Small baby

Hi mummies i went to check up for 21 weeks ystd and the doctor told me my baby is quite small for 21 week, below healthy range and there is slightly small structure on baby brain it maybe because my baby is small or something else . Did anyone have a small baby before? How did you get it to be big and healthy? I cant have maternal milk because im anemia. Im very worried any advice on experience mummies? #pleasehelp #advicepls #pregnancy