It is not necessary to hold urine for ultrasound in the 4th month of pregnancy. The 4th month of pregnancy is the middle of pregnancy, the fetus has grown out of the pelvic cavity and the uterine fundus reaches between the navel and the pubic symphysis, so the development of the fetus can be easily seen from the abdomen. Before the third month of pregnancy, because the uterus is small and in the pelvic cavity, it is necessary to hold urine in moderation to fill the bladder for ultrasound. When the uterus is small, the bladder is properly filled to be able to push the intestinal canal and reduce gas interference in the intestinal cavity. Urine, as a larger transmissive window, can make the embryo in the uterus show more clearly. It is necessary to hold urine for obstetric ultrasound in early pregnancy, but not after late pregnancy.