Prenatal Screening · 11-14 weeks
First-Trimester Combined Screening
The 12-week scan + bloods. NHS routine at 11-14 weeks: nuchal translucency + PAPP-A + free β-hCG combined with your age = risk estimate for Down syndrome, Edwards, Patau. ~85-90% detection rate. Plus what to do if PAPP-A is low or NT raised.
Last reviewed 2 June 2026
First-trimester combined screen (FTCS)
T21 posterior risk — 11-13+6 weeks
yr
MoM
MoM
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Enter maternal age and any markers (in MoM) to estimate T21 posterior risk.
Educational tool only — simplified Bayesian model. For clinical use, the official FMF calculator (fetalmedicine.org) is the gold standard. Detection rate 85-90 % at 5 % FPR for T21 with all 4 markers. Threshold ≥ 1:250 (US) or ≥ 1:150 (UK FASP) flags screen-positive. NIPT now widely available as second-line; some centres use NIPT as primary screen.
What does this mean?
The first-trimester combined screen estimates the probability that the pregnancy is affected by trisomy 21, 18, or 13. It combines four numbers Bayesian-style: your age (the prior risk), the ultrasound nuchal translucency measurement at 11– 13+6 wk (thickened in chromosomal trisomies and some cardiac defects), and two serum analytes (free β-hCG and PAPP-A, which run high/low respectively in T21). Detection rate is ~85–90 % at a 5 % false- positive rate. A “screen-positive” result (typically ≥ 1:250) is NOT a diagnosis — it means the risk is high enough to offer a diagnostic test (CVS, amnio) or a confirmatory screen (NIPT cell-free DNA, ~99 % sensitivity for T21). Most screen-positive pregnancies turn out to be normal. Conversely, a low risk doesn’t guarantee no anomalies — the 20-week anatomy scan and clinical follow-up remain important.
What is the first-trimester combined screen?
Non-invasive screening at 11+0 to 13+6 weeks. Combines:
- Maternal age.
- Nuchal translucency (NT) — ultrasound of fluid pocket at back of baby’s neck.
- Biochemistry — free β-hCG and PAPP-A blood markers.
Result is a RISK ESTIMATE for Down syndrome (e.g. “1 in 1,200”). Detection ~85-90% at 5% false-positive.
When?
11+0 to 13+6 weeks. Results in 1-2 weeks. If missed, quad screen offered at 15-20 weeks instead.
Nuchal translucency
Fluid pocket at back of baby’s neck on ultrasound. Measurement matters.
- Normal: usually 1-2.5 mm.
- Raised (>3.5 mm): higher risk of chromosomal / structural anomalies.
PAPP-A and free β-hCG
- PAPP-A low (<0.5 MoM): T21/T18/T13 risk AND placental issues.
- Free β-hCG high: T21 risk.
- Free β-hCG low: T18 risk.
What does “1 in 1,000” mean?
Higher number = lower risk. NHS UK cut-off: 1 in 150 = screen positive — offered NIPT or diagnostic test. Even “1 in 100” = 99% chance baby doesn’t have T21.
If high risk
- NHS offers FREE NIPT (more accurate refinement).
- If NIPT positive: CVS or amniocentesis for definitive diagnosis.
- Genetic counselling throughout.
Support: ARC, DSA, SOFT.
Low PAPP-A, normal risk for chromosomes
Associated with placental issues. Management:
- Aspirin 150 mg from <16 weeks (PE prevention).
- Growth scans at 28, 32, 36 weeks.
- Uterine artery Doppler at anomaly scan.
Raised NT
NT >3.5 mm needs detailed evaluation:
- Chromosomal testing (NIPT or CVS/amnio).
- Fetal cardiac scan 16-18 weeks.
- Detailed anatomy 20 weeks.
Even if chromosomes normal, ~10-15% have other issues. Many normalise.
Different scenarios — first-trimester screening
Scenario 1: Age 28, normal NT, normal bloods, risk 1 in 5,000
Reassuring. Standard antenatal care.
Scenario 2: Age 40, risk 1 in 80
NHS-funded NIPT. If positive, CVS / amnio. Genetic counselling.
Scenario 3: PAPP-A low (0.3 MoM), normal NT
Aspirin from 12 wk; growth scans 28, 32, 36 wk; uterine Doppler 20 wk.
Scenario 4: NT 4.5 mm raised
NIPT or CVS; fetal cardiac 16-18 wk; detailed anatomy 20 wk; genetic counsel.
Scenario 5: Declining screening
Valid. Still have dating + 20-week anomaly scans.
Care guidance
- Book 11-14 weeks.
- Bloods + ultrasound same day or close.
- Optional — informed choice.
