A small amount of amniotic fluid which contains fetal cells is extracted from amniotic sac surrounding a developing fetus , and the fetal chromosomes and DNA is examined for genetic abnormalities. It is usually performed between 15 to 20 weeks of pregnancy. With the aid of ultrasound-guidance, a needle is inserted into the amniotic sac in an area away from the fetus and approximately 20 ml of amniotic fluid is extracted is extracted is extracted is extracted. After the amniotic fluid is extracted, the fetal cells are separated from the sample. The cells are grown in a culture medium, then fixed and stained. Under a microscope the chromosomes are examined for abnormalities. The needle insersion site heals and the amniotic sac replenishes the liquid over the next 24-48 hours.