What Is First Trimester Screening?
Reviewed by Safi Allah Khan
Medically reviewed by: Safi Allah Khan · Written by the Momi team · Last updated 20 October 2026
Quick answer: First trimester screening is an optional test, done at about 10 to 14 weeks, that combines a blood test with an ultrasound measuring fluid at the back of the baby's neck (nuchal translucency). It estimates the chance of Down syndrome and some other chromosome conditions. It doesn't diagnose; a higher-chance result is followed by further tests if you choose.
Screening is offered to everyone, and whether you have it is up to you. Find your current week with How Many Weeks Pregnant Am I?, and see where screening sits in the pregnancy appointment schedule.
What first trimester screening tests for
ACOG explains that first trimester screening looks at the chance of aneuploidy: when a baby has a missing or extra chromosome. The main conditions screened for are:
- Down syndrome (trisomy 21)
- Edwards syndrome (trisomy 18)
- Patau syndrome (trisomy 13)
ACOG adds that an abnormal nuchal translucency measurement is also linked to some physical differences of the heart, abdominal wall and skeleton. In England, the NHS offers the combined test for Down's, Edwards' and Patau's syndromes, and you can choose which results you want.
The NT scan and the blood test
First trimester (or "combined") screening has two parts, usually done at the same visit.
| Part | What happens | What it measures |
|---|---|---|
| Nuchal translucency (NT) scan | An ultrasound, often at your dating scan | The thickness of a fluid-filled space at the back of the baby's neck |
| Blood test | A sample from your arm | The levels of two substances made in pregnancy |
The results are combined with your age and how many weeks pregnant you are to give a single chance figure. The NHS says you should usually get results within 2 weeks.
Timing
| Test | When | Source |
|---|---|---|
| First trimester screening (blood test + NT) | 10 to 13 weeks | ACOG |
| Combined test at the dating scan | About 10 to 14 weeks | NHS |
| Cell-free DNA (cfDNA, also called NIPT) | From 10 weeks; results in about a week | ACOG |
| Quad (quadruple) blood test, if first trimester screening wasn't possible | 15 to 22 weeks (ACOG) or 14 to 20 weeks (NHS) | ACOG, NHS |
| Anatomy (20-week) scan | 18 to 22 weeks (ACOG) or 18 to 21 weeks (NHS) | ACOG, NHS |
Timing matters because nuchal translucency can only be measured in a narrow window. The NHS notes you won't be offered the combined test if your dating scan happens after 14 weeks; you'll be offered the quadruple blood test instead, which is not quite as accurate.
What is cell-free DNA testing?
Cell-free DNA is a small amount of DNA from the placenta that's found in your blood. ACOG says a blood test from 10 weeks can screen it for Down, Patau and Edwards syndromes and for differences in the number of sex chromosomes. In the US it's often offered as an alternative to, or alongside, first trimester screening. In England, the NHS offers it (as NIPT) after a higher-chance combined test result. A positive cfDNA result should be followed by a diagnostic test.
Understanding your results
Screening results are given as a chance, not a yes or no.
- Lower chance: most people get this result. It doesn't completely rule out the condition.
- Higher chance: the chance is higher than in the general population. It doesn't mean your baby definitely has the condition.
ACOG notes that false-positive and false-negative results can happen with any screening test. If you get a higher-chance result, the NHS says you can choose a more accurate screening test (NIPT), a diagnostic test (CVS or amniocentesis), or no further tests. A doctor, midwife or genetic counselor will talk you through your options and support whatever you decide.
Screening vs diagnostic tests
| Screening tests | Diagnostic tests | |
|---|---|---|
| Examples | Combined test, NT scan, cfDNA, quad test | CVS, amniocentesis |
| What you get | A chance (lower or higher) | A definite answer for the conditions tested |
| How it's done | Blood test and ultrasound | Sample from the placenta (CVS) or amniotic fluid (amnio) |
| Timing | From 10 weeks | CVS 10 to 13 weeks; amnio usually 15 to 20 weeks |
| Risk to pregnancy | None from the test itself | A very small chance of miscarriage |
ACOG says the chance of miscarriage with CVS is slightly higher than with amniocentesis. Both screening and diagnostic testing are offered to everyone who is pregnant.
Deciding whether to have screening
There's no right or wrong answer. ACOG suggests thinking about how you'd use the results. Some parents want to know in advance so they can prepare and plan care. Others prefer not to know, or would not have follow-up tests whatever the result. You can talk it through with your provider or a genetic counselor before deciding. Carrier screening, which checks whether parents carry genes for inherited conditions such as cystic fibrosis or sickle cell disease, is a separate choice that can be made before or during pregnancy.
How Momi helps
Momi tracks your pregnancy week by week from your last period or conception date, so you'll know when you're entering the 10 to 14 week screening window. Add your scan and blood test to visits, with reminders, and store your ultrasound pictures in your bump photo journal. If your due date changes after the dating scan, update it in the app.
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Related: first trimester symptoms · can my due date change? · 12 weeks pregnant · pregnancy trimesters
Sources
- ACOG, Prenatal Genetic Screening Tests
- ACOG, Prenatal Genetic Diagnostic Tests
- NHS, Screening for Down's syndrome, Edwards' syndrome and Patau's syndrome
- NHS, 12-week scan
- NHS, Screening tests in pregnancy
Medical disclaimer: Momi and this website provide general information, not medical advice. Always talk to your doctor or midwife; in an emergency call your local emergency number.
FAQ
Is first trimester screening optional?
Yes. ACOG says screening and diagnostic tests are offered to all pregnant women, and it's your choice whether to have them. The NHS says the same: you can have all, some or none of the screening tests and you can change your mind, though some tests must be done at certain weeks.
When is first trimester screening done?
The blood test and nuchal translucency scan are usually done together between 10 and 13 weeks according to ACOG. In England, the combined test is done at the dating scan, which is between about 10 and 14 weeks. Cell-free DNA testing can be done from 10 weeks.
What does a higher-chance result mean?
It means your baby has a higher chance of the condition than the general population, not that your baby definitely has it. You can choose a more accurate screening test such as cell-free DNA (NIPT), a diagnostic test such as CVS or amniocentesis, or no further tests.
How accurate is first trimester screening?
Screening tests estimate chance, so false-positive and false-negative results can happen. ACOG notes that combining first and second trimester results is more accurate than a single result. Only a diagnostic test like CVS or amniocentesis can tell you for certain. Ask your provider for the detection rates of the test you're offered.
How much does first trimester screening cost?
It depends on where you live and your insurance. In England, all NHS screening tests in pregnancy are free. In the US, costs vary by insurer, plan and test, so check with your insurance and your provider's office before your appointment.
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