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Prenatal Testing Explained

Prenatal Testing Explained

When I was pregnant with Luke I felt very pressured to go through the full gamut of prenatal testing.  Most of the tests were no brainers – low or no risk tests that were clearly helpful.  But a few of them – specifically the screenings – weren’t as obvious and I felt like I wasn’t given enough information to make an informed decision.  So I started asking questions, talking to other women and researching the tests on my own.   In this post I’ve outlined the most common tests you can expect during pregnancy – when they are performed, how they’re done, what they test for and what risks are involved.  I hope this helps you know what to expect and to choose which tests are right for you…

Routine Testing: The tests in this section are considered standard.  Most women will receive them all, some multiple times.  They’re considered very safe and pose little to no risk to mother or baby.

  • Blood Test
    • When is it administered? Typically at your first prenatal visit.
    • How is it done?  A small vial of blood is drawn, usually from your arm.
    • What does it test for? Blood type, Rh factor, glucose, iron and hemoglobin levels, sexually transmitted disease, immunity to rubella (german measles) and toxoplasmosis.
    • Risks? This is a basic and accurate blood test, it poses no risk to mother or baby.
  • Urine Test
    • When is it administered? This is typically done at the first visit and periodically throughout the rest of pregnancy.  Many practitioners request a sample at every prenatal visit.
    • How is it done? You just pee in a little cup.
    • What does it test for?  It helps identify diabetes, urinary tract infections, kidney infections, dehydration and preeclampsia.
    • Risks? None
  • Glucose Testing
    • When is it administered? A preliminary test is done between weeks 16 and 28.  If you test positive, an additional test is administered shortly thereafter.
    • How is it done? The initial test involves drinking a sweet glucose drink and having your blood drawn an hour later.  The follow up test requires a 12-hour fast and multiple blood tests.  They take a baseline (fasting) level, then you drink the glucose and get tested again every hour for three hours.
    • What does it test for? It tests for gestational diabetes (identified by two or more high levels in the second test).
    • Risks? No known risks.
  • Group B Strep Test
    • When is it administered? Between weeks 35 and 37.
    • How is it done? A swab is taken of your vagina and rectum.
    • What does it test for?  It tests for a form of strep that some women carry without symptoms.  It’s typically harmless to adults but can be passed to newborns during labor and cause problems.  If you test positive, you can take antibiotics during labor to prevent your baby from being infected.
    • Risks? This is a standard and very safe test.
  • Ultrasound
    • When is it administered? Believe it or not ultrasounds are not considered medically necessary and just a generation ago most women didn’t receive them unless a problem was suspected.  Nowadays most women get at least a 20-week ultrasound and often several additional ones, even in normal pregnancies.
    • How is it done? An ultrasound uses high-frequency sound waves to scan your belly and create a picture of your baby.  It’s usually performed by moving a probe over your belly and amplifying the image onto a screen.
    • What does it test for? It can be used to confirm a viable pregnancy, estimate gestational age, monitor growth, identify multiples, identify gender, screen for characteristics of Down Syndrome, congenital malformations or structural abnormalities, identify placenta location and monitor amniotic fluid levels.
    • Risks? Ultrasound is generally accepted as safe.  There are no known consequences, however some women worry that the long term effects of frequent ultrasounds have not been studied.


Prenatal Screenings: In my experience these screenings were presented as safe and routine procedures that good mothers ought to do.  Mothers are pressured to have them done without question and as a result they often undergo the screenings without being properly informed of the risks.  Most of the time this goes off without a hitch – you get the screen, it comes back negative, you sleep easy and move on.  But approximately 5% of the time, moms who have perfectly healthy babies receive a false positive and are upset to realize they never really understood how a screening works.

Screenings assess your risk of a problem, they do NOT diagnose a problem.   The threshold for tests is typically around 1 in 250 (you’ll be given your specific risk based on your results) – ie. any test that comes back with a likelihood greater than 1 in 250 is deemed positive/abnormal.  This means that the vast majority of women who test positive actually have nothing wrong.  But they won’t know for sure unless they undergo more invasive testing such as amniocentesis.  This inflicts a ton of moms with unnecessary fear and stress. Being told your baby could have a major problem is terrifying and forces mothers to decide between living with that fear or undergoing potentially risky testing.

