AbstractBackgroundThe placenta is an extraembryonic organ, which is essential to maintain a normal pregnancy. However, placental development in humans is poorly understood because of technical and ethical reasons.MethodsWe analyzed the anatomical localization of each trophoblastic subtype in the cynomolgus monkey placenta by immunohistochemistry in the early second trimester. Histological differences among the mouse, cynomolgus monkey, and human placenta were compared. The PubMed database was used to search for studies on placentation in rodents and primates.Main findingsThe anatomical structures and subtypes of the placenta in cynomolgus monkeys are highly similar to those in humans, with the exception of fewer interstitial extravillous trophoblasts in cynomolgus monkeys.ConclusionThe cynomolgus monkey appears to be a good animal model to investigate human placentation.