Prenatal Screening Tests: NIPT, Ultrasound, and What Results Mean

You are pregnant, and your clinician asks whether you want prenatal screening. You hear terms like NIPT, nuchal translucency, and amniocentesis. Or perhaps a result already came back as "high risk," and you are trying to understand what that means.

This guide explains each test, what it can and cannot tell you, and how to read results.

Screening tests and diagnostic tests are different

This is the most important idea.

• Screening tests estimate the chance that a baby has a condition. They use blood tests, ultrasound, or both. They carry no risk to the pregnancy.
• Diagnostic tests check the baby's chromosomes directly. They can confirm or rule out a condition. They need a sample from the placenta or the fluid around the baby.[1][5]

Screening is a choice. Some people want as much information as possible. Others choose not to test. Both are valid, and your clinician should support your decision.

What your doctor looks at

Who should be offered screening

ACOG in the US recommends offering screening and diagnostic testing to all pregnant people, regardless of age or risk factors.[1] The joint guideline from the Society of Obstetricians and Gynaecologists of Canada (SOGC) and the Canadian College of Medical Geneticists gives the same advice.[2]

The main screening options

• Cell-free DNA screening (NIPT, noninvasive prenatal testing). A blood test from the pregnant person, done from about 10 weeks. It looks at small pieces of DNA from the placenta in the blood.[4]
• First trimester combined screening. An ultrasound to measure the nuchal translucency (fluid at the back of the baby's neck) plus blood tests, usually between about 10 and 14 weeks.[5]
• Second trimester serum screening, often called the quad screen, from about 15 to 22 weeks.
• Integrated or sequential screens, which combine first and second trimester results. Which ones are offered depends on your province, state, or clinic.

How accurate is NIPT?

NIPT is the most sensitive and specific screening test for the common chromosome conditions: trisomy 21 (Down syndrome), trisomy 18, and trisomy 13.[1]

A large study of about 16,000 pregnancies in the general population compared NIPT with standard first trimester screening for Down syndrome:[3]

• NIPT found all 38 pregnancies with Down syndrome in the main analysis.
• Its false-positive rate was about 0.06%, compared with about 5.4% for standard screening.
• When NIPT was positive, about 81% of those pregnancies truly had Down syndrome. With standard screening, about 3.4% did.

This last number is the key. Even with NIPT, some positive results are false. The chance that a positive result is true depends on the condition and on your age. For rarer conditions, such as some sex chromosome differences and small deletions, more positive results are false.

What a high-risk result means

A high-risk or positive screening result is not a diagnosis. Both ACOG and the SOGC say that decisions about the pregnancy should not be made from a screening result alone. A diagnostic test should be offered first.[1][2]

Sometimes NIPT gives no result, often because there is not enough placental DNA. This can happen more often with higher body weight or early testing. A clinician should discuss next steps, because a no-result can sometimes be linked to a chromosome condition.[1]

Diagnostic tests

• CVS (chorionic villus sampling): a small sample from the placenta, usually at about 10 to 13 weeks.
• Amniocentesis: a sample of amniotic fluid, usually from 15 weeks on.

Both carry a very small chance of pregnancy loss, slightly higher with CVS than with amniocentesis.[6] They give a much more certain answer.

The anatomy ultrasound

Most people are offered a detailed ultrasound at about 18 to 22 weeks. It checks the baby's brain, heart, spine, kidneys, limbs, and growth, and the position of the placenta.[2] It can find physical differences that blood tests do not screen for, such as some heart defects and spina bifida.

Canada and the US: same advice, different funding

The clinical advice in both countries is similar: offer screening to everyone, and confirm positive screens with diagnostic testing. The difference is cost. In Canada, each province decides which screens are publicly funded. Some fund NIPT for everyone, and others fund it only after a higher-risk first screen or for some groups. In the US, coverage depends on your insurance plan.

Carrier screening is different

Carrier screening checks the parents, not the baby, for genes that could be passed on, such as those for cystic fibrosis or spinal muscular atrophy. It is ideally done before pregnancy or early in pregnancy. If both parents carry the same condition, diagnostic testing of the pregnancy may be offered. Ask your clinician which carrier tests are offered where you live.

Questions to ask your clinician

• Which screening tests are available to me, and which are covered?
• What conditions does each test check for?
• What is the chance that my positive result is a true positive?
• Should I see a genetic counselor before deciding on next steps?
• What are the risks and timing of CVS or amniocentesis for me?
• What did my anatomy ultrasound show, and do I need follow-up scans?

When to get care quickly

Go to the emergency department or call your care team right away if you have heavy vaginal bleeding, severe belly pain, fever, or fluid leaking from the vagina, including after a CVS or amniocentesis.

Bottom line

Prenatal screening gives you information about chance, not certainty. NIPT is the most accurate screening test for Down syndrome, but a positive result still needs a diagnostic test before any decision. Canadian and US guidelines agree on offering screening to everyone and respecting your choice.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.