Please note – I’m not trying to persuade you against prenatal screening.  Screenings can bring immeasurable relief to expectant mothers and have helped many families prepare for the arrival of a special needs child.  This is a personal decision based on your own risk factors, family history and personal wishes.  What I hope to do here is help moms-to-be understand the limitations of the screening process so they can make an informed decision and be mentally prepared in the event that they receive a positive result.  Here are the most common screenings currently available:

  • First Trimester Screen
    • When is it administered? Between weeks 11 and 14.
    • How is it done? It involves a maternal blood test for pregnancy hormones hCG and PAPP-A and and an ultrasound that measures the fluid at the back of the baby’s neck.
    • What does it test for? This test screens your risk of having a child with chromosomal abnormalities such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18).
    • Risks? This test is the most accurate non-invasive screening method for chromosomal abnormalities and the screen itself is very safe.  The controversy lies in the possibility of receiving a false positive result.  Approximately 5% of women test positive for this screen.  Of these women, only about 1 in 200 (0.5%) actually have a problem, but you won’t know for sure without more invasive testing such as amniocentesis.
  • Triple Screen Test – Also called Triple Test, Multiple Marker Screen and AFP Plus
    • When is it administered? Between weeks 15 and 20.
    • How is it done? This is a maternal blood screening that determines AFP, hCG and Estriol levels.
    • What does it test for? It assesses your risk for chromosomal abnormalities, genetic disorders and neural tube defects.
    • Risks? The screen itself is very safe.  Again the problem is its high rate of false positive results.   This screen gives you a separate risk assessment for Down syndrome, Trisomy 18 and neural tube defects.  Any risk greater than 1 in 250 is considered a positive result so many women who have perfectly healthy babies are testing positive.  This test also has a significant false negative rate – it only detects approximately 70% of babies with Down syndrome and Trisomy 18 and about 85% of babies with neural tube defects.
  • Quad Screen – this is the same test as the Triple Screen (see above) with the addition of testing for Inhibin-A.   It has the same accuracy rates for trisomy 18 and neural tube defects and it’s slightly more accurate at detecting Downs (80% vs. 70%).


Advanced Testing: These tests are typically only performed when a problem is suspected (such as a positive screening result).  They’re highly accurate tests but they’re also invasive and pose slight risks of infection and miscarriage.

  • Amniocentesis
    • When is it administered? Usually between weeks 14 and 20.
    • How is it done? A thin needle is inserted through your abdomen into your uterus where it collects a sample of amniotic fluid.
    • What does it test for? It detects chromosomal abnormalities (ex. Downs), genetic disorders (ex. Cystic fibrosis) and neural tube defects (ex. Spina bifida).  It can also be used to provide DNA for a paternity test.  It has an accuracy rate of approximately 99%.
    • Risks? Amnio poses a 1/200 (0.5%) chance of miscarriage by increasing your risk of infection, inadvertently breaking your water or inducing premature labor.
  • Chorionic Villus Sampling (CVS)
    • When is it administered? Typically between weeks 10 and 13.
    • How is it done? The most common method guides a thin catheter through your cervix.  When it reaches the point where your placenta attaches to the uterus it suctions up some cells.
    • What does it test for? CVS can detect chromosomal abnormalities such as Down syndrome, genetic disorders such as cystic fibrosis and can provide DNA for paternity testing.  It has a 98-99% accuracy rate, although it does not measure the severity of the disorders it tests for.
    • Risks? CVS is an invasive procedure that poses a 1% risk of miscarriage.  It also increases you chance of infection and can cause cramping and pain.  It’s not recommended for women who have active STD’s, who are carrying twins or who have experienced bleeding during pregnancy.
  • Cordocentesis
    • When is it administered? After week 17.
    • How is it done? A thin needle is inserted through the abdomen and uterus and into the umbilical cord where it retrieves a small sample of fetal blood.
    • What does it test for?  It tests for chromosomal abnormalities and blood disorders.  It’s typically used when other tests (such as amnio) were not possible or came back with inconclusive results.
    • Risks? This test has a high level of accuracy, but it does not test for the severity of a condition.  It poses a 1-2% risk of miscarriage and can also cause blood loss at the puncture site, infection and a drop in fetal heart rate.
This summarizes the most common tests and screenings you may encounter during pregnancy.  If you’d like a more expansive guide on what to expect from pregnancy, here are reviews of my three favorite pregnancy books:  I found them very helpful and hope you will too!

To view additional additional family related posts, please see:

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