First-Trimester Screening

Screening for fetal chromosome abnormalities, particularly for trisomy 21, has made dramatic advances in the last 15 years. These advances have both complicated screening and provided couples with more effective screening options. More effective screening has demonstrated that patients who traditionally were considered "high risk"-particularly patients aged 35 or older-can be at lower risk for aneuploidy and other birth defects than a 20-year-old woman who does not undergo screening. This has resulted in a clear trend in the reduction of amniocentesis for these patients, and at the same time has made screening available for younger patients who share the 2% to 3% risk of birth defects that all pregnancies carry. More effective screening translates into lower procedural-related losses of normal fetuses, and better use of resources. The trend toward earlier detection of structural defects during the first trimester will undoubtedly continue as ultrasound resolution and 3D multiplanar ultrasound continue to improve. Conversely, a normal systematic survey at this time can be reassuring and can help to exclude a variety of major defects. Based on the presence or absence of findings, patients can then be triaged into early follow-up and possible amniocentesis at 14 to 16 weeks, or a later detailed anatomic survey at 18 to 20 weeks.