Risk of Ectopic Pregnancy with Mirena After Failed Bilateral Fimbriectomy
In a patient with a history of bilateral fimbriectomy, the absolute risk of any pregnancy—including ectopic pregnancy—with Mirena is extraordinarily low because the fimbriectomy itself already provides highly effective contraception by preventing egg capture. 1
Understanding the Baseline Risk
The clinical context is critical here:
- Bilateral fimbriectomy removes the fimbriated ends of the fallopian tubes, which are essential for ovum capture, and this procedure alone provides highly effective contraception by preventing the egg from entering the fallopian tube 1
- The absolute risk of any pregnancy after complete bilateral fimbriectomy is already extremely low 1
- Additional contraception like Mirena is generally unnecessary if the fimbriectomy was complete and successful 1
Relative vs. Absolute Risk: A Critical Distinction
When pregnancy does occur with an IUD in place, there is an important statistical phenomenon:
- When a woman becomes pregnant during IUD use, the relative likelihood of ectopic pregnancy increases greatly because IUDs prevent intrauterine pregnancies more effectively than ectopic pregnancies 1
- Approximately 1 in 20 pregnancies (5%) will be ectopic when pregnancy occurs with an IUD in place 1
- However, the absolute risk remains extremely low because pregnancy itself is so rare with Mirena 1
Mirena's Contraceptive Efficacy Data
The actual pregnancy rates with Mirena are remarkably low:
- The 5-year failure rate is 0.5-1.1 per 100 users 2
- For extended use (years 6-8), the cumulative failure rate is only 0.68% 3
- The absolute number of ectopic pregnancies is low, as is the rate per 1000 users 2
- In the Mirena Extension Trial through 8 years, only 2 pregnancies occurred among 362 participants, with one being ectopic 3
Clinical Decision-Making for Your Patient
For a patient with bilateral fimbriectomy considering Mirena:
- Mirena may still be appropriate for managing heavy menstrual bleeding or dysmenorrhea, not for contraception 1
- The patient already has dual protection: the fimbriectomy prevents egg capture, and Mirena would provide additional contraceptive mechanisms (thickening cervical mucus, inhibiting sperm motility) 4
- The combined absolute risk of ectopic pregnancy is negligible given both mechanisms of protection
Important Caveats
Verify the fimbriectomy was complete and successful before counseling that additional contraception is unnecessary 1. If there is any uncertainty about the completeness of the surgical sterilization, treat the patient as having normal tubal anatomy for risk assessment purposes.
Counsel the patient to watch for signs of ectopic pregnancy (severe abdominal pain with unusual vaginal bleeding) if she experiences any symptoms suggesting pregnancy, though this risk is extraordinarily low with dual protection 5.