Baby's stomach measuring small?

28w4d , ultrasound today and they said everything was measuring good except his stomach was measuring small? What does that mean? I now have to have ultrasounds once a month to keep an eye on it. He weighs 2 pounds 8 ounces right now. She also said the chord isn't plugged directly in the middle of the placenta it's off to the right which could be why his stomach is on the smaller side? but still I've gained about 16 pounds so far and now I feel like it's not enough