The combined test produces one number, and that number is the source of more anxiety than any other result in early pregnancy. It arrives as a fraction — 1 in 900, 1 in 300, 1 in 90 — with very little explanation of what went into it or what it is claiming.
What went into it is a scan measurement, two blood proteins and your age, run through a calculation that has been in use for decades. What it is claiming is narrower than it looks. This guide covers what is measured, how the figure is produced, where the cut-off sits and why, and what each kind of result actually leads to.
The short answer: the combined test is offered between 11 and 14 weeks (in practice 11 weeks + 2 days to 14 weeks + 1 day) and screens for Down's, Edwards' and Patau's syndromes. It measures the nuchal translucency on the scan plus PAPP-A and free β-hCG in your blood, and combines them with your age and the baby's size into a single chance figure. A result of 1 in 150 or a greater chance is reported as higher chance and leads to NIPT or a diagnostic test being offered. It is a probability, never a diagnosis.
What is actually measured
Three inputs, from two appointments that usually happen the same morning.
1. Nuchal translucency
All babies have a small pocket of fluid at the back of the neck in the first trimester. The sonographer measures its depth in millimetres, in a specific plane, with the baby in a specific position — which is why this measurement takes a while and why the scan sometimes pauses while you are asked to move or cough.
A larger measurement is statistically associated with chromosomal conditions and with some heart defects. It is not read on its own: the same millimetre value means different things at different crown-rump lengths, because the fluid pocket grows with the baby.
2. PAPP-A
Pregnancy-associated plasma protein-A is made by the placenta. Levels tend to run lower in pregnancies affected by the conditions being screened for. Low PAPP-A is also associated with placental problems later in pregnancy, which is a separate finding that some units act on independently.
3. Free β-hCG
The free beta subunit of human chorionic gonadotropin, the hormone a pregnancy test detects. It behaves differently in the three conditions screened for, which is what lets one calculation distinguish between them rather than just flagging "something".
The window is not flexible. Outside roughly 11+2 to 14+1 weeks the nuchal translucency cannot be measured reliably and the blood markers are not informative, so the combined test simply cannot be done. This is why a late booking appointment matters: it is one of the few things in pregnancy that expires.
How the number is produced
The three measurements are not compared to thresholds individually. They are entered into software along with your age, your weight, your gestational age from the crown-rump length, whether this is a twin pregnancy, whether you smoke and whether the pregnancy was conceived by IVF — all of which shift the marker levels for reasons unrelated to the baby's chromosomes.
The output is a chance ratio for each of the three conditions, specific to this pregnancy. Two women with identical nuchal measurements can get very different results, because everything else in the calculation differs.
| Result | How it is reported | What follows |
|---|---|---|
| 1 in 2 to 1 in 150 | Higher chance | A conversation with a specialist, and NIPT or a diagnostic test offered |
| Less than 1 in 150 | Lower chance | No further testing offered; the 20-week scan still happens |
| No result | Test not completed | Usually a nuchal measurement that could not be obtained; the quadruple test may be offered instead |
Why 1 in 150, and what it does not mean
The cut-off is a policy decision, not a biological boundary. It is set where the programme judges the benefit of offering further testing to outweigh the anxiety and the procedure risk of doing so. Move it to 1 in 300 and more affected pregnancies would be identified, at the cost of many more women being sent for invasive tests they did not need.
Two consequences follow, and both are counter-intuitive:
- A higher-chance result usually means the baby does not have the condition. At 1 in 150, 149 out of 150 do not. The word "higher" is doing comparative work, not absolute.
- A lower-chance result is not a guarantee. 1 in 900 is not zero. Screening reduces uncertainty; it does not remove it, and the 20-week scan exists partly for this reason.
Edwards' and Patau's syndromes
The combined test screens for all three conditions at once, but they are reported separately because they behave differently. Down's syndrome (trisomy 21) gets its own chance figure; Edwards' (trisomy 18) and Patau's (trisomy 13) are usually reported as a joint figure, because both are far rarer and their marker patterns are similar.
If the combined test could not be done in time, the quadruple test between 14 and 20 weeks screens for Down's syndrome only. Edwards' and Patau's are then looked for at the 20-week anomaly scan instead, because both usually produce structural findings a detailed scan can see.
What to ask when the result arrives
💬 Four questions worth asking
- What were the individual numbers? The nuchal measurement in millimetres, and PAPP-A and β-hCG as MoM values. The combined figure hides which input drove it.
- Was the PAPP-A low? If so, ask whether it changes your growth monitoring later in pregnancy, separately from the chromosomal question.
- What are my options, and how long do I have? There is almost always more time than the appointment implies.
- What would each next test tell me that this one did not? NIPT and amniocentesis answer different questions with different certainty and different risk.
Our guides to NIPT and cell-free DNA testing and the early pregnancy ultrasound cover the two directions this decision usually goes, and the full schedule of tests and scans puts it in the context of everything else being offered.
