In short
A 1 in 250 result means that of 250 pregnancies with exactly that result, 249 are not affected. The double marker and NT scan estimate a chance, not a diagnosis. Here is how to read the report line by line.
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01
What does the double marker test actually measure?
The double marker is a blood test that measures two substances made in early pregnancy: free beta human chorionic gonadotropin (free beta-hCG) and pregnancy-associated plasma protein A (PAPP-A). Neither number means anything on its own. Screening software combines them with your age, the ultrasound measurement of nuchal translucency, the baby's crown-rump length and the exact gestational age, and returns one figure: the estimated chance that this pregnancy is affected by trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome).
There are typical patterns. In trisomy 21, PAPP-A tends to run low and free beta-hCG high. In trisomy 18, both tend to run low. But these are averages drawn from thousands of pregnancies, and plenty of completely unaffected pregnancies produce the same pattern by chance. That overlap is the reason the report hands you a ratio instead of a yes or a no, and the reason no one can read your two marker values and tell you the answer.
02
Is 1 in 1000 better or worse than 1 in 250?
The larger the second number, the lower the chance. A result of 1 in 1000 is more reassuring than 1 in 250, which is more reassuring than 1 in 50. Read the ratio as a plain sentence: if 250 pregnancies all had this identical result, one would be expected to be affected and 249 would not. Put that way, a result of 1 in 250 is weighted heavily towards an unaffected baby. That reading does not stretch across the whole of the higher-chance range, though. A result at the top of it, such as 1 in 5 or 1 in 2, means a substantial chance, and those figures deserve a prompt appointment with a fetal medicine team rather than the same reassurance.
High risk only means above the cut-off that the screening programme or laboratory has chosen. The NHS programme uses a chance cut-off of 1 in 150 at term for both the combined and quadruple tests: a result between 1 in 2 and 1 in 150 is called higher chance, and 1 in 151 or beyond is lower chance. Other laboratories print other cut-offs, so the same numeric result can be labelled differently on two reports. Before reacting to the label, find the cut-off your report used.
03
What does MoM mean beside each marker?
MoM stands for multiples of the median. A value of 1.0 MoM means your marker sits exactly at the middle of the range for pregnancies at the same gestational day. A PAPP-A of 0.4 MoM means it is at about 40 per cent of that typical value. Raw concentrations cannot be compared between weeks, laboratories or assays, because they change hour by hour in early pregnancy. Converting to MoM is what makes one woman's result comparable with the reference population.
MoM values are corrected before the risk is calculated. Maternal weight, gestational age, parity, smoking, multiple gestation, conception through fertility treatment and diabetes all shift marker levels and, if ignored, shift the detection and false-positive rates too. One study of women with pre-gestational diabetes taking insulin found a 12 per cent reduction in median PAPP-A and an 18 per cent reduction in median hCG compared with women without diabetes. This is why the request form asks for an accurate weight and a firm date.
04
Does a high-chance result mean the baby has a problem?
No. This is a screening test, and screening sorts people into groups that deserve a closer look. Serum marker screening is generally considered to carry about a 5 per cent false-positive rate, which means roughly one in twenty women screened is worried unnecessarily. Against that, combined first-trimester screening picks up close to 90 per cent of pregnancies affected by Down syndrome, which is why it is offered at all. Both facts are true at the same time.
What a higher-chance result changes is the next step, not the diagnosis. Nothing about the pregnancy is different on the day the report arrives from the day before. The reverse also holds: a lower-chance result is not a certificate of a healthy baby. The combined test looks only at three specific chromosomal conditions. It says nothing about structural problems of the heart, spine or kidneys, and nothing about conditions that appear later in pregnancy.
05
What should the nuchal translucency measurement be?
Nuchal translucency is the thin layer of fluid at the back of the baby's neck, seen on ultrasound in a precise mid-line view. Every fetus has some; the question is how much. The measurement is only valid in a narrow window, when the crown-rump length is between 45.0 mm and 84.0 mm, because the fluid layer changes predictably with size. Outside that window the number cannot be fed into the risk calculation at all.
