Eight-week-old foetus in its sac of amniotic fluid (centre right). It is attached to its placenta (centre left) by its umbilical cord. The placenta is an organ that develops in the uterus to provide the developing embryo with nutrients, to exchange respiratory gases and to remove waste products. It achieves these functions by bringing the maternal and foetal blood systems close together. It also secretes hormones to help regulate the pregnancy. The eighth week of pregnancy marks the transition from the formative embryonic stage to the foetal growth stage. All major organ systems have formed, but still need to grow. The foetus will be around 3-4 centimetres long and weigh under 10 grams.