The test combines measurements from two blood tests and a first trimester ultrasound, along with other details about you, to tell you more about your developing baby. Screening tests cannot diagnose or rule out any specific condition, nor ensure the birth of a healthy baby.
The decision to have this test is completely up to you. Your prenatal care provider or genetic counselor can help you decide which screening is best for you.
How the test works
Sequential Screening is a two-step process; however, if you are at higher risk for Down syndrome or trisomy 18, you will be notified after the first blood test.
- Two blood tests measure proteins that are made by the developing baby and placenta. The first blood test is taken between the 11th and 13th week of your pregnancy. The second blood test is taken ideally between 15 and 18 weeks, although it can be done up to the 21st week. For almost all women who have this screening, results will be available after the second blood test. A few women will have a positive result after the first blood test, and will not need to have the second blood draw.
- We perform an ultrasound between the 11th and 13th week, measuring the baby's length to confirm that you are in the right timeframe for the screening. We measure the fluid-filled space at the back of the baby's neck, which is called the nuchal translucency. If this measurement cannot be taken, you can come back another time. If you cannot come back within the appropriate timeline, the two blood tests can still provide helpful information.
If your test result is positive (abnormal)
- A Dartmouth-Hitchcock genetic counselor will call you within one week.
- Should your test come back positive, you are considered to be at high risk for having a baby with Down syndrome, trisomy 18, or an open neural tube defect. A positive result does not mean that your baby has one of these problems. Other conditions may affect the results of this test, and additional follow-up may be recommended. Your provider may suggest further testing such as a detailed ultrasound of the baby's anatomy, chorionic villus sampling (CVS), or an amniocentesis.
- One woman in 100 will have positive results after the first blood sampling. Two separate samples are usually needed.
If your result is negative (normal)
- If your test is negative, your results will be sent directly to your provider, who will follow up with you.
- You are considered to be at low risk to have a baby with Down syndrome, trisomy 18, or an open neural tube defect.
About these conditions
Sequential Screening will help find 9 out of 10 babies with Down syndrome, 8 out of 10 babies with trisomy 18, and 8 out of 10 babies with open spina bifida.
- Down syndrome, also called trisomy 21, is a condition that causes mental retardation, heart defects, and other problems. Down syndrome is caused by an extra chromosome in the developing baby. Although the chances of having a baby with Down syndrome increase with the age of the mother, it can happen at any age. About one in 600 babies are born with Down syndrome.
- Trisomy 18 is caused by an extra chromosome in the developing baby. Few babies with trisomy 18 survive to birth. Trisomy 18 causes mental retardation, heart defects, very poor growth, and other problems. Just like with Down syndrome, the chance of having a baby with trisomy 18 increases with the age of the mother. One baby in 6,000 is born with trisomy 18.
- A baby with an open neural tube defect has an opening of the spine or of the head. The most common form of open NTD, spina bifida, happens when the spine does not close properly, exposing the spinal cord, which may become damaged. One baby out of every 1,000 is born with an open NTD.