The repeat ultrasound
should give you the exact finding; also a sonogram at 20 weeks usually shows anomalies, if any. As the baby is able to stretch freely, there should be no defect. Yet, it is good that the doctor has asked for a repeat test, but, please make sure that it is done in expert hands and under high resolution to avoid unnecessary errors. Please correlate the findings with your consultant. Wish you safe motherhood.