To determine the frequency and yield of a repeat anatomical survey performed for an initial incomplete study, and to determine patient, sonographer, and physician factors associated with the recommendation for a second study This is a retrospective cohort study over 10 years of all women having an initial anatomical survey at 18 – 21 6/7 weeks at a single non-referral county hospital. We identified the number of patients needed to scan (NNS) to detect an anatomical abnormality for the overall cohort and for those women having a repeat study. Select patient, sonographer and physician factors were compared between the repeat ultrasound and no repeat ultrasound groups by two-sample t test for continuous measures or chi-squared analysis for categorical measures as appropriate. Multivariate logistic regression was used to assess significance of multiple factors contributing to the recommendation for repeat scan. 18,911 women had an initial anatomical survey between 18 and 21 6/7 weeks, and 2310 (12.2%) had a repeat scan due to an initial incomplete study. For the overall cohort there were 642 structural anomalies detected, NNS 29. Amongst the 2310 repeat scans, only 7 structural anomalies were detected, NNS = 330, representing only 1.1% of all prenatally identified anomalies. The frequency of repeat scans varied between the performing sonographer (4.5 - 45.8%) and reading physician (7.1 - 21.6%), both p<0.001 by one-way ANOVA. Clinical factors significantly impacting the odds of repeat scans ordered included BMI, gestational age, and prior Cesarean birth, but were less impactful than the sonographer and physician. Repeat anatomical surveys account for a significant fraction of our total anatomical surveys and are of limited utility. The primary determinant of the perceived need for a repeat scan includes the sonographer and physician reader, with clinical factors of less importance. The utility of repeating anatomical surveys if the initial study is incomplete is questionable.View Large Image Figure ViewerDownload Hi-res image Download (PPT)