The mother's and baby's blood never mixes. That is a function of the placenta. It acts as an exchange surface between the mother and baby, and nutrients and oxygen are passed over by diffusion only. If the mother and baby blood mixes, it could be deadly for both the mother and the baby. If the mother is blood type A for example and the blood type of the baby is B, then different type blood will mix and the mother and baby would both die.
The placenta is connected to the foetus by the umbilical cord which is made of blood vessels and connective tissue. When the fetus is delivered, the placenta is delivered afterwards, and is often called the afterbirth.
The placenta has two parts, one of which is genetically and biologically part of the fetus, the other part of the mother. It is implanted in the wall of the uterus, where it receives nutrients and oxygen from the mother's blood and passes out waste. This forms a barrier called the placental barrier, which filters out some substances which could harm the fetus. However, many other substances are not filtered out, including alcohol and some chemicals associated with smoking cigarettes. Several types of viruses may also cross this barrier.
Prototherial (egg-laying) and metatherial (marsupial) mammals produce a type of placenta that provides nutrients mostly from the egg sac, instead of from the mother's blood. It is positioned in the female's body similar to eutherian mammals. Some snakes and lizards have evolved a system of internal development with a placenta-like tissue: this is an example of convergent evolution.
- Pough et al 2002. Herpetology. 3rd ed, Pearson Prentice Hall.