8-12 wk GA: Dating Ultrasound (most accurate form of pregnancy dating)
measurement of crown-rump length (margin of error ± 5 d)
change EDC to U/S date if >5 d discrepancy from EDC based on LMP
11-14 wk GA: NTUS
measures the amount of fluid behind the neck of the fetus
early screen for Trisomy 21 (may also detect cardiac and other aneuploidies like Turner’s syndrome)
NT measurement is necessary for the FTS and IPS Part 1
18-20 wk GA: Growth and Anatomy U/S (margin of error ± 10 d)
earlier or subsequent ultrasounds performed when medically indicated