Normal Progesterone Levels in First Trimester Pregnancy
In viable first trimester pregnancies, serum progesterone levels typically range from approximately 57.5 nmol/L (18.3 ng/mL) at 5 weeks to 80.8 nmol/L (25.7 ng/mL) at 13 weeks, showing a linear increase throughout the first trimester. 1
Evidence-Based Reference Ranges
Normal Viable Pregnancy Values
Median progesterone levels increase linearly from 57.5 nmol/L to 80.8 nmol/L (approximately 18-26 ng/mL) between 5 and 13 weeks gestation in uncomplicated pregnancies 1
The mean serum progesterone in viable first trimester pregnancies is approximately 46.5 ± 7.4 ng/mL (146 nmol/L) in symptomatic women who ultimately maintain viable pregnancies 2
Asymptomatic women with viable pregnancies demonstrate mean progesterone levels of 22.1 ng/mL (69.4 nmol/L) in early first trimester 3
Critical Thresholds for Viability Assessment
Progesterone levels ≥20 ng/mL (62.8 nmol/L) are 98.9% specific for viable pregnancy, making this an excellent threshold for reassurance 2
Levels ≥10 ng/mL (31.4 nmol/L) are 93.3% specific for viable pregnancy, though less definitive than the 20 ng/mL cutoff 2
Progesterone <90.62 nmol/L (28.8 ng/mL) is associated with increased miscarriage risk, with each 1 nmol/L decrease below this threshold conferring a 3% increased risk of pregnancy loss 4
Important Clinical Caveats
Guideline Recommendations Override Single Values
ACOG and SMFM recommend against using progesterone supplementation based solely on low serum progesterone levels in asymptomatic women, as there is no evidence of effectiveness for preventing first trimester miscarriage 5
ACOG advises against relying on a single progesterone value in isolation and recommends correlating with ultrasound findings and clinical presentation 5
Local laboratory reference ranges should be consulted as values may vary between institutions 5
Progesterone Cannot Be Used as Absolute Cutoff
While progesterone <5 ng/mL is highly suggestive of nonviable pregnancy, case reports document viable pregnancies progressing with progesterone as low as 1.2 ng/mL 6
A single serum progesterone level <5 ng/mL is suggestive but not diagnostic of nonviable pregnancy, and routine intervention based solely on this threshold may interrupt desired viable pregnancies 6
Threatened Miscarriage Context
Women presenting with threatened miscarriage (bleeding/pain) have median progesterone levels approximately 10 nmol/L lower at every gestational week compared to asymptomatic women (41.7-78.1 nmol/L vs 57.5-80.8 nmol/L) 1
In threatened miscarriage, mean progesterone in those who ultimately miscarry is 9.9 ± 4.8 ng/mL versus 46.5 ± 7.4 ng/mL in those with ongoing viable pregnancies 2
Women who eventually miscarry show only marginal, non-significant increases in progesterone (19.0-30.3 nmol/L) from 5-13 weeks, contrasting with the linear rise seen in viable pregnancies 1
Practical Clinical Algorithm
For asymptomatic women:
- Progesterone ≥20 ng/mL (62.8 nmol/L): Highly reassuring for viability 2
- Progesterone 10-20 ng/mL (31.4-62.8 nmol/L): Correlate with ultrasound and clinical findings 5, 2
- Progesterone <10 ng/mL (31.4 nmol/L): High concern for nonviability, but confirm with serial hCG and ultrasound before intervention 5, 2
For symptomatic women (bleeding/pain):
- Progesterone ≥20 ng/mL: Reassuring despite symptoms 2
- Progesterone <10 ng/mL: 79.3% sensitive for nonviable pregnancy, but requires ultrasound confirmation 2
Critical pitfall: Never use progesterone alone to diagnose pregnancy failure or initiate intervention—always correlate with transvaginal ultrasound findings and serial hCG measurements 5, 6