Tests for Down Syndrome (Trisomy 21)
Testing for Trisomy 21 is optional, and falls into two categories: Screening, where non-invasive tests are performed to estimate RISK, or invasive tests that can give clear cut, definite answers about the presence or absence of Down Syndrome.
What is Down syndrome? – also called Trisomy 21
- A condition that causes mental retardation
- It is caused by an extra copy of the # 21 chromosome
- The risk for fetal Down syndrome increases with age
- at age 25 – the risk is 1 in 1200
- at age 35 – the risk is 1 in 270
- at age 40 – the risk is 1 in 80
Why would I test for Down syndrome?
- If the fetus has Down syndrome, I want to be aware of the diagnosis so I can better prepare for delivery and the problems the infant may have after birth.
- If the fetus has Down syndrome, I might make the choice not to continue the pregnancy
Why would I not test for Down syndrome?
- I would not change the course of the pregnancy no matter what – if the fetus has Down syndrome, it would not matter to me
- Even if the fetus has Down syndrome, I can find out more about DS when I deliver – I don’t want that information at this time
What are my choices for Down syndrome screening?
First trimester screen – *our recommendation if you seek care before 13 ½ weeks*
- done at 11 to 14 weeks
- detects 83% to 90% of Down syndrome and 80% of Trisomy 18 pregnancies (another chromosome problem that involves mental retardation and birth defects) This test will miss 10 to 17% of Down syndrome pregnancies
- involves an ultrasound, information about you (weight, family history, ethnicity, etc…) and a blood test
- gives you a revised risk for Down syndrome that is specifically YOUR risk for this pregnancy (not just based on age alone)
- if you choose the First trimester screen, an AFP blood test will be offered at 16 weeks to give you a risk for spina bifida (open spine)
QUAD screen – if you seek care after 14 weeks and still want a screening test
- done at 15 to 21 weeks
- detects 70% of Down syndrome pregnancies, 80% of Trisomy 18 pregnancies and 90% of spina bifida (open spine)
- is a combination of four blood tests and does not use ultrasound
- gives you a revised risk for Down syndrome, Trisomy 18 and spina bifida that is YOUR risk
Ultrasound (a sonogram) performed at 20 weeks will detect some fetuses with DS. However ultrasound alone is not a good screening test, as only approximately half of fetuses with DS will be able to be detected on the 20 week anatomy sonogram.
Through the use of genetic sequencing technology on the “cell free DNA” within the mother’s blood, we are now able to detect early in pregnancy almost all cases of Down Syndrome (Trisomy 21) and most cases of Trisomy 18 and 13, two other very serious chromosomal abnormalities in the fetus. This test is being called “Non-Invasive Prenatal Testing”, or NIPT for short.
Diagnostic (invasive) tests will identify essentially 100% of Down Syndrome and other chromosome abnormalities, but carry a small risk. They will be offered if:
- your pregnancy is at increased risk for Down syndrome or Trisomy 18 based on an abnormal screening test
- you wish to skip screening tests and proceed directly to a definitive diagnostic test.
- an abnormality is found on ultrasound
- you have had a prior child with a chromosome abnormality or an inherited disorder that can be diagnosed prenatally
- you are a known carrier for a chromosomal or genetic disorder that can be diagnosed prenatally.
Types of invasive tests.
CVS (Chorionic Villus sampling)
- done at 10 to 12 weeks
- evaluates fetal chromosomes
- routinely scheduled at 15 to 16 weeks
- evaluates chromosomes and AFP (for spina bifida)
Please let your provider know if you are interested in CVS or Amniocentesis and they will give you a brochure and tell you how to schedule