HCG Levels at Week 1 and Week 2 of Pregnancy After Plan B Use
Plan B (levonorgestrel) does not affect HCG levels if pregnancy has already been established—HCG follows normal pregnancy patterns regardless of prior emergency contraception use. 1
Understanding the Timeline
When discussing "week 1" and "week 2" of pregnancy, clarification is essential:
- Gestational age dating (standard obstetric dating) starts from the first day of the last menstrual period, meaning "week 1-2" would be before conception even occurs 2
- Post-conception dating is more relevant here—actual weeks after fertilization and implantation 2
Expected HCG Levels in Very Early Pregnancy
At Implantation (Day 6-12 Post-Conception)
- HCG becomes detectable in serum at approximately 6-12 days after conception, when the blastocyst implants 1
- Serum HCG levels >5 mIU/mL indicate pregnancy, while levels ≤5 mIU/mL may occur in healthy non-pregnant individuals 1
- Most qualitative urine pregnancy tests detect HCG at concentrations of 20-25 mIU/mL 1
Week 1 Post-Conception (Approximately Day 7-14)
- Day 16 post-conception (end of first week of implantation): median HCG in viable pregnancies is approximately 182-223 mIU/mL 3
- Low HCG levels (<25th percentile, approximately <182 mIU/mL) at day 16 are associated with 16.7% miscarriage risk, even after cardiac activity is confirmed 3
- HCG levels can range from as low as 357 mIU/mL to over 1,800 mIU/mL in normal early pregnancies 4
Week 2 Post-Conception (Approximately Day 14-21)
- A gestational sac becomes visible on transvaginal ultrasound when HCG reaches 1,000-2,000 mIU/mL 2, 5
- By HCG level of 1,800 mIU/mL or greater, a gestational sac is consistently detected on ultrasound in normal intrauterine pregnancies 4
- HCG doubles approximately every 48-72 hours in viable early pregnancy, with the minimal rise being 53% over 48 hours 6
Critical Clinical Considerations
HCG Kinetics Matter More Than Single Values
- A single HCG measurement has limited diagnostic value—serial measurements 48 hours apart provide more meaningful clinical information 2
- The minimal rise for a viable intrauterine pregnancy is 24% at 1 day and 53% at 2 days 6
- The median slope for HCG rise is 50% after 1 day, 124% after 2 days, and 400% after 4 days 6
Important Pitfalls to Avoid
- Do not assume negative pregnancy based on timing alone—most qualitative pregnancy tests require an additional 11 days past expected menses to detect 100% of pregnancies 1
- HCG can remain elevated for several weeks after pregnancy termination (spontaneous or induced), potentially causing false-positive tests 1
- Different HCG assays have varying sensitivities—using the same laboratory for serial measurements is recommended 2
- 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
When to Obtain Serial HCG Testing
- Obtain repeat serum HCG in exactly 48 hours when initial ultrasound is indeterminate or pregnancy location cannot be confirmed 2
- Continue serial measurements until HCG rises to a level where ultrasound can confirm intrauterine pregnancy (>1,000-1,500 mIU/mL) 2
- Arrange specialty consultation if HCG plateaus (defined as <15% change over 48 hours) for two consecutive measurements 2
Age-Related Considerations
- Serum HCG increases with age in non-pregnant women—women >55 years can have HCG up to 13.1 IU/mL without pregnancy 7
- A cutoff of 14.0 IU/mL should be used when interpreting HCG results in women >55 years of age 7
- Pregnancy is unlikely in perimenopausal women (41-55 years) with HCG between 5.0-14.0 IU/mL if serum FSH is >20.0 IU/mL 7