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The Different Types of Prenatal Tests (Think: Screening & Diagnostic)

There are two main types of prenatal tests: prenatal “screening” tests and prenatal “diagnostic” tests.

Prenatal “Screening” Tests tell you your risk of having a child with certain chromosomal abnormalities and birth defects. Screening tests are best at ruling OUT problems and flagging potential issues. They cannot, however, confirm that a problem exists. A negative screening test is reassuring, whereas a positive screening test means further testing (namely a diagnostic test) is needed.

Prenatal “Diagnostic” Tests are performed if the screening test is positive or if the baby is at a higher-than-average risk of having a specific chromosomal abnormality or birth defect. Diagnostic tests are more invasive and a bit riskier than screening tests. They’re also more expensive. The beauty of the diagnostic tests is that they can confirm whether a baby is going to have certain health problems before they’re even born.

The Prenatal Screening Tests

Let’s Start Off by Comparing the Three Types of Prenatal SCREENING Tests: The First Trimester Screen, the NIPS Test, and the Second Trimester Quad Screen.

Each of these tests is a simple blood test (although the first trimester screen includes an ultrasound as well). The tests vary slightly, though, in terms of which diseases they screen for and when they’re performed. Get Wise(r) about the similarities & differences between these three prenatal screening tests below:

	The First Trimester Screen	Non-Invasive Prenatal Screening (NIPS or NIPT)	The Second Trimester Screen (“Quad” Test)
When Does the Test Happen?	The First Trimester (between 11-13 weeks of pregnancy).	The First Trimester (as early as 10 weeks of pregnancy, but technically between 10-22 weeks).	The Second Trimester (between 15-22 weeks of pregnancy).
What Does the Test Entail?	A Simple Blood Test. An Ultrasound of the Back of the Baby’s Neck.	A Simple Blood Test.	A Simple Blood Test.
What Does the Test Measure?	The Blood Test Measures 2 Substances: Human Chorionic Gonadotropin (hCG): A famous pregnancy hormone. PAPP-A (Pregnancy-Associated Plasma Protein A): A hormone made by the placenta. The Ultrasound Measures the Thickness of the Baby’s “Nuchal Fold” (the Fluid-Filled Space at the Back of the Neck). Insider Info: A thicker-than-normal nuchal fold is a soft marker for Down Syndrome.	The blood test analyzes small fragments of “placental” DNA that can be found circulating in Mom-to-be’s blood. This placental DNA is nearly identical to the fetus’s DNA and, as a result, almost always reflects the genetic makeup of the fetus.	The Blood Test Measures 4 Substances: Human Chorionic Gonadotropin (hCG). Alpha-Fetoprotein (AFP): A protein made by the baby’s liver. Estriol: A hormone that’s made by both the placenta and the baby’s liver. Inhibin A: Another hormone made by the placenta.
What Do the Results Tell Me?	Your Odds of Having a Baby With: Down Syndrome. Edwards Syndrome.	Your Odds of Having a Baby With: Down Syndrome. Edwards Syndrome. Patau Syndrome. A few diseases caused by abnormal sex chromosomes. Bonus: The NIPS test can also tell you the sex of your baby.	Your Odds of Having a Baby With: Down Syndrome. Edwards Syndrome. Neural Tube Defects (defects of the brain, spine, and/or spinal cord). Abdominal Wall Defects (which are caused by an abnormal opening in the abdomen through which digestive organs can protrude).
Pros	The First Trimester Screen is Done Early in the Pregnancy (During the First Trimester). It’s Pretty Darn Accurate and Detects: 85% of Down Syndrome (Trisomy 21) Cases.¹ 90% of Edwards Syndrome (Trisomy 18) Cases.²	The NIPS Test Can Be Done as Early as 10 Weeks of Pregnancy. It Can Tell You the Sex of Your Baby. It’s Super Accurate and Detects: 99% of Down Syndrome (Trisomy 21) Cases. ~91% of Edwards Syndrome (Trisomy 18) Cases. ~91% of Patau Syndrome (Trisomy 13) Cases.³	In Addition to Screening for Down Syndrome and Edwards Syndrome, The Second Trimester Screen Looks For: Neural Tube Defects. Abdominal Wall Defects. It’s Fairly Accurate and Detects: 80% of Down Syndrome (Trisomy 21) Cases. 80% of Edwards Syndrome (Trisomy 18) Cases. 85% of Neural Tube Defects.⁴
Cons	The First Trimester Test Doesn’t Screen For: Neural Tube Defects. Abdominal Wall Defects.	The NIPS Test Doesn’t Screen For: Neural Tube Defects. Abdominal Wall Defects. It’s More Expensive Than the First Trimester Screen & the Second Trimester Screen. It Isn’t Offered by Every Doctor.	The Second Trimester Screen Can’t Be Done Until the Second Trimester. It Has a Higher Rate of False Positives (in Which the Test is Wrongly Positive) Than the Other Two Tests.

