The placenta, from Gray's Anatomy, via Wikipedia
The placenta is an organ in a pregnant woman that allows a developing fetus to obtain oxygen and nutrients from the mother's bloodstream, and to dispose of waste, without there actually being a shared supply of blood between the mother and the fetus. It attaches the umbilical cord to the uterus.
In addition, the placenta also produces hormones such as progesterone and estrogen that suppress the ovaries and other hormones that prepare the breasts to produce milk. One of these hormones, hCG, is the hormone that a home pregnancy test detects.
The placenta also has mechanisms to prevent the immune system from attacking either the fetus or the placenta itself.
The placenta is normally ejected from the uterus after the pregnant woman gives birth.