β-hCG Levels by Week of Early Pregnancy
Quantitative β-hCG values have enormous individual variability and should never be used alone to establish gestational age or assess pregnancy viability—ultrasound dating by crown-rump length is the gold standard, and serial β-hCG measurements over 48 hours provide far more clinical value than any single absolute number. 1
Why Absolute β-hCG Values Are Unreliable for Dating
- The American College of Radiology explicitly recommends using ultrasound dating, not hCG levels, to establish accurate gestational age because hCG levels have poor accuracy for dating beyond 6 weeks 1
- A single β-hCG measurement cannot distinguish viable intrauterine pregnancy (median ≈1,304 mIU/mL), embryonic demise (≈1,572 mIU/mL), or ectopic pregnancy (≈1,147 mIU/mL) because the ranges overlap substantially 1, 2
- At 6 weeks gestation, viable pregnancies can range from as low as 1,094 mIU/mL to well over 25,000 mIU/mL, demonstrating enormous between-patient variability 3
General Pattern of β-hCG Throughout Pregnancy
- β-hCG becomes detectable 6–9 days after conception, with levels initially rising above 5 mIU/mL to confirm pregnancy 1, 4
- Levels rise rapidly in early pregnancy, peak around 8–12 weeks of gestation, then decline steadily through week 16 and beyond 1, 4
- Free β-hCG, intact hCG, and hyperglycosylated hCG all decrease by approximately 20–40% from 11 to 13 completed weeks 1, 4
Threshold Values for Ultrasound Correlation (Not Dating)
These thresholds indicate when structures can be seen, not when pregnancy should be a certain age:
- Gestational sac threshold: Visible as low as 390 mIU/mL in some viable pregnancies, but 99% visualization occurs only at ≥3,510 mIU/mL 3
- Yolk sac threshold: Can appear at 1,094 mIU/mL, but 99% visualization requires ≥17,716 mIU/mL 3
- Fetal pole threshold: May be seen at 1,394 mIU/mL, but 99% visualization requires ≥47,685 mIU/mL 3
- The traditional discriminatory threshold of 3,000 mIU/mL has virtually no diagnostic utility for predicting ectopic pregnancy (positive likelihood ratio 0.8, negative likelihood ratio 1.1) 1
Clinical Algorithm: How to Actually Use β-hCG
For Pregnancy of Unknown Location
- Obtain baseline quantitative serum β-hCG when pregnancy location cannot be confirmed by ultrasound 1
- Repeat β-hCG exactly 48 hours later—this interval is evidence-based for characterizing ectopic risk and viable pregnancy probability 1, 5
- Interpret the 48-hour change:
For Ultrasound Timing
- Schedule transvaginal ultrasound when β-hCG reaches 1,000–3,000 mIU/mL range, as a gestational sac is typically visible at these levels if intrauterine pregnancy is present 1
- Never defer ultrasound based on "low" β-hCG in symptomatic patients—approximately 22% of ectopic pregnancies occur at β-hCG <1,000 mIU/mL, and rupture can occur at any level 1, 2
Critical Pitfalls to Avoid
- Do not compare a patient's β-hCG to population averages or online calculators—only the 48-hour rate of rise is clinically meaningful 1
- Do not use β-hCG value alone to exclude ectopic pregnancy when ultrasound is indeterminate (Level B recommendation, American College of Emergency Physicians) 1
- Do not diagnose pregnancy failure based on a single low β-hCG—serial measurements and repeat ultrasound are required in hemodynamically stable patients 1
- Different hCG assays detect different isoforms with varying sensitivities; using the same laboratory for serial measurements is recommended 6
Special Considerations
- In IVF pregnancies, initial β-hCG values are higher: fresh day-3 embryos median 400 mIU/mL, frozen day-3 median 600 mIU/mL, frozen day-5 (blastocyst) median 937 mIU/mL at first measurement 7
- Markedly elevated β-hCG (>100,000 mIU/mL) at 6 weeks may indicate gestational trophoblastic disease or multiple gestation 1
- For aneuploidy screening at 11–13 weeks, β-hCG is reported as multiples-of-median (MoM) adjusted for gestational age and maternal factors, not as absolute values 1