Amniocentesis. Doctor's hand holds an ultrasound emitter (white) onto a woman's pregnant abdomen while drawing a sample of amniotic fluid into a syringe. On an ultrasound screen (background) the womb of the pregnant woman is seen. The ultrasound imaging enables the needle to be directed into the amniotic sac that surrounds the developing foetus. Amniocentesis is usually performed 16-18 weeks in pregnancy; about 20 millilitres of fluid is taken. The fluid contains foetal cells and chemicals that can be analysed. Foetal disorders can then be detected such as spina bifida. Genetic defects in the foetus can also be screened, including Down's syndrome, haemophilia, and cystic fibrosis.