearance to delivery was 95minutes (range 10–510minutes). The median score of severity of meconium uptake was significantly higher than in the negative controls. There was no correlation between the interval of meconium appearance to delivery and score of severity of meconium uptake (p=0.76). Inflammatory lesions were present in 12 (27%) of 44 cases and vascular lesions in 11 (25%) of 44. Conclusions. Duration of placental exposure to meconium in vivo was not related to meconium uptake by macrophages where exposure was <8.5hours.