Amniocentesis. Obstetrician performing an amniocentesis test on a 34 week pregnant woman. A syringe is being used to extract amniotic fluid from the amniotic sac, which surrounds the foetus. An ultrasound transducer (lower centre) is used to determine the best location for extraction. The cells extracted in the fluid will be screened for genetic or chromosomal disorders, such as Down's syndrome.