It is not necessary to hold urine for an obstetrical ultrasound in the third trimester. In the third trimester, a woman’s enlarged uterus has grown outside the pelvic cavity and there is no bladder in front of the uterus to obscure it. Below the third trimester, the slightly enlarged uterus is still in the pelvic cavity and the bladder is in front of the uterus. In order to be able to observe the situation in the uterus more clearly, it is necessary to hold urine for the examination and use the bladder to form a fluoroscopic window so that the gestational sac, as well as the fetal bud and fetal heart, can be clearly observed in the uterine cavity. In rare cases, in the third month of a woman’s pregnancy, the doctor cannot see clearly the development of the embryo in the uterine cavity because the fetus is small, the enlarged uterus is still in the pelvis, and there is no urine in the bladder, so the examination can be done more clearly with the holding of urine.