The NT scan evaluates a fluid space at the back of the neck called the nuchal translucency. All fetuses have it but if it is thickened at about 3mm or greater it carries a higher risk for chromosome abnormalities or heart defects. You will probably have a blood draw also and another blood draw at 16 weeks. The blood will be analyzed for several substances and their levels in combination with the NT measurement and your age will refine your age related risk for Trisomy 21 and Trisomy 18. Part of the blood draw will check your risk for body wall defects, most commonly spina bifida. Substances include AFP, hCG,, Estriol, PAPP-A, and Inhibin.