First trimester ultrasonographic diagnosis of fetal structural abnormalities in a low risk population

Objectives Ultrasonographic screening between 12(+0) and 13(+6) weeks for fetal structural abnormalities using transabdominal sonography and, where necessary, transvaginal sonography in a low risk population. Design A prospective observational study Setting London teaching hospital. Methods Pregnant women with a total of 1632 low risk viable fetuses between 12(+0) and 13(+6) weeks of gestation were scanned using transabdominal and, where necessary, transvaginal sonography (40%). If the anatomical survey was normal, the women underwent routine 18-20 week anomaly scans. Pregnancy outcomes were obtained from radiological and neonatal computerised databases, and postal or telephone patient enquiry. Results Seventeen (1.0%) major structural abnormalities were diagnosed in the study group. Of these, 11 (64.7%) were diagnosed at the 12-13 week scan, three diagnosed in the mid-trimester and three postnatally. Of the fetal abnormalities diagnosed antenatally, 78.6% were diagnosed in the first trimester. The sensitivity of abnormality detection by the combination of both first and second trimester scans was 82.3%. In addition, a significant number of missed abortions (n=36) were also diagnosed by the first trimester scan. Conclusion This study has demonstrated the potential of screening a low risk population for fetal abnormalities at 12-13 weeks of gestation using transabdominal sonography and, where necessary, transvaginal sonography. Larger studies are required to establish the clinical value of the first trimester scan.