Plan B and HCG Levels: Direct Answer
Plan B (levonorgestrel) does not affect HCG levels at any time point, including after 1 month, because it works by preventing ovulation before fertilization occurs and does not interfere with existing pregnancy or HCG production. 1
Mechanism of Plan B and HCG
Levonorgestrel emergency contraception works exclusively before ovulation and fertilization occur. The FDA-approved mechanism shows that levonorgestrel blocks or delays the release of an egg from the ovary, and it will not work if you are already pregnant. 1 Specifically:
- Plan B prevents pregnancy by stopping ovulation before it happens—it does not affect an existing pregnancy or HCG production. 1
- If fertilization and implantation have already occurred, Plan B has no effect on the pregnancy or HCG levels. 1, 2
- There is no evidence for an anti-implantation effect of levonorgestrel, meaning it cannot disrupt an established pregnancy or alter HCG production. 2
Expected HCG Levels in Normal Pregnancy
If pregnancy has occurred (meaning Plan B failed or was not taken), here are the expected HCG levels:
At 4 Weeks Gestation (from LMP)
- A gestational sac of 1-3 mm can be detected at approximately 31 days (4.4 weeks) with a mean HCG level of 730 IU/L (range 467-935 IU/L). 3
- HCG levels at this stage typically range from approximately 500-1,000 IU/L. 3
At 6 Weeks Gestation
- The yolk sac becomes visible at approximately 36 days (5.1 weeks) with a mean HCG level of 4,130 IU/L (range 1,120-7,280 IU/L). 3
- Fetal heart motion becomes visible at approximately 41 days (5.9 weeks) with a mean HCG level of 12,050 IU/L (range 5,280-22,950 IU/L). 3
- By 6 weeks, HCG levels typically range from 5,000-20,000 IU/L. 3
At 8 Weeks Gestation
- HCG levels peak around 8-11 weeks of pregnancy, typically reaching 50,000-100,000 IU/L or higher. 4
- An intrauterine pregnancy should be clearly visible on transvaginal ultrasound when HCG exceeds 3,000 IU/mL. 4
Clinical Interpretation After Plan B Use
If you took Plan B and are testing HCG one month later:
- A negative HCG test means Plan B successfully prevented pregnancy. 1
- A positive HCG test means pregnancy occurred despite Plan B use (failure rate is approximately 1-2 out of 8 women who would have gotten pregnant). 1
- The HCG level itself reflects only the pregnancy status and gestational age—Plan B has no effect on the HCG number. 1, 2
Important Clinical Pitfall
Do not confuse the timing of when Plan B was taken with when pregnancy testing should occur. You should get a pregnancy test if your period is delayed beyond 1 week after the expected time, as this may indicate Plan B failed and pregnancy occurred. 1 The HCG levels at that point will reflect the actual gestational age of the pregnancy, not any effect from the levonorgestrel itself. 1, 3
In perimenopausal women over age 40, baseline HCG can be slightly elevated (up to 7.7 IU/L) even without pregnancy, which is a separate consideration unrelated to Plan B use. 5