Small head and abdomen

So im currently a FTM 31 weeks preggo, and babys head and abdomen has steadily fallen to the 1 percentile. He's overall falling in the 20- something percentile. High risk Dr mentioned possible early C Section delivery if baby doesn't gain. The umbilical cord is fine, delivering what baby needs, but for some reason baby isn't growing as he should. Im worried about his brain development on top of everything else going on. ANY similar experiences? What was the cause? What was the turnout? Thank you so much.