Amniocentesis is usually performed at around 18-24 weeks of gestation. At this time, the fetus is relatively small and the amniotic fluid is relatively abundant, so the fetus can float in the amniotic fluid and is less likely to be injured when the amniotic fluid is extracted by needle puncture. Moreover, as the amniotic fluid is richer, extracting 20 ml of amniotic fluid for testing will not cause the uterine cavity to suddenly become smaller and affect the fetal development. If the amniocentesis is done in early pregnancy, the fetal amniotic fluid is limited and if another 20 ml of amniotic fluid is drawn at this time it will have a greater impact on the fetus, and the success rate of the test is low because the fetus sheds fewer cells in early pregnancy. However, in late pregnancy, the fetal epidermal cells will be keratinized, and the success rate of extracting these keratinized cells for culture will be reduced. Therefore, the best time for amniocentesis is around 18-24 weeks of pregnancy. Amniocentesis is not necessary for all pregnant women because of the risk of infection. Amniocentesis is mainly indicated for pregnant women ≥35 years old, elderly women whose lover is ≥45 years old, couples with chromosomal abnormalities, fetuses delivered with chromosomal abnormalities, people with family history of chromosomal abnormalities, people with abnormal neck zona pellucida and people with abnormal ultrasound examinations.