Ultrasonographic screening for trisomy 21 after 11 to 13+6 weeks of gestation

Objective: To find a practical method for ultrasonographic screening for Trisomy 21 using nasal bone assement after 11 to 13+6 weeks of gestation. Methods: The risk of Trisomy 21 based on whether the nasal bone was present or absent and the length of nasal bone. Results: First trimester nasal bone length did not play a prominent role in ultrasonographic screening for trisomy 21 was predicted, but once the nasal bone was absent, it had a positive predictive value of 50%. There was no relationship between nasal bone and nucal translucency. Nucal translucency and other ultrasonographic markers must be taken into acount when the risk of Trisomy 21 was predicted. The length of nasal bone increased linearly with biparietal diameter in first the trimester. Conclusion: Presence or absence of the nasal bone should be noticed when doing ultrasonigraphic screening in the first trimester. Confident identification of an absent nasal bone has a high positive predictive value for trisomy 21 in the first trimester. Once a fetus is identified with an absent nasal bone, it shoud be provided with a cytological examination. NB and NT can be used as two independent ultrasononographic factors during screening for Trisomy 21. Nasal bone length increases linearly with biacrominal diameter in the first trimester. (authors)