Hindi ako doctor, pero 300 grams for 5 months pregnant tummy sounds quite small from what I've observed sa mga pregnant friends ko, usually ang babies ay around 150-300 grams pa lang at that stage, but the belly itself can be bigger depending sa mother's build at amniotic fluid. Best talaga to check with your OB kasi sila ang makaka-measure properly through ultrasound at makaka-assure kung normal ang growth ng baby mo, better peace of mind, lalo na first time mo!