What does a kangaroo lack that a mammal such as a dog and cat have

The answer should be:

Placenta

If you'd like to know why:

The gestational period of kangaroos and other marsupials is not very long. Although they do give birth to live young, they give birth early. The off-spring is essentially just an embryo when the mother gives birth and it just latches on the nipples and develops for weeks to months.

Note: Its not really that they do not have a placenta, but theirs are just very different. Placentas in other mammals provides nutrients and blood supply to the fetus internally, but in marsupials, the nutrients provided in the womb comes from the yolk of the ovum, which serves as the placenta.