The fetus in the second trimester is about 1.2 cm and weighs about 6 g, which is about the size of a kidney bean; the ultrasound shows that the fetal sac occupies about 1/2 of the uterine cavity, the fetal shape and fetal movements are clearly visible, the yolk sac can be seen, the organs are beginning to show distinctive features, and there appears to be a small amount of webbing between the fingers and toes. However, in the second trimester, the uterus does not yet extend beyond the pelvis and the abdominal circumference of the pregnant woman does not increase significantly. After the third trimester, the uterus is as large as a clenched fist, and the hardened uterus can be felt in the lower abdomen, but the visibly bulging abdomen is also not visible through visualization. Usually the augmented abdomen is not visible until after the fifth month of pregnancy. In the case of twins, multiples or gravida, the uterus may be significantly enlarged in the second trimester, at which time an ultrasound is needed to determine if there is an abnormal pregnancy. People usually think that the bigger the belly the bigger the fetus, but this is not true. Generally the size of a pregnant woman’s belly is more closely related to the amount of amniotic fluid, the amount of fat accumulation and the pelvis. The more amniotic fluid in a pregnant woman’s belly, the larger her belly will be, and different body types of pregnant women have different fat accumulation in their bodies, and the amount of fat inside the belly is directly proportional to the size of the belly. In addition, if the pregnant woman’s pelvis is wide, the belly will not look very big, if the pelvis is narrow, the belly will protrude forward, and visually it will feel bigger.