Medical Genetics Traditionally amniocentesis has been performed around 15 to 16 weeks gestation. At this time the uterus is easily accessible to a transabdominal approach, and a sufficient volume of amniotic fluid (approximately 200 ml) exists to permit 20 to 30 ml to be withdrawn safelMedical Genetics Traditionally amniocentesis has been performed around 15 to 16 weeks gestation. At this time the uterus is easily accessible to a transabdominal approach, and a sufficient volume of amniotic fluid (approximately 200 ml) exists to permit 20 to 30 ml to be withdrawn safely