What this test measures
Inhibin A is a glycoprotein produced by the corpus luteum in early pregnancy and predominantly by the placenta from the second trimester onwards. It acts as a feedback regulator of FSH. In a pregnancy with Down syndrome (trisomy 21), maternal serum Inhibin A is approximately twice the level of an unaffected pregnancy at the same gestational age.
Inhibin A is the fourth marker in the Quadruple Marker test (Quad Screen), where it is combined with AFP, unconjugated estriol (uE3), and total/free β-hCG to estimate Down syndrome and trisomy 18 risk between 15 and 22 weeks of gestation. Adding Inhibin A raises Down syndrome detection from ~70% (Triple Marker) to ~80% at the same false positive rate.
Why it matters
For Indian patients who miss the first-trimester (Dual Marker + NT) screening window — common because the first booking visit is often delayed — the Quadruple Marker between 15 and 22 weeks is the main second-trimester option. Inhibin A is what makes the Quad Marker meaningfully more sensitive than the older Triple Marker for Down syndrome detection.
Inhibin A is rarely ordered as a standalone test outside this context; the panel result and overall risk calculation is what guides decisions, not the absolute Inhibin A number.
How to prepare
No fasting required. The test is timed to the same gestational window as the rest of the Quad screen (15+0 to 22+0 weeks, best at 16–18 weeks). Bring an LMP date, recent dating ultrasound, IVF/IUI status, ethnicity, weight, and prior pregnancy history.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Inhibin A (pg/mL (or MoM))[1][2] | Median MoM = 1.0 (range 0.5–2.0 typical) | Low Inhibin A MoM (<0.5) is associated with trisomy 18. | High Inhibin A MoM (>2.0) is associated with trisomy 21 (Down syndrome) and with placental dysfunction (pre-eclampsia, IUGR) later in pregnancy. |
Inhibin A pattern by trisomy
| Condition | Typical Inhibin A pattern | Other Quad markers |
|---|---|---|
| Unaffected pregnancy | MoM ~ 1.0 | All markers MoM ~ 1.0 |
| Trisomy 21 (Down) | ↑ (~2 MoM) | ↑ β-hCG, ↓ AFP, ↓ uE3 |
| Trisomy 18 (Edward) | ↓ (~0.5 MoM) | ↓ all markers |
| Open neural tube defect | Normal | ↑ AFP |
| Placental dysfunction | ↑ (later in pregnancy) | AFP often elevated too |
Frequently asked questions
Why is Inhibin A part of the Quadruple Marker?
Inhibin A is approximately twice as high in Down syndrome pregnancies as in unaffected ones at the same gestational age. Adding it to the Triple Marker (AFP + uE3 + β-hCG) raises Down syndrome detection from ~70% to ~80%.
Is Inhibin A ordered as a standalone test?
Rarely. It is almost always part of the Quadruple Marker panel. A solo Inhibin A reading without the other markers and risk calculation has limited clinical use.
When is it tested?
Between 15+0 and 22+0 weeks, ideally at 16–18 weeks. This is the window for the Quadruple Marker.
Do I need to fast?
No.
A high Inhibin A — should I be worried?
A standalone high value is not enough to make a decision. The combined risk score (with AFP, uE3, β-hCG, maternal age, gestational age) is what your doctor uses. If the combined risk is high, NIPT or amniocentesis follows.
Can Inhibin A predict pre-eclampsia?
A high mid-trimester Inhibin A has been associated with later pre-eclampsia and IUGR in research studies, but it is not used as a standalone predictor in routine practice. Your obstetrician will combine it with blood pressure, uterine artery Doppler, and PlGF/sFlt-1 if needed.
Is Inhibin A used in ovarian cancer testing?
Yes — Inhibin A and B are tumour markers for certain ovarian sex-cord stromal tumours (granulosa cell tumour). The assay is the same; the clinical context is different.
How is the report given?
Inhibin A is reported as an absolute value (pg/mL) and as a MoM (Multiple of the Median) for your gestational age. The lab software then integrates it into the combined risk for the Quad Marker.
Related Pregnancy / Prenatal tests
Tests commonly ordered alongside INHIBIN A, or that help interpret an unexpected result.
Sources & references
- ACOG — Screening for Fetal Chromosomal Abnormalities · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Second Trimester Screening · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Inhibin · accessed 2026-05-30T00:00:00.000Z
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