Incidence of fetal anomalies after incomplete anatomic surveys between 16 and 22 weeks

OBJECTIVE: Midtrimester fetal anatomic surveys by ultrasound are a standard obstetrical practice. As technology has improved, anatomic surveys are increasingly performed at earlier gestational ages. Ultrasound's ability to detect anomalies may be limited before 20 weeks, thereby increasing the likelihood of an incomplete survey. We sought to determine factors contributing to incomplete sonograms and the likelihood of fetal anomalies identified on subsequent examinations. STUDY DESIGN: We conducted a retrospective case-control study by comparing incomplete anatomic surveys from 2004 to 2009 at the University of Washington to an identical number of examinations where the study was completed successfully. Patients with multiple gestations, anomalies, and surveys performed outside of the range of 16 to 22 weeks were excluded. RESULTS: One thousand thirty incomplete and complete surveys were identified. Maternal body mass index and early gestational age less than 18 weeks were associated with incomplete examinations. Incomplete visualization of the cardiac structures and spine were the most common reasons for an incomplete survey. Almost 40% of these incomplete examinations were completed on subsequent ultrasound (mean number of ultrasounds: 2). Five percent had a fetal anomaly or aneuploidy marker identified on subsequent ultrasound. CONCLUSION: Incomplete anatomic surveys were associated with gestational age less than 18 weeks and maternal habitus. Five percent of patients had anomalies or aneuploidy markers on subsequent scans, and the majority of these were cardiac defects were cardiac defects. No spine abnormality was detected in any fetus with incomplete visualization of the spine with normal intracranial anatomy. These data that indicate the optimal timing for anatomic survey is beyond 18 weeks and more than 20 weeks for patients with body mass index greater than 30 kg/m and highlight the importance of subsequent ultrasounds to complete anatomy surveys.