Q. Do all mammals have a placenta?
Mammals of the monotreme group (platypus, echidnas) are oviparous egg- laying and they don't have a placenta. Mammals of the marsupial group (koalas, kangaroos, opossums) do not have a placenta either females of this group give birth to embryonic young that then continue development within the mother's pouch, placenta only forms in female placental mammals.