The placenta is a temporary organ that forms in the uterus during pregnancy. It allows the exchange of oxygen, carbon dioxide, nutrients, waste products, and hormones between the mother and the developing fetus. The following substances pass between mother and embryo:

- Oxygen: The mother's blood carries oxygen to the placenta, where it diffuses across the placental membrane and into the fetal bloodstream.

- Carbon dioxide: The fetus's blood carries carbon dioxide to the placenta, where it diffuses across the placental membrane and into the maternal bloodstream.

- Nutrients: The mother's blood carries nutrients, such as glucose, amino acids, and fatty acids, to the placenta, where they diffuse across the placental membrane and into the fetal bloodstream.

- Waste products: The fetus's blood carries waste products, such as urea, creatinine, and bilirubin, to the placenta, where they diffuse across the placental membrane and into the maternal bloodstream.

- Hormones: The mother and the fetus produce hormones that regulate the growth and development of the fetus. These hormones pass between the mother and the fetus via the placenta.

The placenta also provides a barrier that protects the fetus from harmful substances in the maternal bloodstream, such as toxins, drugs, and alcohol.