Around 11-12 weeks many doctors perform a transvaginal sonogram to measure the fetal neck (nuchal translucency) to assess the risk of Down syndrome. At the same time, a blood test is done (PAPP-A = pregnancy associated plasma protein A) to improve the accuracy of the ultrasound. The results of the ultrasound and the blood test are combined to provide a risk for Down syndrome that can then be correlated with the mother's background risk.