What to Know About Prenatal Testing

Angela Cabotaje Fact Checked
© MaaHoo Studio / Stocksy United

When you’re pregnant, you’re suddenly faced with a whole lot of unknowns. Will your baby be healthy? What will your birth be like? When will your morning sickness finally go away?

While there’s no way to predict how the next three trimesters are going to go, there are steps you can take to be prepared for whatever happens next. One of those steps is a variety of prenatal tests to monitor the growth of your baby, check for birth defects and detect chromosomal abnormalities or genetic disorders.

“It’s our responsibility to educate,” explains Dr. Edith Cheng, who is the service chief of obstetrics and the medical director of the Maternal and Infant Care Clinic at University of Washington Medical Center – Montlake and the Prenatal Genetics and Fetal Therapy Program. “Every woman or every couple should be informed, and we make it a point to discuss the screening options available to them during pregnancy.”

Here are the basics of prenatal testing, what it involves and what it can mean for you and your baby. After all, knowledge is power.

What is prenatal testing?

There are two main kinds of prenatal testing: screening tests and diagnostic tests.

Screening tests are not invasive and involve an ultrasound or blood draw. These tests can tell you the chances of your baby having certain chromosomal disorders or birth defects.

Diagnostic tests, on the other hand, can involve more invasive procedures like taking a sample of your placenta or amniotic fluid. These types of tests are usually only required if you receive a positive result on a screening test.

While no test is 100% accurate, diagnostic tests are more precise than screening tests and can indicate whether or not your baby has a chromosomal issue, birth defect or inherited disease.

What are the types of prenatal screening tests?

You know that first appointment with your OB-GYN or midwife where you get an ultrasound and are finally able to see your growing baby for the first time? While it’s certainly a special moment, it’s not purely for sentimental value alone.

“This is the basic screening in the first trimester, usually between 10 and 13 weeks,” Cheng explains. “It confirms the viability of the pregnancy, the dates and also helps us identify if there are twins. It also has the capability to screen for some major birth defects.”

During this screening — called the nuchal translucency (NT) ultrasound — the ultrasound technician will measure the thickness at the back of your baby’s neck. Using this measurement and other factors like your age and weight, they’ll calculate the mathematical risk of your baby having certain chromosome problems.

The next screening test, called a quadruple or “quad” screen, usually occurs in the second trimester between 15 and 22 weeks. This involves taking a blood sample and measuring it for four pregnancy hormones. Combining information from the blood analysis and ultrasound, your maternity experts can then calculate your baby’s chances of having conditions like Down syndrome (trisomy 21), Edwards syndrome (trisomy 18) or spina bifida, a birth defect when the spine and spinal cord don’t form properly.

What is a cell-free DNA test?

Aside from the traditional prenatal screening tests, you may also be offered something called a cell-free DNA test, which involves a blood draw as early as 10 weeks into your pregnancy. This test, introduced in the last five years or so, analyzes DNA from your placenta that has entered your bloodstream.

Not only does a cell-free DNA test tell you if your baby has certain chromosomal issues like Down syndrome, Edwards syndrome or Patau syndrome (trisomy 13), it also tells you the gender of your little one. While this test is highly accurate, its drawback is that it can’t detect structural birth defects.

Your insurance company may not consider a cell-free DNA test as part of standard maternity care. If that’s the case, Cheng says the traditional NT screening and quad screen is a well-accepted method for screening.

“If you don’t get cell-free DNA testing, it might feel like you’re having to settle for an inferior test, but that’s not the case,” she explains. “We’ve been doing quad screenings for 30 years, and it’s still a good test. You’re not missing anything — you’re just doing things a little differently.”

What are the types of prenatal diagnostic tests?

While prenatal screening tests are helpful in identifying the possibility that your baby has a suspected abnormality, they can’t tell you with 100% accuracy. That’s why maternity experts use diagnostic tests to confirm or refute the suspected condition that the screening test was positive for.

One diagnostic test that all expecting moms receive — regardless of your screening results — is an anatomy ultrasound at 18 to 22 weeks. This ultrasound checks for heart defects, cleft lip or palate, and other structural birth defects. It also looks at your placenta, uterus and your amniotic fluid levels. If you’re eager to learn the gender of your little one, this is when the ultrasound technician can fill you in.

Other diagnostic tests are only recommended if you have a positive result on a prenatal screening, meaning your odds for a chromosomal disorder are higher than normal.

“Once there’s a positive screening test result, we have a discussion with genetic counselors, we may do more ultrasounds and we talk about the option of invasive testing to confirm,” Cheng explains.

If you’re between 11 and 14 weeks pregnant, your OB-GYN or maternal fetal medicine specialist might suggest a chorionic villus sampling (CVS) to take a small sample of your placenta. If you’re already further along in your pregnancy, between 16 and 22 weeks, your doctor might instead recommend an amniocentesis (amnio) to take a sample of amniotic fluid.

Both diagnostic tests can be used to diagnose chromosome problems and inherited conditions like muscular dystrophy or hemophilia. While these more invasive tests can be scary, they’re highly accurate and have a low risk of miscarriage: 1 to 2% for a CVS and 0.25% for an amnio.

What are the treatments for babies with birth defects or chromosome problems?

After diagnostic testing, your doctor may confirm that your baby has an issue. This is difficult news for anyone to learn, but there are resources available to support you and your family.

If your baby has a birth defect, maternal fetal medicine specialists and pediatric specialists will work with you to discuss all options and coordinate medical care for you and your baby. Ask your doctor how you can learn more during your pregnancy so you can make informed decisions about surgery or other care options after birth.

While there are no treatments for babies with chromosome problems, your doctor will be able to direct you to organizations and support groups that can help. They can provide you with details about how to care for your child and also suggest ways you can emotionally and mentally prepare.

If you’ve had a previous child with a birth defect or chromosomal abnormality — or have a family history of an inherited disease — Cheng also encourages you to consider preconception counseling before you try having another baby.

“We invite everyone to come in for preconception counseling, even if you’re healthy, if you want to be more informed,” she says. “Our genetic counselors help you assess things ahead of time so you can know if you’re at an increased risk and to help you prepare.”