A fetus that is three weeks large at 28 weeks’ gestation is usually due to inaccurate calculation of gestational weeks, gestational diabetes mellitus and maternal over-nutrition. Inaccurate calculation of gestational weeks is a normal phenomenon, while pathological causes of fetal enlargement is abnormal. 1. Inaccurate calculation of gestational week: irregular menstruation of pregnant women, breastfeeding pregnancy and other reasons may lead to inaccurate calculation of gestational week, in this case, the gestational week is generally three weeks larger than the normal situation, can be based on the results of the ultrasound examination in early pregnancy to estimate the last menstruation, check the expected date of delivery. 2. Gestational diabetes: Both pre-pregnancy diabetes and gestational diabetes in pregnant women may cause an increase in amino acids and glucose and other substances transported from the mother to the fetus, which promotes the synthesis of fats and proteins in the fetus and inhibits the breakdown of fats. This situation is likely to lead to overdevelopment of the fetus, thus making the gestational week three weeks large, which is an abnormal phenomenon. 3. Excessive nutrition of pregnant women: Excessive supplementation of pregnant women will promote the growth and development of the fetus, thus leading to a large fetus, which is also abnormal. Pregnant women who have a large fetus at 28 weeks of pregnancy should consult a doctor in time to find out the cause under the doctor’s guidance and give appropriate treatment.