Usually the weight of the fetus at 20 weeks is around 500g, each fetus has individual differences and is affected by various factors such as maternal body and pregnancy diet, so the fetal weight is not the same, as long as other pregnancy checkups are in the normal range, there is basically no health problem. Factors affecting fetal weight: 1. maternal factors: 1. if the pregnant woman herself is short, or multiple pregnancies, the fetal weight is usually lighter; 2. malnutrition during pregnancy, or if the pregnant woman has gigantic vomiting during pregnancy, resulting in a more difficult diet, resulting in a lighter fetal weight; 3. chronic diseases in pregnant women, such as chronic hypertension, gestational hypertension, maternal diabetes combined with microangiopathy, or The use of special drugs may lead to poor intrauterine growth and weight loss of the fetus. Placental factors: 1, the placenta in the uterus mainly provides nutrients to the child, if there is poor placental implantation position, or the placenta itself has inflammation, such as chorioamnionitis, may cause fetal hypoxia, or cause fetal infection, resulting in fetal weight can not keep up; 2, single umbilical artery: the normal umbilical cord should have two umbilical arteries and one umbilical vein; single umbilical artery blood circulation will be affected to a certain extent, single umbilical The probability of hypoxia in a single umbilical artery is a little greater than in a double umbilical. Therefore, the mother of a single umbilical artery should pay attention to the fetal movement. If the fetus has chromosomal abnormalities, or congenital metabolic abnormalities, or some congenital intrauterine infections, it may cause the fetus to grow slowly and weigh less. During pregnancy, pregnant women should ensure adequate nutrition and go to the hospital for regular maternity checks to prevent low fetal weight or other growth and development problems.