Do marsupials and placental mammals both develop their young at least partly within their bodies in a uterus?

Yes, both marsupials and placental mammals develop their young partly within their bodies in a uterus. However, the key difference lies in the duration and nature of this development. In placental mammals, the young are nourished through a complex placenta for a longer gestational period, while marsupials give birth to relatively undeveloped young that continue to grow and develop in a pouch after birth.