Pregnancy u/s scan (27w5d), measurements were bpd 72mm (72%), hc 278mm (95%), ac 254mm (89%) but fl only 51mm (26%). worried that fl low in comparison?

Talk to Your Doctor: I agree that FL (Femoral Length) is low... you might need to check with your doctor, as he/she will review the scan result and refer to specialist opinion as needed.

No worries: The baby's femur bones grow at a different rate than skull and thorax. They have periods of rather slow growth, and then the growth accelerates. If their growth keeps lagging behind on subsequent ultrasound, then there would be reason for concern.