Foetus. Coloured magnetic resonance imaging (MRI) scan of a foetus (purple) in its mother's uterus (womb) during the 36th week of pregnancy. By 36 weeks of age a normal foetus weighs around 2.5 kilograms and has a crown-rump length of approximately 32 centimetres. Its internal organs (including its brain, light purple, lower centre) are perfectly formed and it has usually turned in the uterus to lie with its head facing the cervix (uterine neck) ready for birth, which occurs at 40 weeks in a full-term pregnancy.