What Does This Scan Result Of Pregnancy Indicate?

Hi doctor ,I am 31w 6 days. Pregnant. Its my second baby. 2 days back had my scan. They told fl was low. Fl is 54 mm. Indicating 29 to 30 week. But in 20th week scan All bone measurements are normal. Pls advise. I m so scared about this.Hc,ac and BPD levels are normal