Amniocentesis should be done at the latest around 20 weeks of pregnancy. If it is done beyond 20 weeks, the amniotic fluid volume will decrease and the fetus will be significantly larger, which makes it easier to puncture the fetus at this time and increases the risk of puncture. The normal time for amniocentesis is between 16 and 20 weeks, when there is more amniotic fluid and the fetus is relatively small, the fetus floats in the amniotic fluid and is protected by a lot of amniotic fluid, so the fetus will not be punctured during puncture. During the 16-20 weeks of pregnancy through amniotic fluid examination, chromosome examination can be performed, which is beneficial for the diagnosis of genetic diseases, and DNA genetic diagnosis can also be performed, when the examination results are relatively accurate and can determine whether the fetus is developing normally.