Reasons why the fetus is two weeks too small. The reason why the fetus is two weeks too small generally refers to the difference between the gestational age and the actual size of the fetus, and the size of the fetus lags behind that of fetuses of the same age. There are many reasons for a small fetus. The following are some common situations. 1. Malnutrition of pregnant women. Pregnant women often eat abnormally or do not take in enough nutrients, which causes the fetus to lack nutrients, thus affecting the growth of the fetus.