Unconjugated Estriol (uE3) Unit Converter

Q: How do you convert unconjugated oestriol from ng/mL to nmol/L?

Multiply by 3.46753, which is one nanogram per millilitre divided by oestriol's molecular weight of 288.39 g/mol. So 1.5 ng/mL is 5.20 nmol/L, and dividing by the same factor goes back. Nanograms per millilitre and micrograms per litre are identical, and nanograms per decilitre are a hundred times larger. Note that this applies to unconjugated oestriol, not total oestriol, which is a different and much larger measurement.

Q: Why does my report give a MoM instead of a concentration?

Because the concentration on its own does not carry the information. The population median unconjugated oestriol roughly trebles between 15 and 20 weeks of pregnancy, so the same number can be perfectly ordinary at one gestation and distinctly low at another. A multiple of the median is the measured value divided by the screening laboratory's median for that exact gestational age, then corrected for maternal weight, ethnicity, diabetes, smoking and multiple pregnancy. Screening risks are calculated from MoM values across all four analytes — never from raw concentrations.

Q: What is unconjugated oestriol measured for?

Almost exclusively for the second-trimester quadruple test, performed at about 15 to 20 weeks with alpha-fetoprotein, hCG and inhibin A. A low uE3 contributes to a raised risk estimate for Down syndrome and, more strongly, for trisomy 18. The test reports a risk rather than a diagnosis, detecting roughly 80% of Down syndrome pregnancies at about a 5% false-positive rate. Serial oestriol measurements were once used to monitor foeto-placental wellbeing in late pregnancy, but that use has been replaced by ultrasound and cardiotocography.

Q: What does a very low or undetectable uE3 mean?

First, check the gestational dates and ask whether the mother is taking corticosteroids — those explain most such results. Beyond that, an undetectable uE3 in an otherwise normal-looking pregnancy points classically to placental steroid sulfatase deficiency, the X-linked condition that causes ichthyosis in an affected boy. A very low uE3 accompanied by a low AFP and low hCG raises the possibility of Smith-Lemli-Opitz syndrome, a recessive disorder of the last step of cholesterol synthesis. Trisomy 18, foetal demise, anencephaly and congenital adrenal hypoplasia also lower it, because oestriol synthesis depends on an intact foetal adrenal, foetal liver and placenta.

Q: Is unconjugated oestriol the same as total oestriol?

No. Unconjugated oestriol is the free steroid and is the analyte used in prenatal screening; total oestriol includes the glucuronide and sulphate conjugates, is measured on a different assay, and gives a much larger number. Using one where the other is expected produces a nonsensical MoM. If a report does not say "unconjugated", it is worth checking which was measured before converting or interpreting anything.

Convert unconjugated oestriol between ng/mL, µg/L, ng/dL and nmol/L — and then read the sentence that matters more than the arithmetic: without the gestational age and the conversion to a multiple of the median, a raw uE3 concentration means almost nothing.

Last updated 14 Sep 2026

Unconjugated Estriol (uE3) converter

Mass ⇄ molar

Unconjugated oestriol (uE3)

ng/mL × 3.46753 = nmol/L. ng/mL and µg/L are identical; ng/dL is the ng/mL figure × 100. None of these is the number the screening report acts on — that is a multiple of the median (MoM).

5.20nmol/LExample

Unconjugated oestriol 1.5 ng/mL at 16 weeks’ gestation

The conversion, and the conversion that actually counts

nmol/L = ng/mL × 3.46753
ng/mL = nmol/L ÷ 3.46753
because 3.46753 = 1 ng/mL (10⁻⁶ g/L) ÷ 288.39 g/mol, the molecular weight of oestriol

