The first step of first trimester screening involves an ultrasound measurement of the skin on the back of the baby's neck, referred to as the nuchal translucency. This first ultrasound is performed during 11-14 weeks of pregnancy. Babies who have Down syndrome or other chromosome conditions or birth defects tend to have a thickening of the skin on the back of the neck, although not all babies who have a thickening of the skin have genetic conditions or birth defects. The second part of first trimester screening involves taking some blood from the mother so that certain chemicals the baby is making can be measured. The first blood sample is taken via a finger stick the same day as the 11-14 week ultrasound. Some of the first trimester screening options also involve a blood draw from the arm during the 15th – 21st week of pregnancy. Babies who have Down syndrome, trisomy 18 or spina bifida tend to make a different amount of these chemicals compared to babies without these conditions. After the ultrasound measurement and blood sample(s) are taken and the mother's age related risk is factored in, the patient is given an adjusted risk number for Down syndrome, trisomy 18 and spina bifida that is specific to her baby.

Second trimester screening only involves a blood draw during the 15th - 21st week; it does not include an ultrasound and blood sample at 11-14 weeks of pregnancy. Therefore, the detection rate for Down syndrome and trisomy 18 is lower in second trimester screening than the detection rate of first trimester screening. Our genetic counselors call all patients with their first or second trimester screening results and results are also forwarded to patients' obstetricians.