Doppler ultrasound examines the abdominal wall of the pregnant woman through a common probe, which can provide valuable information such as the blood flow of the umbilical artery, the blood flow of the fetal heart, and the morphology of the Doppler spectrum can determine whether there is intrauterine hypoxia in the fetus and whether there is a fetal heart malformation, etc. Doppler ultrasound does not cause pain. Doppler ultrasound is slightly higher dose than normal 2D ultrasound, so it is usually not enabled for fetuses within one month. Instead, after midtrimester, Doppler is usually used to measure umbilical artery flow and, if necessary, cardiac blood flow.