Deciding before the result, not after
The single thing that makes this easier is thinking through the branches before you accept the test rather than while holding a piece of paper with a fraction on it. There are only three, and none of them is wrong:
You would want a definite answer. Then a higher-chance result leads to NIPT and, if that is also higher chance, to CVS or amniocentesis. Knowing this in advance makes the pathway feel like a plan rather than an escalation.
You would want to know but would not act on it. Information to prepare with is a legitimate reason to test, and it usually points towards NIPT rather than an invasive procedure, because the risk calculation is different when the result will not change what you do.
You would rather not know. Then declining the screening entirely is more coherent than accepting it and hoping for a reassuring number. This is a common and reasonable choice, and it does not affect anything else you are offered.
When to call your midwife
Call the same day if: you have vaginal bleeding, severe or one-sided abdominal pain, shoulder-tip pain, faintness or a fever. None of these relate to screening results, but they are the reasons to ring during the weeks this page covers, and an ectopic pregnancy in particular needs same-day assessment.
A screening result is never an emergency, however alarming the number looks. If a higher-chance result arrives by phone or letter and you cannot reach anyone to discuss it, that is worth chasing the next working day — but nothing about the pregnancy changes in the meantime.
Frequently Asked Questions
When is first trimester screening done?
The combined test is done between 11 and 14 weeks — laboratories usually state the window precisely as 11 weeks + 2 days to 14 weeks + 1 day. It cannot be done outside it: the nuchal measurement is only interpretable while the baby is a specific size, and the two blood markers are only informative in that window. If you miss it, the quadruple test between 14 and 20 weeks screens for Down's syndrome alone.
What does the combined test measure?
Three things combined into one figure: the nuchal translucency, a pocket of fluid at the back of the baby's neck measured on the scan; and two proteins in your blood, PAPP-A (pregnancy-associated plasma protein-A) and free β-hCG. Those are adjusted for your age, the baby's size and your gestational week, and the result is a single chance figure.
What does a result of 1 in 150 mean?
It means that out of 150 pregnancies with exactly this combination of measurements, one would be expected to have the condition and 149 would not. In the NHS programme, 1 in 150 or a greater chance — meaning anything from 1 in 2 to 1 in 150 — is reported as "higher chance" and leads to a further test being offered. Anything less likely than that, such as 1 in 400, is reported as lower chance.
Is a higher chance result a diagnosis?
No, and the distinction matters more here than anywhere else in pregnancy. Screening estimates a probability; it cannot confirm or exclude anything. Most higher-chance results turn out to be pregnancies without the condition. The result is a reason to be offered NIPT or a diagnostic test, not a finding in itself.
Can I decline first trimester screening?
Yes. It is offered, never required, and declining it does not affect the rest of your antenatal care or your access to the 20-week scan. The question worth answering first is what you would do with each possible result — that is usually clearer than trying to decide whether you "want to know".
What is a normal nuchal translucency measurement?
The measurement is not read as a pass or fail on its own. It is fed into the calculation alongside your age, the baby's crown-rump length and the two blood markers, and a value that would be unremarkable at 11 weeks can be significant at 13, because the nuchal fluid changes with size. Ask for the figure and what it was expected to be at your baby's measurement rather than comparing it to a number from a forum.
Does a low PAPP-A matter on its own?
A low PAPP-A contributes to the chromosomal screening figure, and it is also associated with a higher chance of placental problems later — growth restriction and pre-eclampsia. Some units respond by arranging extra growth scans in the third trimester. It is not a diagnosis of anything, and most pregnancies with a low PAPP-A proceed normally, but it is worth asking whether it changes your monitoring plan.
What happens after a higher chance result?
You will be offered a conversation with a specialist and a choice of next steps: NIPT, a blood test that is far more accurate than screening but still not diagnostic; or a diagnostic test — chorionic villus sampling from about 11 weeks, or amniocentesis from about 15 — which gives a definite answer at the cost of a small miscarriage risk. Detailed ultrasound in a fetal medicine unit may also be offered. Declining all of them is a valid choice.
First trimester screening — at a glance
The combined test runs between 11 and 14 weeks and produces one chance figure per condition from three inputs: the nuchal translucency measured on the scan, and PAPP-A and free β-hCG measured in your blood, adjusted for your age and the baby's size. A result of 1 in 150 or greater chance is reported as higher chance and leads to NIPT or a diagnostic test being offered; anything less likely is reported as lower chance and no further testing follows. Neither result is a diagnosis — most higher-chance pregnancies are unaffected, and a lower-chance result is not a guarantee. The window cannot be extended, and the decision is easier if you work out what you would do with each possible answer before the test rather than after it.
Inside Baby Novum: the lab results journal stores each screening result with its date, the gestational week it was taken at and a photo of the report, and the calendar marks the scan appointment with a reminder. Everything is encrypted on the device — a screening result is about as private as data gets, and there is no server holding a copy.