Prenatal testing is a broad term for the blood tests, ultrasounds, and procedures offered during pregnancy to check on a baby’s health and development. Some tests screen for chromosomal conditions like Down syndrome, others look for structural issues like heart defects or spinal abnormalities, and a smaller number can deliver a definitive diagnosis. Every pregnant person should be offered both screening and diagnostic options, and every person has the right to accept or decline any of them.
Understanding the difference between a screening test and a diagnostic test is the single most important thing to know before making decisions. A screening test tells you whether your baby has a higher or lower risk of a particular condition. It cannot tell you yes or no. A diagnostic test is more invasive but gives you a definitive answer. Most people start with screening and only move to diagnostic testing if the results raise a concern.
First Trimester Screening
The earliest round of screening typically happens between weeks 10 and 13. Two options dominate this window: cell-free DNA screening (sometimes called NIPT) and the nuchal translucency ultrasound. They can be done separately or together.
Cell-free DNA screening is a simple blood draw from your arm. It analyzes tiny fragments of placental DNA circulating in your bloodstream and checks for extra copies of specific chromosomes. For Down syndrome (trisomy 21), it detects 99% of cases with a false positive rate below 1%. For Edwards syndrome (trisomy 18), detection drops slightly to 97%, and for Patau syndrome (trisomy 13), it’s about 92%. These are impressive numbers for a screening test, but they are still screening numbers. A positive result means the probability is high, not that the condition is confirmed.
The nuchal translucency scan measures a pocket of fluid at the back of the baby’s neck using ultrasound. A measurement of 3.0 mm or above, or above the 98th percentile for the baby’s size, is considered abnormal and prompts closer follow-up. Measurements at or above 3.5 mm are associated with significantly higher rates of detectable abnormalities and often lead to an earlier, more detailed anatomy evaluation in the second trimester.
Second Trimester Screening
Between roughly 15 and 22 weeks, a second round of screening options becomes available. The quad screen is a blood test that measures four substances: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), a form of estrogen called estriol, and inhibin-A. Abnormal levels of any of these, either too high or too low, can signal an increased risk for Down syndrome, Edwards syndrome, or neural tube defects like spina bifida. Higher-than-expected inhibin-A, for example, is specifically linked to a greater risk of Down syndrome.
The quad screen is less accurate than cell-free DNA for chromosomal conditions, so many providers now use it primarily when first trimester screening wasn’t done or when looking specifically for neural tube defects, which cell-free DNA doesn’t cover.
The Anatomy Scan
The detailed anatomy ultrasound, usually scheduled between 18 and 22 weeks, is probably the appointment most parents look forward to and feel nervous about in equal measure. It’s not a quick peek. The sonographer systematically evaluates nearly every visible structure in the baby’s body.
The checklist includes the skull’s shape and integrity, the brain’s ventricles and cerebellum, the face and upper lip (checking for cleft lip), the spine in both directions, and the heart’s chambers and outflow tracts. The lungs, diaphragm, stomach, liver, kidneys, bladder, and abdominal wall are all examined. Each limb is measured and assessed individually, down to the hands and feet, checking bone appearance and joint movement. The scan also measures the baby’s head circumference, abdominal circumference, and femur length to track growth.
This ultrasound catches a wide range of structural problems that blood tests cannot detect, from heart defects to kidney abnormalities to limb differences. It’s considered a screening tool rather than a diagnostic one because some conditions are difficult to visualize and can be missed, but it remains one of the most comprehensive single evaluations in all of prenatal care.
Diagnostic Tests: Amniocentesis and CVS
When screening raises a red flag, diagnostic testing can give a clear yes or no. The two main options are chorionic villus sampling (CVS) and amniocentesis.
CVS is performed earlier, typically at 10 to 12 weeks. A small sample of tissue from the placenta is collected, either through the cervix or through the abdomen with a needle. Because the placenta shares the baby’s genetic makeup, the sample can be analyzed for chromosomal conditions with high accuracy. The procedure-related miscarriage risk for CVS is approximately 0.5% to 1.0%, or roughly 1 in 200 to 1 in 100. There is also a very small risk of limb differences, estimated at 1 in 3,000 to 1 in 1,000. One practical trade-off: CVS results occasionally come back ambiguous due to issues like cells from the mother mixing into the sample, and retesting or follow-up amniocentesis may be needed.
Amniocentesis is performed later, usually between 14 and 20 weeks. A thin needle guided by ultrasound withdraws a small amount of amniotic fluid, which contains fetal cells. The procedure-related miscarriage risk has dropped considerably from older estimates. Contemporary data puts the additional risk attributable to the procedure at about 0.06%, far lower than the traditionally quoted 0.5%. The absolute miscarriage rate between 16 and 24 weeks for women who undergo amniocentesis is about 1%, but most of that reflects the baseline miscarriage rate at that stage of pregnancy rather than the procedure itself.
Both tests analyze fetal chromosomes directly, which is why they are definitive in a way that no blood test or ultrasound can be.
Routine Blood Work and Other Tests
Prenatal testing isn’t limited to genetics and anatomy. At your first prenatal visit, a standard panel checks your blood type, Rh factor, antibody status, and complete blood count. You’ll also be tested for hepatitis B and C, HIV, syphilis, rubella immunity, and infections like gonorrhea and chlamydia. Depending on your background and risk factors, your provider may add tests for conditions like sickle cell trait or other inherited blood disorders.
Later in pregnancy, additional routine tests follow a predictable schedule. Between 24 and 28 weeks, a glucose tolerance test screens for gestational diabetes. Between 28 and 32 weeks, sexually transmitted infection testing is repeated. At 36 to 37 weeks, a swab screens for group B strep, a common bacterium that can pose risks to the baby during delivery. These tests aren’t optional extras. They catch conditions that, left undetected, could lead to serious complications.
Timing at a Glance
- 10 to 13 weeks: Cell-free DNA screening, nuchal translucency ultrasound, CVS (if diagnostic testing is chosen)
- 14 to 20 weeks: Amniocentesis (if diagnostic testing is chosen), quad screen
- 18 to 22 weeks: Detailed anatomy ultrasound
- 24 to 28 weeks: Glucose tolerance test, repeat blood count
- 28 to 32 weeks: Repeat STI screening
- 36 to 37 weeks: Group B strep screen
Making Decisions About Testing
No single path through prenatal testing is right for everyone. Some people want every available test as early as possible. Others prefer only the anatomy scan and routine blood work. Your choices may depend on your age, family history, personal values, and how you would use the information. A positive screening result doesn’t obligate you to pursue diagnostic testing, and a diagnostic result doesn’t obligate you to take any particular action.
What matters most is that you understand what each test can and cannot tell you before you agree to it. A screening test with a “positive” result doesn’t mean your baby has a condition. It means the probability is elevated enough to warrant a closer look. Many people receive a concerning screening result, go on to diagnostic testing, and learn that everything is fine. Knowing this ahead of time can save a lot of unnecessary anxiety between appointments.