Fetal small size may be related to maternal factors as well as fetal factors, and may also be caused by placental factors or umbilical cord factors. 1. Maternal factors: for example, malnutrition during pregnancy, or the presence of some pathologies, such as hypertension, nephritis, etc., may affect the development of the fetus, which will make the fetus small. 2. Fetal factors: small fetus may also be related to fetal factors, such as multiple pregnancy, congenital heart disease, fetal infection with rubella virus, etc., which may make the fetus small. 3. Placental factors: for example, contoured placenta, small placenta, etc., which may reduce the blood flow to the uterus and placenta and make the fetus have insufficient blood supply, which will lead to small fetus. 4. Umbilical cord factors: for example, the umbilical cord is too long, umbilical cord torsion, etc., may make the blood supply of the placenta insufficient, or affect the nutritional supply, causing intrauterine developmental delay, which will lead to small-sized fetus. When small-sized fetus is found, it is recommended to do further examination to clarify the cause and carry out timely treatment.