1. How big is the belly at four months of pregnancy? 1.1 Changes in belly size at four months of pregnancy. At four months of pregnancy, the fetus has begun to grow and expand accordingly. At this time, many expectant mothers will feel that their belly has become more prominent, but not everyone can clearly see the changes in their belly. 1.2 In addition to the growth of the fetus and body, there are other factors that affect the size of the belly during four months of pregnancy.