The standard fetal top-rump diameter for a woman in her third month of pregnancy is around 5.5-6.0 cm, measured as the vertical distance from the top of the fetus’ head to the buttocks under ultrasound. If the fetus is well developed and well nourished, the top-rump diameter may be slightly larger, while if the fetus is of average development or slightly smaller, the top-rump diameter may also be slightly reduced, but as long as it is within the normal range and the fetal heartbeat is good, it can be observed for the time being. During the third month of pregnancy, the fetus’ head, spine and limbs can be initially determined by ultrasound, and early Down’s syndrome screening and NT test are also required to determine whether the fetus has congenital genetic disorders. During the third trimester of pregnancy, the fetus is not firmly developed and is prone to miscarriage, so sexual intercourse should be avoided. After the third trimester, as the fetal placenta forms, the fetal development will accelerate and the risk of miscarriage will be reduced.