- NIPT if high-risk (NHS-funded).
- Low PAPP-A: aspirin + growth surveillance.
- Raised NT: structural + genetic follow-up.
- 20-week anomaly scan still essential.
- Genetic counselling available.
- Support: ARC, DSA, SOFT.
Sources
- NHS Fetal Anomaly Screening Programme (FASP).
- ACOG Practice Bulletin 226 (2020).
- RCOG / BMFMS. Joint position on antenatal screening.
- FMF. Combined screening guidance.
- Bilardo CM, et al. ISUOG Practice Guidelines: 11-14 week ultrasound. UOG 2023.
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Frequently asked questions
What is the first-trimester combined screen?
A non-invasive screening test offered at 11+0 to 13+6 weeks pregnancy. Combines: (1) MATERNAL AGE; (2) NUCHAL TRANSLUCENCY (NT) — ultrasound measure of fluid pocket at back of baby's neck; (3) BIOCHEMISTRY — two blood markers: free β-hCG (pregnancy hormone) and PAPP-A (pregnancy-associated plasma protein A). Result is a RISK ESTIMATE — e.g. '1 in 1,200' — for trisomy 21 (Down syndrome). Detection rate ~85-90% at 5% false-positive rate. NHS standard; ACOG and NICE endorse. AKA 'combined test', 'nuchal scan', 'FTCS'.
When is the scan done?
Between 11+0 and 13+6 WEEKS — usually 12-14 weeks. EARLIER than 11 weeks: nuchal translucency hard to measure accurately. LATER than 13+6: NT measurement less reliable. MOST UK pregnancies: 12-week scan (also called 'dating scan' or 'combined screening'). UK NHS PATHWAY: blood test for biochemistry (PAPP-A + free β-hCG) usually same day or 1-2 days before ultrasound. RESULTS: ~1-2 weeks later by post / phone / NHS app.
What is nuchal translucency (NT)?
The pocket of FLUID at the back of baby's neck — visible on ultrasound from 11-13+6 weeks. ALL BABIES have it; the measurement matters. NORMAL NT: <3 mm (usually 1-2.5 mm at 11-13 weeks). RAISED NT (>3.5 mm): higher risk of chromosomal conditions (T21, T18, T13), heart defects, structural anomalies. ULTRASOUND TECHNIQUE matters — needs correctly trained sonographer. PRIVATE scans often have more time / better image quality. NT alone has ~70% detection rate for T21; combined with bloods improves to 85-90%.
What does PAPP-A and β-hCG mean?
TWO blood markers measured from maternal blood at 10-14 weeks. INTERPRETED relative to median for gestational age (MoM = Multiples of the Median). PAPP-A (Pregnancy-Associated Plasma Protein A): made by placenta. LOW PAPP-A (<0.5 MoM) associated with T21, T18, T13, AND with placental issues (PE, growth restriction, preterm birth) — even if chromosomes normal. FREE β-hCG: pregnancy hormone. HIGH free β-hCG associated with T21. LOW associated with T18. CALCULATED RISK combines maternal age + NT + both markers.
What does '1 in 1,000' or '1 in 100' mean?
PROBABILITY estimate for trisomy 21. '1 IN 1,000' means: out of 1,000 women with this exact risk profile, 1 baby would have T21. Higher number = LOWER risk (1 in 1,000 = good); lower number = higher risk (1 in 50 = high). NHS UK CUT-OFF: 1 in 150 = 'screen positive' / 'high risk' — offered NIPT or diagnostic test. NOT THE SAME AS DEFINITIVE — '1 in 100' means 99% chance baby DOESN'T have T21.
What if I'm 'high risk'?
MEANS: risk ≥1 in 150 for T21. NHS PATHWAY: (1) NIPT offered FREE; (2) IF NIPT POSITIVE → CVS or amnio for definitive diagnosis; (3) GENETIC COUNSELLING throughout. DOESN'T MEAN baby has T21 — '1 in 100' still 99% chance no T21. NIPT or diagnostic test gives more certainty. SUPPORT: ARC (Antenatal Results & Choices UK), Down's Syndrome Association, SOFT for T18/T13. TAKE TIME — these decisions are personal.
What if PAPP-A is low but I'm not high-risk?
LOW PAPP-A (<0.5 MoM) without raised T21 risk: associated with PLACENTAL ISSUES — increased risk of pre-eclampsia (~2x), growth restriction (~2x), preterm birth (~2x), stillbirth (slightly raised). MANAGEMENT: (1) ASPIRIN 150 mg from <16 weeks for pre-eclampsia prevention; (2) GROWTH SCANS at 28, 32, 36 weeks; (3) Uterine artery Doppler at anomaly scan. RCOG / NICE guidelines support this. Most pregnancies with low PAPP-A are healthy.
