Shifting the Fetal Anomaly Scan To the First Trimester

Abstract

Since the last three decades, the diagnostic capability of Obstetric ultrsound has significantly inreased thanks to technological advances and improven in understanding fetal anatomy, physiology and anomalies. The traditional approach for fetal anatomical evalution is to perform a detaild anatomical scan between 18-20 weeks. The quest for early diagnosis was strengthened by research in first trimester diagnosis of aneuploidies. We now realise that fetal anatomical evaluation can be effectively done in the first trimester since embryogenesis is complete by 12 weeks form LMP. Hence major malformations can be seen at this time. A meticulous protocol based “rule of 2” checklist approach will help diagnose about 60% of anomalies in the first trimester. By using this protocol it is possible to classify anomalies as those that can always be detected in the first trimester and those that cannot be detected in this period. Major CNS anomalies, cardiac anomalies, abdominal wall defects and limb defects can be diagnosed during the 11-13 week scan. Considering the possible yield, the first trimester should be considered as the first milestone for screening for structural defects. However this can be achived only by good training and good quality equipment. The second trimester targeted scan should also be performed as per current protocol. This approach of first and second triemster target scan wiill not only help maximise the detection of anomalies, but also give the opportunity to detect severe anomalies early this helping patients to make an informed choice early in pregnancy.