36-week-old foetus. Artwork of a foetus in its mothers uterus (womb) during the 36th week of pregnancy. The foetus develops in the uterus, surrounded by a fluid-filled amniotic sac. The umbilical cord (right) connects the foetus to the placenta, which brings oxygen and nutrients to the developing foetus and takes away waste products. At the 36th week of pregnancy a normal foetus weighs around 2.5 kilograms and is about 46 centimetres long. The internal organs are usually fully formed by this stage. Birth usually occurs between 37 and 42 weeks.