MoM = measured uE3 ÷ the median uE3 for that exact gestational age

MW 288.39: oestriol (E3), C₁₈H₂₄O₃ — the 16α-hydroxylated oestrogen, the least potent of the three and overwhelmingly the most abundant in pregnancy
unconjugated, not total: uE3 is the free steroid. Total oestriol includes the glucuronide and sulphate conjugates and is a far larger number measured on a different assay. They are not interchangeable, and prenatal screening uses the unconjugated form
ng/mL = µg/L: numerically identical. ng/dL is a hundred times larger, so 1.5 ng/mL is 150 ng/dL
MoM is the unit of interpretation: the measured concentration divided by the laboratory’s own median for that gestational age, usually with corrections for maternal weight, ethnicity, diabetes and multiple pregnancy. Screening risks are calculated from MoM, never from ng/mL — and a MoM cannot be derived from a concentration without knowing the gestation and the laboratory’s medians
why gestation dominates: the population median uE3 roughly trebles between 15 and 20 weeks. A concentration that is a perfectly ordinary 1.0 MoM at 15 weeks is a markedly low MoM at 20 weeks. Dating error is therefore a screening error

Worked example

Unconjugated oestriol 1.5 ng/mL at 16 weeks' gestation
1.5 ng/mL = 1.50 µg/L = 150 ng/dL — the same concentration written three ways
1.5 × 3.46753 = 5.20 nmol/L (5.201 before rounding)
Now the step that decides everything, and that this page cannot do for you. If the screening laboratory's median uE3 at 16 weeks is 1.5 ng/mL, this result is 1.0 MoM — entirely unremarkable
Take the identical 1.5 ng/mL at 20 weeks, where the median might be around 3.5 ng/mL, and it is roughly 0.43 MoM — low enough to contribute meaningfully to a raised trisomy 18 risk and, below 0.25 MoM, to trigger a different investigation entirely
Same number, same units, same patient, two very different reports — because the interpretation belongs to the gestational age, not to the concentration

The quadruple test, and what each analyte contributes

The quadruple test is performed between about 15 and 20 weeks and reports a risk, not a diagnosis: at a typical 1 in 150 to 1 in 250 cut-off it detects around 80% of Down syndrome pregnancies for a false-positive rate of roughly 5%. Every one of those four numbers goes into the calculation as a multiple of the median, adjusted for gestational age, maternal weight, ethnicity, diabetes and plurality. In most healthcare systems cell-free DNA screening has now displaced or follows it, but the quadruple test remains the reason uE3 is measured at all.
Analyte	Down syndrome (T21)	Edwards syndrome (T18)	Open neural tube defect
Alpha-fetoprotein (AFP)	Low, around 0.75 MoM	Low	High — usually ≥2.0–2.5 MoM, the analyte the NTD screen rests on
Total or free β-hCG	High, around 2.0 MoM	Low	—
Unconjugated oestriol (uE3)	Low, around 0.73 MoM	Low, and lower than in T21	Sometimes low
Inhibin A	High, around 1.8 MoM	Little contribution	—

Why a raw uE3 concentration cannot be read on its own

This is why the page prints no reference interval. Nothing meaningful can be said about 1.5 ng/mL, or 5.2 nmol/L, until it is divided by the right median — and the right median is a property of the pregnancy and the laboratory, not of the analyte.
What is missing	Why it breaks the interpretation
Gestational age	The population median rises steeply through the second trimester, roughly trebling between 15 and 20 weeks. Without an accurate gestation — ideally by ultrasound rather than by dates — the same concentration can be normal or markedly low
The laboratory’s own medians	MoM is calculated against medians derived from that laboratory’s own assay and population. Medians from another laboratory or another platform give a different MoM from the identical concentration
Maternal weight	Larger maternal blood volume dilutes all four analytes, and MoM values are weight-corrected before the risk is calculated
Ethnicity, diabetes, smoking, plurality, assisted conception	Each shifts the medians, and each is a standard correction in a screening algorithm
The other three analytes	Risk comes from the pattern across AFP, hCG, uE3 and inhibin A together. A single low uE3 in isolation is not a screening result

