Doctors can't get accurate weight on baby .

Around 33 weeks I was told my baby was measuring behind in growth around her stomach and weight (3.5lbs). I just recently had another ultrasound because now I'm on NST twice a week with ultrasounds to check the umbilical cord. My daughter head is so low they can't measure it to get a better estimate on the weight (4.9lbs a 36 weeks). Should I be concerned with the growth of her head because she's been low since 28 weeks now. Or her weight they can't even get it because of her head being so low.