What if NT is raised?
NT >3.5 mm: more detailed evaluation. RISKS: (1) CHROMOSOMAL: T21, T18, T13, Turner — confirmed by NIPT or CVS/amnio; (2) STRUCTURAL: heart defects, diaphragmatic hernia, skeletal dysplasias — detailed fetal anatomy ultrasound; (3) GENETIC SYNDROMES: Noonan etc.; (4) INFECTIONS: parvovirus, CMV. EVEN IF CHROMOSOMES NORMAL: ~10-15% have other structural / genetic issues. FOLLOW-UP: NIPT or CVS/amnio; detailed fetal cardiac scan; 20-week anomaly scan. MAY RESOLVE — many raised NTs become normal-looking babies.
Is the first-trimester screen accurate?
DETECTION RATE for T21 ~85-90% at 5% false-positive rate. NOT 100% — some T21 babies will be missed (10-15% false negative). LOWER detection in twins, very obese mothers, smokers. NIPT is more accurate (>99% for T21) but doesn't replace structural assessment of nuchal scan. COMBINED screen also looks for T18, T13 (good detection ~85%). DOES NOT TEST FOR: most structural problems, single-gene conditions, microdeletions, neural tube defects (AFP from quad screen does).
Can I refuse the first-trimester screen?
ABSOLUTELY. SCREENING is OPTIONAL — NHS / NICE / ACOG all explicitly state it's the parent's choice. WHY SOMEONE MIGHT DECLINE: (1) wouldn't change pregnancy decisions; (2) prefer not to know risk numbers; (3) religious / personal beliefs; (4) anxiety about waiting for results. CAN STILL HAVE: dating scan (without NT measurement) for viability + gestational age. 20-WEEK SCAN remains routine. CAN CHANGE MIND later. INFORMED choice essential.
NIPT vs first-trimester screen
NIPT MORE ACCURATE (>99% for T21 vs ~85-90% combined screen). NIPT has fewer false positives. NIPT is BLOOD test — no ultrasound needed. FIRST-TRIMESTER COMBINED also does dating, viability, structural early-anomaly check via ultrasound. BEST APPROACH: HAVE BOTH if accessible — NHS does combined first, NIPT if high-risk. PRIVATE: many parents have nuchal scan (for anatomy) + NIPT (for chromosomes) together. NEITHER replaces 20-week anomaly scan.
What does maternal age contribute?
OLDER MATERNAL AGE = HIGHER BASELINE RISK for T21. Age 20: ~1 in 1,500; Age 30: ~1 in 940; Age 35: ~1 in 350; Age 40: ~1 in 85; Age 45: ~1 in 30. Age is FACTORED INTO risk calculation. BUT: most T21 babies born to YOUNGER mothers because more pregnancies overall. Maternal age alone is poor predictor — that's why combined screening exists.
Can I have the screen with twins?
YES — slightly different methodology. RISKS calculated separately per baby ('twin-specific'). NT measured for both. PAPP-A / β-hCG average across twins (less reliable in twins than singletons). DETECTION RATE lower in twins (~70-80% vs 85-90% singletons). NIPT for twins available privately (not routinely NHS). DICHORIONIC (separate placentas) easier; MONOCHORIONIC (shared placenta) more complex. SPECIALIST scanner usually does twins.
Will my insurance / NHS cover this?
UK NHS: YES, included in routine antenatal care. Free. PRIVATE: typically £150-300 for combined screen package. US: usually covered by insurance for prenatal care. CANADA / AUSTRALIA: standard prenatal care. WORTH CHECKING what's offered in your area + private alternatives. EVEN if NHS, MANY parents pay for additional private nuchal scan for image quality + time + early genetic discussion.
What's the next step after the result?
(1) LOW RISK (<1 in 150): standard antenatal care continues; 20-week anomaly scan; routine monitoring. (2) HIGH RISK (≥1 in 150): NHS offers FREE NIPT OR diagnostic test directly. (3) UNCERTAIN RESULTS: discussed with consultant. (4) LOW PAPP-A: aspirin + growth surveillance. (5) RAISED NT: detailed fetal anatomy ultrasound + NIPT/diagnostic. ALWAYS: genetic counsellor available; ARC for support; specialist clinic referral if needed.
How does this relate to other calculators on BumpBites?
Companion: /calculators/nipt-cfdna for follow-up testing; /calculators/quad-screen for second-trimester alternative; /calculators/aspirin-pe-prevention for low PAPP-A management; /calculators/pregnancy-week for gestation timing; /calculators/pregnancy-bmi; /calculators/twin-probability for multiples; /calculators/fetal-weight for growth surveillance.