A very low uE3: the findings that are specifically associated with it

Oestriol in pregnancy is made by the placenta from 16α-hydroxy-DHEA-sulfate supplied mainly by the foetal adrenal, so uE3 is a readout of an intact foeto-placental unit: foetal adrenal, foetal liver and placenta all have to be working. That is why a very low or undetectable uE3 is worth a specific thought rather than a shrug — but also why maternal steroids and incorrect dating must be excluded first.
Condition	Mechanism	Typical uE3
Steroid sulfatase deficiency (X-linked ichthyosis)	The placental enzyme that desulphates DHEA-sulfate, the precursor for placental oestriol synthesis, is absent — usually from a contiguous STS gene deletion at Xp22.31. The pregnancy is otherwise normal and the affected baby is a boy with ichthyosis	Very low, often undetectable or well under 0.15 MoM
Smith-Lemli-Opitz syndrome	Autosomal recessive defect of 7-dehydrocholesterol reductase, the last step of cholesterol synthesis. Without cholesterol the foeto-placental unit cannot make steroid precursors, so uE3 collapses — classically with a low AFP and low hCG as well	Very low, typically <0.3 MoM
Trisomy 18 (Edwards syndrome)	Foetal adrenal hypoplasia reduces the DHEA-sulfate supplied to the placenta	Low, with low AFP and low hCG
Foetal demise, anencephaly, congenital adrenal hypoplasia, placental aromatase or sulfatase defects	All interrupt the same foeto-placental precursor pathway	Low to undetectable
Maternal corticosteroid treatment	Suppresses foetal adrenal DHEA-sulfate production	Low — a benign and important explanation to exclude before investigating
Wrong dates	The commonest explanation of all	Apparently low; resolves when the gestation is corrected by ultrasound

A screening analyte that is not interpreted as a concentration

Unconjugated oestriol is reported in nanograms per millilitre, micrograms per litre, nanograms per decilitre and nanomoles per litre. Nanograms per millilitre and micrograms per litre are identical, nanograms per decilitre are a hundred times larger, and the molar conversion runs through oestriol’s molecular weight of 288.39: one nanogram per millilitre is 3.46753 nanomoles per litre, so 1.5 ng/mL is 5.20 nmol/L. Note that this is the unconjugated steroid; total oestriol, which includes the glucuronide and sulphate conjugates, is a much larger number from a different assay and is not interchangeable with it.

The arithmetic is the easy part, and it is also the part that matters least. Unconjugated oestriol is measured for one purpose in routine practice — as one of the four analytes of the second-trimester quadruple test, alongside alpha-fetoprotein, hCG and inhibin A — and in that test no result is ever interpreted as a concentration. Each analyte is converted to a multiple of the median for the exact gestational age, using the screening laboratory’s own medians and its own assay, and then corrected for maternal weight, ethnicity, diabetes, smoking, plurality and mode of conception before a risk is calculated from the pattern across all four. A raw uE3 number without a gestational age and a set of medians attached to it is close to meaningless, which is why this page prints no reference interval under the answer.

The reason gestation dominates so completely is that the population median uE3 rises steeply through the screening window, roughly trebling between 15 and 20 weeks. A concentration that is an unremarkable 1.0 MoM at 15 weeks is a markedly low MoM at 20 weeks. Incorrect dating is consequently the single commonest cause of an apparently abnormal screening result, and it is the reason gestational age is established by ultrasound rather than by menstrual dates wherever possible. In the quadruple test a low uE3 contributes to a raised risk of Down syndrome, at a median of about 0.73 MoM, and to a raised risk of trisomy 18, where it is lower still and accompanied by a low AFP and a low hCG.

Oestriol in pregnancy is synthesised by the placenta from 16α-hydroxy-DHEA-sulfate supplied largely by the foetal adrenal gland and hydroxylated in the foetal liver, so the measurement reads out an intact foeto-placental unit. That gives a very low or undetectable uE3 a specific differential of its own, separate from aneuploidy risk. Placental steroid sulfatase deficiency, the enzyme defect underlying X-linked ichthyosis, removes the step that liberates DHEA from its sulphate and characteristically produces an undetectable uE3 in an otherwise normal pregnancy. Smith-Lemli-Opitz syndrome, a recessive defect of the final step of cholesterol synthesis, starves the whole pathway of substrate and gives a very low uE3 with a low AFP and low hCG. Foetal demise, anencephaly, congenital adrenal hypoplasia and maternal corticosteroid treatment do the same thing by other routes. Before any of those is pursued, though, the dates should be checked and maternal steroid exposure excluded — because those two account for most of what looks at first like a low oestriol.