The threshold quoted depends on the source, and it is worth knowing that. One widely used clinical reference treats a measurement above 3 mm as abnormal. The NHS programme handbook notes that measurements equal to or greater than 3.5 mm may be associated with trisomies 21, 18 and 13. Either way, the measurement is never read alone: it is combined with the two blood markers and maternal age. Small differences in technique change the ratio, which is why sonographers who perform it are audited. One further point matters if the measurement is large: when nuchal translucency is 3.5 mm or more and the chromosome result then comes back normal, the raised measurement still carries a higher chance of a heart defect and of other structural problems, so a detailed anomaly scan and a scan of the baby's heart are usually arranged. A normal chromosome result does not cancel a raised nuchal translucency.
06
Is the next step NIPT or amniocentesis?
Non-invasive prenatal testing (NIPT), which reads cell-free DNA in the mother's blood, is the usual next offer after a higher-chance combined or quadruple test result. It is a much better screening test, but it is still a screening test. In one large referral laboratory series, the positive predictive value was 86.1 per cent for trisomy 21, 57.8 per cent for trisomy 18, 25.0 per cent for trisomy 13, and as low as 20 per cent for monosomy X. So a positive NIPT for Down syndrome is usually correct, while a positive result for trisomy 13 usually is not.
Only chorionic villus sampling or amniocentesis gives a diagnosis, because only they sample the baby's own cells. The authors of that series are explicit that a positive NIPT result should always be confirmed by an invasive diagnostic test on amniotic fluid, chorionic villi or cord blood before any decision is made. Both procedures carry a small procedure-related risk of miscarriage; recent reviews put it lower than the figures quoted a generation ago, and your fetal medicine unit should give you its own number. Declining further testing is also a legitimate choice.
07
In which weeks is the test valid?
First-trimester combined screening runs from about 10 weeks to about 14 weeks, and what decides eligibility on the day is the ultrasound rather than the calendar: the crown-rump length has to be between 45.0 mm and 84.0 mm, which corresponds to roughly 11 to 14 weeks. One widely used clinical reference quotes the window as 10 weeks to 13 weeks 6 days, while the NHS takes the blood sample from 10 weeks and sets the scan by crown-rump length. Done too early, the markers and the nuchal measurement cannot be interpreted. Done too late, the fluid layer can no longer be measured reliably, and the laboratory will refuse to issue a risk figure rather than issue a wrong one.
Missing the window is not the end of screening. The quadruple test, which measures alpha-fetoprotein, hCG or free beta-hCG, inhibin-A and unconjugated oestriol, is offered from 14 weeks 2 days to 20 weeks 0 days. It screens for Down syndrome and is less accurate than the combined test, and it uses the same 1 in 150 cut-off. Edwards and Patau syndromes are then looked for at the 20-week scan instead.
08
The result is lower chance. What happens now?
Routine antenatal care continues unchanged. The next milestone is the mid-pregnancy scan at around 20 weeks, which is a structural examination: the brain, face, spine, heart, abdominal wall, kidneys and limbs. That is a different job from the first-trimester test, and it is the reason a lower-chance double marker result does not make the 20-week scan optional. Screening for the three chromosomal conditions is finished at this point; screening for the structure of the baby has not yet started.
Keep the report itself, not just the summary line. The individual MoM values, the nuchal translucency measurement in millimetres and the gestational age used are what a fetal medicine specialist will want if any question comes up later in the pregnancy. Photographing the page on your phone takes a few seconds and saves repeating work in a hospital that does not hold your first-trimester records.
Sources
- StatPearls: Prenatal Genetic Screeningncbi.nlm.nih.gov
- NHS Fetal Anomaly Screening Programme handbook: screening for Down's, Edwards' and Patau's syndromesgov.uk
- NHS: Screening for Down's syndrome, Edwards' syndrome and Patau's syndromenhs.uk
- Contingent screening models for first-trimester Down syndrome screeningpmc.ncbi.nlm.nih.gov
- Positive predictive value estimates for non-invasive prenatal testingpmc.ncbi.nlm.nih.gov
- First-trimester aneuploidy screening markers in women with pre-gestational diabetespmc.ncbi.nlm.nih.gov
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