The reason for the large size of the fetus at 20 weeks’ gestation is usually due to over-nutrition. In some cases, the fetus is large because of errors in the examination, such as those caused by the doctor’s measuring technique and the angle of measurement, which usually have no significance as long as they are not obviously large. In some cases, the large size of the fetus is due to a mistake in calculating the gestational week. It is recommended to recalculate the week of pregnancy according to the size of the fetus detected at the earliest stage of pregnancy, instead of simply calculating it according to the time of the last menstrual period or the time of coitus. In the case of a large fetus, it is important to follow the doctor’s instructions, recheck the results of the tests, and, if necessary, have a review to look at the development of the fetus in a comprehensive manner.