With so many options for prenatal screening and diagnostic testing, choosing your right fit can seem overwhelming. As healthcare providers for women in pregnancy, we hope to help you better understand what options are available, and do so in a more manageable way.
The first step is to know yourself. What type of information do you want to know about your pregnancy? How much and when do you want it? How comfortable are you with the chance of a false positive or false negative? What might you do with the information you get?
Thinking about these questions will help you and your partner decide which options are best for you.
What Are Screening Tests?
Screening tests come in many forms, including ultrasounds, blood work and even reviewing your personal and family medical history.
The earliest screening test—Pan-Ethnic Carrier Screening—can be performed even before conceiving. This testing is designed to tell potential parents if they’re carriers of autosomal recessive conditions prevalent in the general population. A carrier is an individual who’s not affected but has the potential to pass on a disease if they have children with someone who’s a carrier of the same condition. Knowing this information can help determine if your children are at risk for specific conditions that can be tested for during or after pregnancy, such as cystic fibrosis or sickle cell disease.
Some prenatal testing can be performed as early as 10–14 weeks in the pregnancy, such as First Trimester Screening and Non-Invasive Prenatal Screening (NIPS). And it’s possible to perform pre-implantation genetic diagnosis for those using IVF (in vitro fertilization) to become pregnant.
- First Trimester Screening combines an ultrasound to measure the nuchal translucency (space behind the back of a baby’s neck) and blood work to give a risk calculation for conditions like Down syndrome (Trisomy 21) and Trisomy 18 (Edwards syndrome). It has an 80–90% detection rate.
- NIPS is a blood test that looks at the baby’s DNA that’s circulating through the mother’s blood and typically tests for Down syndrome, Trisomy 18 and Trisomy 13 (Patau syndrome). It can also tell the baby’s sex. NIPS has a 99% detection rate for Down syndrome, but lower for Trisomy 18 and 13. Unlike first trimester screening, NIPS is recommended only for women who are over 35, or who are carrying a baby with a known ultrasound abnormality, have had a positive first trimester screening test, or had a prior pregnancy with a chromosomal abnormality.
If you don’t want to have first trimester screening or don’t know you’re pregnant until after 14 weeks, you can opt for Second Trimester Screening. This test is done at 15–22 weeks and is a blood test that looks for blood markers that can provide risks for Down syndrome, Trisomy 18 and Open Neural Tube defects like spina bifida. It has an 80% detection rate for Down syndrome.
Another option is to combine aspects of first and second trimester screening—called Sequential or Integrated Screening. This involves blood tests in the first and second trimester to give a higher detection rate (around 90–95%).
Screening tests report results as “low” or “high” risk and are not definitive, but can be very reassuring to families with no other risk factors. If you receive a “high risk” result, your doctor may talk with you about considering diagnostic testing to confirm or negate your results.
It’s important to know that these tests don’t screen for every condition, so additional testing may be recommended depending on your specific situation and history. For example, mothers with certain medical conditions—high blood pressure, diabetes, lupus, family histories—or prior history of miscarriage, premature birth or a child with birth defects may be counseled about additional testing or screenings.
What Are Diagnostic Tests?
If you’re the type of person who needs definitive information or is considered high risk because of a fetal or maternal issue, diagnostic testing may be an appropriate option. There are two main types of prenatal diagnostic testing—chorionic villus sampling (CVS) and amniocentesis.
- CVS is typically done between 10–13 weeks. This procedure involves placing a small needle through the abdomen or a catheter transvaginally through the cervix to collect a sample of the placenta. This test is 98% accurate in looking at the baby’s DNA for chromosomal changes—like Down syndrome—and specific genetic conditions that may run in a family or are suspected by ultrasound findings. CVS testing has a 1 in 400 risk for complications—such as infection or miscarriage—and is the earliest diagnostic testing that can be done in a pregnancy.
- Amniocentesis can be performed after 15 weeks. This procedure involves placing a thin needle through the abdomen and collecting a tablespoon of amniotic fluid from around the baby. It’s over 99% accurate in detecting genetic conditions and can also tell about infections, open neural tube defects and some metabolic functions of the baby. Amniocentesis has a 1 in 500 risk for complication.
These risk numbers for complications vary by medical practice, with more experienced practices and facilities often reporting much lower risks.
What About Ultrasound?
Ultrasound can be both a screening and diagnostic test. In the first trimester, ultrasound is used to confirm the due date and number of babies. The nuchal translucency measurement is also used to determine pregnancies at high risk for chromosome differences or structural malformations.
In the second trimester, when the baby is more developed, ultrasound can detect major structural birth defects like congenital heart disease, spina bifida or problems with the stomach or bowel. Babies are also evaluated for smaller differences on ultrasound, called markers, that can be seen in some genetic conditions like Down syndrome.
If a difference is seen on ultrasound, you may be referred for genetic counseling or to see a high-risk obstetric practice to discuss further testing options.
What’s Right For You?
See what we mean? With this smorgasbord of prenatal screening and testing options you may feel that it’s impossible to pick the right one. We suggest working with your doctor and a genetic counselor to explore what’s best for you.
Genetic counselors are healthcare professionals who explain screening and testing options, help families make decisions around testing, and interpret results. You can find one close to you by searching online for the National Society of Genetic Counselors, NSGC.org.
Screening for Zika Virus?
If there’s a risk for the Zika virus, both prenatal screening and diagnostic tests are available. Women at risk for Zika virus are those who’ve traveled to or live in areas with outbreaks. Screening tests include looking for antibodies in the mother’s blood indicating a recent or past infection or evaluating urine and blood to see if any Zika virus is identified.
If a screening test is positive, diagnostic testing like amniocentesis can see if the baby is infected. Often, babies who have a congenital infection, like Zika, may have differences—like smaller head size, differences in brain development, calcifications in the brain or abdomen—that can be seen on ultrasounds.
Julie S. Moldenhauer, MD (right), is the medical director of the Garbose Family Special Delivery Unit and a Maternal Fetal Medicine/Reproductive Genetics Specialist at the Center for Fetal Diagnosis and Treatment at The Children’s Hospital of Philadelphia.
Erica Schindewolf (left) is a licensed certified genetic counselor specializing in reproductive genetics at the Center for Fetal Diagnosis and Treatment at the Children’s Hospital of Philadelphia.