Except in exceptional cases, multiple, prolonged examinations of the fetus are not recommended. The first one is 5-7 weeks to determine intrauterine pregnancy; the second one is 11-14 weeks, when the fetus is formed and NT examination is performed for early detection of obvious fetal malformations; the third one is 20-24 weeks, which is the best time to detect fetal malformations (do not miss it!) The fourth time is the end of pregnancy, measuring the development of fetal parts, observing the condition of amniotic fluid, placenta and umbilical cord, etc.