HCG Levels by Week of Gestation
Plan B Does Not Affect HCG Levels in Established Pregnancy
Levonorgestrel (Plan B) is an emergency contraceptive that prevents pregnancy primarily by delaying ovulation, but it does not affect HCG levels if pregnancy has already been established. If conception has occurred, HCG levels will follow normal pregnancy patterns regardless of prior Plan B use. 1
Normal HCG Progression in Early Pregnancy
Detection and Early Levels
- HCG can be detected as early as 6 days after conception, with serum levels rising rapidly in early pregnancy 2
- Most qualitative urine pregnancy tests detect HCG at concentrations of 20-25 mIU/mL, though pregnancy detection rates vary based on timing relative to missed menses 1
- Serum HCG levels >5 mIU/mL indicate pregnancy, while levels ≤5 mIU/mL may occur in healthy non-pregnant individuals 1
Week-by-Week HCG Patterns
Weeks 4-5 (Days 28-35):
- A gestational sac becomes visible on transvaginal ultrasound when HCG reaches 1,000 mIU/mL 3
- At 32 days gestational age, a gestational sac should be present in all normal pregnancies 3
Weeks 5-6 (Days 36-42):
- A yolk sac becomes visible when HCG reaches 7,200 mIU/mL 3
- Between 36-40 days gestational age, a yolk sac should be visible in every normal pregnancy 3
- Some pregnancies may show a visible yolk sac with HCG levels between 1,000-7,200 mIU/mL 3
Weeks 6-7 (Days 42-49):
- Cardiac activity becomes detectable when HCG exceeds 10,800 mIU/mL 3
- After 40 days gestational age, an embryo with cardiac activity should be identified in all normal pregnancies 3
Weeks 8-10 (Days 56-68):
- HCG levels peak between 56-68 days of gestation, representing the highest concentrations during pregnancy 2
Week 18 and Beyond:
- HCG reaches its nadir (lowest point) at 18 weeks gestation 2
- No secondary rise in HCG occurs in the third trimester, contrary to findings from older, less specific immunoassays 2
Clinical Considerations
Discriminatory Thresholds for Ultrasound
- The traditional discriminatory threshold of 3,000 mIU/mL has virtually no diagnostic utility for predicting ectopic pregnancy (positive LR 0.8, negative LR 1.1) 1
- A threshold of 25,000 mIU/mL identifies 88% of intrauterine pregnancies on bedside ultrasound 1
- Median HCG levels do not significantly differ between intrauterine pregnancy (1,304 mIU/mL), embryonic demise (1,572 mIU/mL), or ectopic pregnancy (1,147 mIU/mL) at initial presentation with indeterminate ultrasound 1
Fetal Sex Differences
- Female fetuses are associated with higher maternal HCG levels as early as 2 weeks after conception, with this difference persisting throughout the first trimester 4, 5
- This sex-specific difference affects fetal growth patterns, with high HCG in late first trimester associated with increased growth in female but not male fetuses 4
Important Caveats
- HCG can remain elevated for several weeks after delivery or abortion (spontaneous or induced), potentially causing false-positive pregnancy tests 1
- Qualitative pregnancy tests may require an additional 11 days past expected menses to detect 100% of pregnancies 1
- Elevated HCG unrelated to pregnancy can occur with pituitary dysfunction or neoplasia 1