PAPP-A MoM Calculator
PAPP-A MoM Calculator
Work out a PAPP-A multiple of the median from your result and your laboratory’s median for the same gestational age, or enter the reported MoM directly, and see what a low value means for growth surveillance. The median belongs to your laboratory or screening programme — this page does not supply one, and a MoM on its own is not a Down’s syndrome risk.
PAPP-A multiple of the median
Result ÷ median → MoMPAPP-A 1,050 mIU/L; illustrative median 3,000 mIU/L
Multiple of the median
- median
- your laboratory’s or programme’s median for that gestational day, on the same assay and in the same unit
- adjusted MoM
- the MoM on a screening report is also corrected for maternal weight, ethnicity, smoking, IVF and diabetes; a simple division is not
- 0.415
- 5th centile; RCOG Green-top 31 (2013) major risk factor for a small-for-gestational-age baby
Worked example
PAPP-A 1,050 mIU/L; illustrative median 3,000 mIU/L
1,050 ÷ 3,000 = 0.35 MoM
0.35 is below 0.415, so this is a low PAPP-A
The median here is an illustration. With a real laboratory median, the answer is only as good as that median
What a PAPP-A MoM can and cannot tell you
| Question | Can a MoM answer it? | Where the answer comes from |
|---|---|---|
| Is PAPP-A low for this gestation? | Yes | This page, with your laboratory’s median |
| Should growth be monitored more closely? | Yes, below 0.415 MoM | Your unit’s fetal growth pathway |
| What is the chance of Down’s, Edwards’ or Patau’s syndrome? | No | The combined test: age, nuchal translucency, free beta-hCG and PAPP-A together |
| What is the PAPP-A in another unit? | Not needed for a MoM | The PAPP-A unit converter |
Why your MoM may differ from the one on the report
| Difference | Effect |
|---|---|
| Adjustment for maternal weight | Heavier women have lower concentrations; the report corrects for it |
| Ethnicity, smoking, IVF, diabetes | Each shifts PAPP-A; the report corrects for them |
| Gestational day | The median changes daily in the first trimester; a median from the wrong day is wrong |
| Assay | Medians are assay-specific; a median from another laboratory does not apply |
A ratio to a median that only your laboratory has
PAPP-A rises steeply through the first trimester, so a concentration means nothing without the gestational day it was taken on. Screening programmes deal with this by dividing each result by the median for that exact day in their own population, measured on their own assay. The answer is a multiple of the median: 1.0 is typical, 0.5 is half the usual level. That median is specific to the assay, the population and the day, which is why this page will not supply one. A table of medians copied from somewhere else would give a precise-looking MoM that is simply wrong for your result. Enter the median your laboratory or screening programme uses, in the same unit as the result.
There is a second difference worth knowing. The MoM printed on a screening report has also been adjusted for maternal weight, ethnicity, smoking, IVF conception and diabetes, each of which moves PAPP-A. A straight division does not do that, so a MoM calculated here can differ from the reported one. If the report gives a MoM, that is the number to use; enter it with a median of 1 to see how it reads.
What a low MoM means is the part people are usually looking for. Low PAPP-A reflects the placenta, and is associated with a small-for-gestational-age baby, pre-eclampsia and stillbirth. RCOG Green-top Guideline 31 (2013) made a PAPP-A below 0.415 MoM, the 5th centile, a major risk factor for a small baby and recommended serial growth scans. That guideline has since been replaced by a 2024 edition; NHS pathways built on it still use about 0.415 but differ on the scan schedule and on aspirin, so the schedule is your unit’s to set. What a MoM is not is a trisomy risk. That comes from the combined test, which puts PAPP-A together with free beta-hCG, nuchal translucency and maternal age. For the other biochemical marker of the combined test, the free beta-hCG MoM calculator works the same way. For units see the PAPP-A unit converter and free beta-hCG unit converter; for pre-eclampsia, the pre-eclampsia laboratory criteria interpreter and sFlt-1/PlGF ratio calculator.
Frequently asked questions
How do you calculate a PAPP-A MoM?
Divide the PAPP-A result by the median PAPP-A for the same gestational day, measured by the same laboratory on the same assay and in the same unit. A result of 1,050 against a median of 3,000 is 0.35 MoM. The median must come from your laboratory or screening programme; medians are not transferable between assays or populations.
What PAPP-A MoM is considered low?
Below 0.415 MoM, the 5th centile. RCOG Green-top Guideline 31 (2013) treated that as a major risk factor for a small-for-gestational-age baby and recommended serial growth scans. The 2024 edition replaced that guideline, and NHS pathways that implement it still use about 0.415 (some round to 0.42), with differing scan schedules.
Does a low PAPP-A mean my baby has Down’s syndrome?
No. A PAPP-A MoM is one input to the combined screening test, which calculates a chance from PAPP-A, free beta-hCG, nuchal translucency and maternal age together. A low PAPP-A on its own is not a trisomy risk. What it does flag is placental function, which is why it leads to closer monitoring of the baby’s growth.
What happens if my PAPP-A is low?
Usually extra ultrasound scans to check the baby’s growth in the second half of pregnancy, and in some units low-dose aspirin. The 2013 RCOG guideline recommended serial scans with umbilical artery Doppler from 26–28 weeks; current NHS pathways commonly scan every 4 weeks from 28 weeks. Most pregnancies with low PAPP-A have a normally grown baby; the scans are to find the ones that do not.
Why is my calculated MoM different from the one on my report?
Because the reported MoM is adjusted for maternal weight, ethnicity, smoking, IVF conception and diabetes, and a simple division is not. The gestational day and assay also have to match exactly. If your report gives a MoM, use that number.
Related calculators
References
- Royal College of Obstetricians and Gynaecologists. The Investigation and Management of the Small-for-Gestational-Age Fetus. Green-top Guideline No. 31, 2nd edition, February 2013 (minor revisions January 2014). "A low level (< 0.415 MoM) of the first trimester marker PAPP-A should be considered a major risk factor for delivery of a SGA neonate"; odds ratios 2.7 and 3.66 for birthweight below the 10th and 3rd centiles; women with a major risk factor referred for serial ultrasound of fetal size and umbilical artery Doppler from 26–28 weeks. Superseded by the 2024 edition below.
- Morris RK, Johnstone E, Lees C, Morton V, Smith G; Royal College of Obstetricians and Gynaecologists. Investigation and Care of a Small-for-Gestational-Age Fetus and a Growth Restricted Fetus (Green-top Guideline No. 31). BJOG. 2024;131(9):e31–e80. doi:10.1111/1471-0528.17814. The current edition. Its full text could not be consulted for this page.
- Hywel Dda University Health Board. Fetal Growth Surveillance Guideline, version 3.0 (2025), citing Green-top Guideline 31, 3rd edition (2024): "Low serum PAPP-A (<0.415 MoM) in first trimester" as a high-risk factor, with growth scans 4-weekly from 28 weeks until birth. An example of a local implementation, not a national standard.
- Royal Cornwall Hospitals NHS Trust. Fetal Growth Restriction: Risk Assessment, Prevention, Surveillance, version 3.3: PAPP-A below the 5th centile (<0.42 MoM) as high risk, with growth scans at 28, 32, 36 and 40 weeks and aspirin 150 mg daily from 12 to 36 weeks. A second local implementation, showing the variation between units.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.