Genetic testing during pregnancy can reveal whether your baby has chromosomal conditions like Down syndrome, inherited disorders like cystic fibrosis or sickle cell disease, and structural problems like neural tube defects. The specific results you get depend on which type of test you choose, and there are several, each with different timing, accuracy, and scope.
One crucial distinction runs through all of prenatal genetic testing: screening tests tell you the probability of a condition, while diagnostic tests give you a definitive yes or no. Understanding this difference shapes how you interpret every result you receive.
Screening Tests vs. Diagnostic Tests
A screening test tells you whether your baby has a higher or lower risk of a specific condition. It cannot confirm that condition exists. A diagnostic test is more invasive but provides a clear answer. Both are offered to all pregnant patients regardless of age or risk factors, and you can accept or decline any of them after discussing your options with your provider.
This matters in practical terms because a “positive” screening result does not mean your baby has a condition. It means the risk is elevated enough that a diagnostic test is worth considering. Many people who get a positive screening result go on to have a normal diagnostic result.
Carrier Screening: Your Own Genetic Profile
Carrier screening looks at your DNA (and often your partner’s) rather than the baby’s. The goal is to find out whether you carry a gene variant for a condition that could be passed to your child. You can be a carrier without having any symptoms yourself. If both parents carry a variant for the same recessive condition, each pregnancy has a 25% chance of producing a child with that disorder.
The most commonly screened conditions include cystic fibrosis, spinal muscular atrophy, and sickle cell disease, though expanded panels now test for dozens or even hundreds of conditions at once. Cystic fibrosis affects about 1 in 2,500 non-Hispanic white individuals and is less common in other ethnic groups. Spinal muscular atrophy carrier testing detects more than 90% of carriers in most populations, though the detection rate drops to about 71% in African Americans due to a genetic variation that makes the standard test less sensitive.
Carrier screening can be done before or during pregnancy with a simple blood draw. It’s a one-time test, since your carrier status doesn’t change between pregnancies. If both partners turn out to be carriers for the same condition, diagnostic testing on the baby (through CVS or amniocentesis) can determine whether the baby inherited two copies of the gene variant.
NIPT: Screening for Chromosomal Conditions
Noninvasive prenatal testing, or NIPT, is a blood test you can take as early as 10 weeks into pregnancy. It analyzes fragments of your baby’s DNA that circulate in your bloodstream and screens for the most common chromosomal conditions: Down syndrome (trisomy 21, caused by an extra copy of chromosome 21), trisomy 18, and trisomy 13. It also checks for extra or missing copies of the X and Y chromosomes, which means it can identify conditions like Turner syndrome and reveal fetal sex.
For Down syndrome, NIPT catches about 95% of affected pregnancies and has a very high specificity of 99.95%. But the positive predictive value, which is the chance that a positive result is actually correct, is lower than most people expect. In one large study, only about 68% of positive NIPT results for Down syndrome were confirmed as true positives. That means roughly 1 in 3 people who get a positive NIPT result for Down syndrome are carrying a baby that does not have the condition. This is why NIPT is classified as a screening test, not a diagnostic one.
Some labs offer expanded NIPT panels that screen for microdeletion syndromes, which are conditions caused by small missing segments of chromosomes. The most well-known is 22q11.2 deletion syndrome (also called DiGeorge syndrome). The accuracy of these expanded screens is considerably lower. In the general pregnant population, the positive predictive value for microdeletion results ranges from roughly 9% to 20%, meaning the majority of positive results turn out to be false alarms. For this reason, any positive NIPT result, whether standard or expanded, should be confirmed with a diagnostic test before making any decisions.
Blood Tests for Neural Tube Defects
A separate blood test, typically drawn between 15 and 20 weeks of pregnancy, measures a protein called alpha-fetoprotein (AFP) in your blood. Higher-than-expected levels can signal an open neural tube defect, such as spina bifida or anencephaly, which together account for about 94% of neural tube defects. The first AFP blood test can miss up to 20% of affected pregnancies, so an abnormal result is usually followed by a detailed ultrasound and potentially amniocentesis, where AFP measured directly in the amniotic fluid correctly identifies more than 98% of open neural tube defects.
CVS: Early Diagnostic Answers
Chorionic villus sampling is a diagnostic test performed between weeks 10 and 13 of pregnancy. A small sample of tissue from the placenta is collected and analyzed for the baby’s chromosomes and, when needed, for specific gene mutations. Because it provides a full chromosomal picture (called a karyotype), CVS can definitively diagnose Down syndrome, trisomy 18, trisomy 13, and sex chromosome abnormalities. It can also diagnose single-gene conditions like cystic fibrosis, muscular dystrophy, and hemoglobin disorders through direct DNA analysis.
The main advantage of CVS is timing. Results are available in the first trimester, giving families more time to plan. The miscarriage risk associated with CVS is less than 1%. One limitation: CVS cannot test for neural tube defects, because that requires measuring AFP in the amniotic fluid, which isn’t collected during this procedure.
Amniocentesis: The Most Comprehensive Test
Amniocentesis is usually performed between 15 and 18 weeks of pregnancy, though it can sometimes be done earlier. A thin needle withdraws a small amount of amniotic fluid, which contains fetal cells that can be analyzed for chromosomal abnormalities, genetic mutations, and neural tube defects. This makes amniocentesis the most comprehensive single diagnostic test available during pregnancy.
Like CVS, amniocentesis produces a karyotype that definitively identifies chromosomal conditions. It can also diagnose inherited disorders like cystic fibrosis and hemophilia through DNA analysis. The added benefit over CVS is the ability to measure AFP levels in the fluid itself, making it especially useful for families with a history of neural tube defects. The miscarriage risk is comparable to CVS, generally less than 1%.
What Testing Cannot Tell You
Genetic testing during pregnancy is powerful, but it has clear limits. Screening tests can only flag increased risk for conditions they’re designed to detect. They don’t scan the entire genome for every possible problem. Even diagnostic tests like amniocentesis and CVS analyze specific chromosomes or genes and won’t catch every genetic variation that could affect your child’s health.
No test can predict how severe a condition will be in a specific child. Two babies with the same chromosomal finding can have very different outcomes. For conditions like Down syndrome or cystic fibrosis, there’s a wide spectrum of how individuals are affected, and prenatal testing alone can’t tell you where on that spectrum your child might fall.
Testing also won’t detect conditions caused by new (de novo) mutations that arise spontaneously unless those mutations happen to show up on the specific tests being run. And many common health conditions, from autism to heart disease, involve complex interactions between many genes and environmental factors that current prenatal tests don’t address.