Pregnancy on Birth Control Pills: Failure Rates and Causes
Combined oral contraceptives (COCs) are >99% effective with perfect use, but typical-use failure rate is ~7% (7 in 100 women/year). Pregnancy can occur due to:
- Missed pills: Especially first-week misses or extending the hormone-free interval.
- Vomiting/diarrhea: Within 3–4 hours of ingestion reduces absorption.
- Drug interactions: Enzyme inducers (rifampin, certain anticonvulsants, St. John's wort) lower hormone levels.
- Obesity: Higher BMI may reduce efficacy.
- Rare ovulation breakthrough: Even with perfect use, follicular development occasionally occurs.
What to Do
If you suspect pregnancy, take a urine hCG test. If positive, stop the pill and contact your provider. If negative but symptoms persist, retest in a week. Consider long-acting reversible contraceptives (IUD, implant) for higher efficacy.
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