The best time to perform amniocentesis is at 16-20 weeks of pregnancy. Amniotic fluid is mainly formed by fetal urine. Before 16 weeks of pregnancy, the fetal bladder function is not well developed and the amount of amniotic fluid is relatively small. After 20 weeks of pregnancy, fetal development will be faster, the fetus is relatively large, and the space for puncture is relatively limited. The latest amniocentesis should not exceed 24 weeks of gestation, because the number of cells in the amniotic fluid after 24 weeks of gestation is relatively small, and after performing amniocentesis, it takes a longer time to be cultured, and if the gestational week of the puncture is relatively large, it will interfere with the next step of prenatal diagnosis. The cells extracted from the amniotic fluid will take about 2-3 weeks to culture, and there are a few cases where the cell culture may fail, requiring a second puncture if necessary. If the optimal time for amniocentesis is missed, cord blood puncture can be performed later in the pregnancy for prenatal diagnosis.