Frequently asked questions

How do you convert unconjugated oestriol from ng/mL to nmol/L?

Multiply by 3.46753, which is one nanogram per millilitre divided by oestriol’s molecular weight of 288.39 g/mol. So 1.5 ng/mL is 5.20 nmol/L, and dividing by the same factor goes back. Nanograms per millilitre and micrograms per litre are identical, and nanograms per decilitre are a hundred times larger. Note that this applies to unconjugated oestriol, not total oestriol, which is a different and much larger measurement.

Why does my report give a MoM instead of a concentration?

Because the concentration on its own does not carry the information. The population median unconjugated oestriol roughly trebles between 15 and 20 weeks of pregnancy, so the same number can be perfectly ordinary at one gestation and distinctly low at another. A multiple of the median is the measured value divided by the screening laboratory’s median for that exact gestational age, then corrected for maternal weight, ethnicity, diabetes, smoking and multiple pregnancy. Screening risks are calculated from MoM values across all four analytes — never from raw concentrations.

What is unconjugated oestriol measured for?

Almost exclusively for the second-trimester quadruple test, performed at about 15 to 20 weeks with alpha-fetoprotein, hCG and inhibin A. A low uE3 contributes to a raised risk estimate for Down syndrome and, more strongly, for trisomy 18. The test reports a risk rather than a diagnosis, detecting roughly 80% of Down syndrome pregnancies at about a 5% false-positive rate. Serial oestriol measurements were once used to monitor foeto-placental wellbeing in late pregnancy, but that use has been replaced by ultrasound and cardiotocography.

What does a very low or undetectable uE3 mean?

First, check the gestational dates and ask whether the mother is taking corticosteroids — those explain most such results. Beyond that, an undetectable uE3 in an otherwise normal-looking pregnancy points classically to placental steroid sulfatase deficiency, the X-linked condition that causes ichthyosis in an affected boy. A very low uE3 accompanied by a low AFP and low hCG raises the possibility of Smith-Lemli-Opitz syndrome, a recessive disorder of the last step of cholesterol synthesis. Trisomy 18, foetal demise, anencephaly and congenital adrenal hypoplasia also lower it, because oestriol synthesis depends on an intact foetal adrenal, foetal liver and placenta.

Is unconjugated oestriol the same as total oestriol?

No. Unconjugated oestriol is the free steroid and is the analyte used in prenatal screening; total oestriol includes the glucuronide and sulphate conjugates, is measured on a different assay, and gives a much larger number. Using one where the other is expected produces a nonsensical MoM. If a report does not say “unconjugated”, it is worth checking which was measured before converting or interpreting anything.

Related calculators

References

Wald NJ, Rodeck C, Hackshaw AK, Walters J, Chitty L, Mackinson AM. First and second trimester antenatal screening for Down’s syndrome: the results of the Serum, Urine and Ultrasound Screening Study (SURUSS). J Med Screen. 2003;10(2):56–104. doi:10.1177/096914130301000202
Craig WY, Haddow JE, Palomaki GE, et al. Identifying Smith-Lemli-Opitz syndrome in conjunction with prenatal screening for Down syndrome. Prenat Diagn. 2006;26(9):842–849. doi:10.1002/pd.1518
Bradley LA, Palomaki GE, Knight GJ, et al. Undetectable maternal serum unconjugated estriol levels in the second trimester: risk of perinatal complications associated with placental sulfatase deficiency. Am J Obstet Gynecol. 1997;176(3):531–535. doi:10.1016/S0002-9378(97)70543-2
American College of Obstetricians and Gynecologists. Practice Bulletin No. 226: Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol. 2020;136(4):e48–e69. doi:10.1097/AOG.0000000000004084

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.