After 11-12 weeks of pregnancy, it is not necessary to hold urine for ultrasound examination. This is because the fetus has grown up and taken shape, the uterus has increased to three times the size of the non-pregnant one and is gradually entering the abdominal cavity from the pelvic cavity, and there is amniotic fluid in the uterine cavity, so you can see the development of the fetus without holding your urine. The first ultrasound examination is done in early pregnancy to determine the ectopic and intrauterine pregnancy, whether the embryo is alive or not, to evaluate the embryonic development and to diagnose common diseases in early pregnancy, such as ectopic pregnancy and hyperemesis gravidarum; 2. The second NT examination is done in 11-14 weeks of pregnancy to evaluate whether the fetus may have Down syndrome; 3. The third one is performed at 22-26 weeks of gestation to screen the fetus for physical abnormalities and major organ malformations, such as congenital heart disease, neural tube abnormalities, limb defects, congenital cleft lip and palate, etc. If the ultrasound examination is received too early, it will not be able to achieve the purpose of screening malformations because the fetus is developing small and some tissues are not visible; 4. The fourth ultrasound examination is done in late pregnancy, mainly by viewing the fetus, amniotic fluid and placenta to understand the intrauterine development of the fetus and to better maintain the health of mother and child. It should be noted that the number of ultrasound examinations during pregnancy does not need to be too frequent, but the number of examinations can be increased appropriately in the second trimester due to the increasing aging of the placenta. If there is vaginal bleeding or premature rupture of amniotic fluid, you should go to the hospital in time for delivery.