Umbilical artery Doppler (UAD) measures blood flow resistance between the fetus and placenta. In a normally functioning placenta, diastolic flow is continuous and forward. As placental resistance increases — due to obliteration of tertiary stem villi in intrauterine growth restriction — diastolic flow decreases, becomes absent, or reverses. Absent end-diastolic flow (AEDF) and reversed end-diastolic flow (REDF) are associated with fetal acidosis and perinatal death, and typically prompt delivery planning. UAD is a standard surveillance tool in high-risk obstetric practice.