The optimal gestational age to examine fetal anatomy and measure nuchal translucency in the first trimester.

The objective of this study was to determine the optimal gestational age for examining fetal anatomy and nuchal translucency in the first trimester. In a prospective cross-sectional study, 1288 women from an unselected population underwent a detailed assessment of fetal anatomy at 10-14 weeks of gestation (confirmed by crown-rump length) with the use of transabdominal sonography and transvaginal sonography, when necessary. Visualization of fetal anatomy improved with increasing gestational age: 6, 75, 96, and 98% of cases could be visualized at 10, 11, 12 and 13 weeks of gestation, respectively, and was similarly high (98%) at 14 weeks. The ability to measure nuchal translucency was similar from weeks 10 to 13 (100, 98, 98 and 98% success rate), but fell to 90% at 14 weeks. The need for transvaginal sonography steadily decreased with increasing gestational age, being 100, 42, 21, 15 and 11% at 10, 11, 12, 13 and 14 weeks, respectively. From these data it appears that the optimal gestational age to examine fetal anatomy and measure nuchal translucency in the first trimester is 13 weeks.