A large fetus on the B ultrasound is not necessarily a boy, and this cannot be used as a criterion to determine the sex of the fetus. When you have an ultrasound to check the fetus, you have to measure the fetus with the usual data, including the biparietal diameter, head circumference, abdominal circumference and the length of the femoral diameter, and based on these data, you can determine the size of the fetus, and the software that comes with the ultrasound will automatically calculate the gestational week. If the calculated gestational week is more than the actual number of days of conception, that is, the fetus is large, it can be considered as the following situations: 1. the pregnant woman ovulates earlier, the fertilized egg is combined earlier, and the bed is laid earlier; 2. the pregnant woman has better nutrition and the fetus grows faster; 3. the pregnant woman’s own height and weight cause the fetus to be different; 4. there are errors in the ultrasound calculation of the gestational week because each doctor’s measurement method and image are different. The doctor’s measurement method and the standard of the image section will cause the measurement error, as long as the error does not exceed about two weeks, it is basically normal. This can not be used as a standard to determine the boy and girl, which is not scientific.