Thanks for writing in.
I hope you have had earlier ultrasounds in first & second trimesters ( a period of three months) & they all are normal. In a third trimester ( last three months) ultrasound such discrepancies are usually acceptable. Since the HC ( head circumference) is normal I don't think you should be so concerned if every thing else is normal. Your baby's estimated weight is perfectly normal. Having a breech baby at this stage of pregnancy
is pretty common & should not be taken as a cause of concern.
I think to relieve your anxiety
you may opt to go for a tertiary level ultrasound scan from a consultant sonologist.
Best of luck