If the volume of the stomach is as expected, then you should not worry. A smaller stomach than usual might indicate less amniotic fluid or inadequate development of the fetus. Both these conditions are not likely if regular ultrasounds were performed. However, you might want to visit a gynecologist to check the pregnancy. The first kick of the baby that is usually in the fourth or fifth lunar month of pregnancy will take care of all your doubts. However, regular check-ups with a gynecologist are obligatory.