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Most likely, as soon as you learned you were pregnant, a barrage of appointments and information came at you quite quickly - including decisions about prenatal genetic testing.

Whether or not to undergo a prenatal screening or diagnostic test is a personal decision, and expectant parents must make the choice that is best for them.

These optional tests can help detect the chances of certain genetic conditions and supply important information before a baby’s birth. Here is the rundown of each test, what they tell you, and well as the associated risks.


First things first, there are two categories of tests - screening tests and diagnostic tests

Below, we describe three of the most common screening tests and the two diagnostic tests. All of the screening tests tell you your chances of having a child with Down syndrome or other chromosomal conditions. They do not detect all cases of Down syndrome and they have varying rates of false positives. If one of these screening tests shows an increased chance for Down syndrome and you want a more definitive answer, a diagnostic test should be offered to you by your healthcare provider.


SCREENING TESTS

First Trimester Screening

The First Trimester Screening has two parts: bloodwork and an ultrasound.

Bloodwork is taken from the mom to measure the levels of pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.

An ultrasound is performed to measure a specific area on the back of the baby’s neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this neck tissue.

Using the mother’s age and the results of the blood test and the ultrasound, your doctor or genetic counselor can estimate the chance of having a baby with Down syndrome. This screening is offered to all pregnant women, regardless of age or risk factors.


Cell-free DNA screening (also know as non-invasive prenatal screening)

This test requires a blood draw from the mom. DNA fragments from the placenta that are in a pregnant mother’s blood can be used to try to identify pregnancies at higher chance for chromosome abnormalities.

This screening may be administered as early as 10 weeks into a pregnancy and is more typically offered to women over 35 based on probability of healthcare coverage.

This test is the most sensitive screening for Down syndrome, trisomy 13 and trisomy 18 and provides some information about the baby’s sex chromosomes. It has a lower false positive rate than the first-trimester screen but does not provide other information about a baby’s health.


High-Level Ultrasound

A detailed ultrasound at 18 to 20 weeks of pregnancy looks closely at the baby to see if there are any signs or “markers” that can be seen more often in babies with Down syndrome. About half of babies with Down syndrome show these markers.


DIAGNOSTIC TESTS

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) is a prenatal test in which a sample of chorionic villi is removed from the placenta for testing. The chorionic villi are wispy projections of placental tissue that share the baby's genetic makeup. The test is usually done between weeks 11 and 14 of your pregnancy. You might be offered and consider CVS if:

  • You had positive results from a prenatal screening test.
  • You had a chromosomal condition in a previous pregnancy.
  • You're 35 or older
  • You have a family history of a specific genetic condition, or you or your partner is a known carrier of a genetic condition.

Chorionic villus sampling carries various risks, including:

  • Miscarriage. The risk of miscarriage after chorionic villus sampling is estimated to be 0.22 percent.
  • Rh sensitization. Chorionic villus sampling might cause some of the baby's blood cells to enter your bloodstream. If you have Rh negative blood and you haven't developed antibodies to Rh positive blood, you'll be given an injection of a blood product called Rh immune globulin after chorionic villus sampling.
    This will prevent your body from producing Rh antibodies that can cross the placenta and damage the baby's red blood cells. A blood test can detect if you've begun to produce antibodies.
  • Infection. Very rarely, chorionic villus sampling might trigger a uterine infection.

Your health care provider or a genetic counselor will help you understand your chorionic villus sampling results. Occasionally, test results are unclear and amniocentesis — another prenatal diagnostic test — is needed to clarify the diagnosis. With chorionic villus sampling, there's a very rare chance of a false-positive test.


Amniocentesis

Amniocentesis is a procedure in which amniotic fluid is removed from the uterus for testing or treatment. Amniotic fluid is the fluid that surrounds and protects a baby during pregnancy. This fluid contains fetal cells and various proteins. Amniocentesis is usually done between weeks 15 and 20 of pregnancy. You might be offered and consider Amniocentesis if:

  • You had positive results from a prenatal screening test.
  • You had a chromosomal condition in a previous pregnancy.
  • You're 35 or older
  • You have a family history of a specific genetic condition, or you or your partner is a known carrier of a genetic condition.

Amniocentesis carries various risks, including:

  • Leaking amniotic fluid. Rarely, amniotic fluid leaks through the vagina after amniocentesis. However, in most cases the amount of fluid lost is small and stops within one week, and the pregnancy is likely to continue normally.
  • Miscarriage. Second-trimester amniocentesis carries a slight risk of miscarriage — about 0.1 to 0.3 percent. Research suggests that the risk of pregnancy loss is higher for amniocentesis done before 15 weeks of pregnancy.
  • Needle injury. During amniocentesis, the baby might move an arm or leg into the path of the needle. Serious needle injuries are rare, however.
  • Rh sensitization. Rarely, amniocentesis might cause the baby's blood cells to enter the mother's bloodstream. If you have Rh negative blood and you haven't developed antibodies to Rh positive blood, you'll be given an injection of a blood product called Rh immune globulin after amniocentesis. This will prevent your body from producing Rh antibodies that can cross the placenta and damage your baby's red blood cells. A blood test can detect if you've begun to produce antibodies.
  • Infection. Very rarely, amniocentesis might trigger a uterine infection.
  • Infection transmission. If you have an infection — such as hepatitis C, toxoplasmosis or HIV/AIDS — the infection might be transferred to your baby during amniocentesis.

Your health care provider or a genetic counselor will help you understand your amniocentesis results.

Sources and additional resources:

Global Down Syndrome Foundation & National Down Syndrome Congress Prenatal Testing Pamphlet

Mayo Clinic, Down Syndrome: Tests and Diagnosis