The Bottom Line: Which prenatal screening test you end up doing (if any) is up to you and your doctor. Many couples choose to do the first trimester screen or the NIPS test (over the second trimester screen) because these tests are done earlier in the pregnancy. This gives couples more time to discuss their options. When comparing the first trimester screen and the NIPS test, the NIPS test appears to be more accurate, but it’s also more expensive and isn’t offered everywhere.

The Prenatal DIAGNOSTIC Tests

If your screening test is positive or you’re at a higher-than-average risk of having a baby with a chromosomal abnormality (because of your age or your family history) your doctor will recommend a diagnostic test. The two main prenatal diagnostic tests are:

1. Chorionic Villus Sampling (CVS)

AND

2. The “Genetic” Amniocentesis

Double Take: There’s another type of amniocentesis that’s used during pregnancy called the “maturity” amniocentesis. Instead of looking at the baby’s chromosomes, however, the maturity amniocentesis assesses how mature the fetus’s lungs are towards the end of pregnancy (in case the baby needs to be delivered early).

Let’s Compare and Contrast The Two Prenatal Diagnostic Tests in the Chart, Below:

	Chorionic Villus Sampling (CVS)	The “Genetic” Amniocentesis
When Does the Test Happen?	The First Trimester (between 10-13 weeks of pregnancy). Results are available within 7-10 days.	The Second Trimester (between 15-20 weeks of pregnancy). The results come back in about 10 days.
What Happens During the Test?	A needle is inserted through Mom-to-be’s abdomen (or a catheter is inserted through her cervix) and a sample of cells from the placenta is extracted. Reality Check: Although the procedure sounds painful, it’s not that bad.	A needle is inserted through Mom-to-be’s abdomen under ultrasound guidance to collect a small amount of amniotic fluid from the amniotic sac (the sac the fetus hangs out in). The fluid is then sent to a genetics lab to be analyzed.
What Will the Test Tell Me?	The CVS test reveals your baby’s “karyotype” (i.e. their chromosomal make-up). The karyotype will tell you if your baby has extra or missing chromosomes (or pieces of chromosomes).	Like the CVS test, the “Genetic” Amniocentesis will show you your baby’s karyotype.
Pros	The CVS test is super accurate and identifies 99% of all chromosomal abnormalities. It’s the earliest diagnostic test that can be done.	Like the CVS procedure, the “Genetic” Amniocentesis is very accurate and identifies 99% of all chromosomal abnormalities. It also tests for neural tube defects. The amniocentesis procedure has a lower miscarriage rate than the CVS test (but not by much).
Cons	There’s a slightly higher risk of miscarriage with the CVS procedure (vs. with the amniocentesis). The CVS test does NOT assess for neural tube defects. If the test is done before 9 weeks (which it won’t be), there’s a risk of limb deformities. That’s why the procedure is done at 10 weeks or later.	The “Genetic” Amniocentesis is performed later in the pregnancy than the CVS test.

Summary: Both the CVS test and the “Genetic” Amniocentesis are reliable prenatal diagnostic tests. The CVS test can be done earlier in the pregnancy which tends to make it a more desirable option for expectant couples. It does have a slightly higher rate of miscarriage, though, so this is a factor when weighing the two options.

The Bottom Line

Use the info above to help you make an informed decision about what type of prenatal testing you want to do (if any).

View Sources

Footnotes
1. GenPath Diagnostics. Accessed April 2021. “Combined First Screen.” https://www.genpathdiagnostics.com/hcp/womens-health/prenatalscreening/maternal-serum-screening/combined-first-screen/
2. GenPath Diagnostics. Accessed April 2021. “Combined First Screen.” https://www.genpathdiagnostics.com/hcp/womens-health/prenatalscreening/maternal-serum-screening/combined-first-screen/
3. Bloomlife. Accessed April 2021. “Who Should Have Access to Non-invasive Prenatal Testing? Everyone.” https://bloomlife.com/preg-u/non-invasive_prenatal_testing/
4. What to Expect. October 2020. “The Quad Screen?” https://www.whattoexpect.com/pregnancy/pregnancy-health/prenatal-testing-quad-screen/ [Accessed April 2021]

Content
1. Kiefer, Amy PhD, and Molly Dickens, PhD. 2017. “EBook: The Complete Guide to Prenatal Testing.” https://expectingscience.com/wp-content/uploads/2017/10/bloomlife_prenatal_testing_ebook.pdf [Accessed February 2021]
2. ACOG. October 2020. “Prenatal Genetic Screening Tests.” https://www.acog.org/womens-health/faqs/prenatal-genetic-screening-tests?utm_source=redirect&utm_medium=web&utm_campaign=int [Accessed February 2021]
3. Prenatal Information Research Consortium. April 2016. “ACOG Issues New Prenatal Testing Guidelines.” https://prenatalinformation.org/2016/04/29/acog-issues-new-prenataltesting-guidelines/ [Accessed February 2021]

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Dr. Blair Farr is a board-certified doctor and Mom of two. Her goal is to use her personal & professional experiences to help others find more support, peace, and joy on their own pregnancy & parenting journeys.