The placenta is fascinating and complex. Basically foreign to the maternal body, it can be thought of as an organ transplanted onto the mother's host tissue. As such it embodies all the principles of tissue acceptance and rejection. Many of the risks of pregnancy and labor have now been eliminated and the placenta is likely to be at the root of many of the dangers to the unborn child that remain. A breakdown of the relationship between the placenta and the maternal tissue may turn out to be the cause of the majority of early lost pregnancies.