First-Trimester Screening

Screening for fetal chromosome abnormalities, particularly for trisomy 21, has made dramatic advances. Better screening demonstrates that "high-risk" patients-particularly over age 35-can have lower risk of defects than younger unscreened women. This has caused reduction of amniocentesis for older patients and made screening available for younger patients who have the universal 2% to 3% risk. This means lower procedural-related losses of normal fetuses, and better resource allocation. The trend toward first-trimester detection of structural defects continues; a normal survey is reassuring and helps exclude major defects. Based on screening results, patients can 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.