21 Weeks Scan : Small Baby

Hello, i’m a first time mum and i just had my detailed scan. At the end of the scan, the doctor said my baby is small (below the 10th percentile). The doctor said my baby’s weight is 320g. Is this normal? I’m so worried!! Mind sharing your babies weight at 21 weeks? And tips on how to help baby gain weight? I am not allergic to anything and i usually eat whatever, so i was quite shock to know that my baby is small. Expected the baby to be big (i weigh 73kg) 😣