A 28-year-old woman on combined oral contraceptive pills for 2 years reports breakthrough bleeding. Which of the following is the most common cause of breakthrough bleeding in OCP users?

Explanation

## Breakthrough Bleeding in OCP Users **Key Point:** **Missed pills or poor compliance** is the most common cause of breakthrough bleeding in OCP users, accounting for the majority of cases. ### Pathophysiology of Breakthrough Bleeding Breakthrough bleeding (unscheduled vaginal bleeding) occurs when: 1. **Missed or delayed pills** → Loss of gonadotropin suppression 2. **Follicular activity resumes** → Estrogen production from developing follicles 3. **Endometrial proliferation** → Unstable endometrium → Bleeding ### Differential Diagnosis of Breakthrough Bleeding | Cause | Frequency | Mechanism | Management | |-------|-----------|-----------|-------------| | **Missed/delayed pills** | Most common | Loss of contraceptive effect | Reinforce compliance | | Low progestin formulation | Common | Inadequate endometrial suppression | Switch to higher progestin dose | | Estrogen-induced proliferation | Rare | High-dose estrogen | Switch to lower estrogen dose | | Infection (STI, endometritis) | Occasional | Endometrial inflammation | Treat infection | | Endometrial pathology | Rare | Polyps, fibroids, malignancy | Imaging/biopsy | **High-Yield:** In a compliant user with breakthrough bleeding, consider **low-dose OCP formulation** and switch to a pill with higher progestin content. In a non-compliant user, the first step is **counseling on adherence**. **Clinical Pearl:** The first question to ask a woman with breakthrough bleeding on OCPs is: **"Are you taking your pills on time every day?"** — this simple history often reveals the diagnosis. **Mnemonic:** **MISSED** — **M**issed pills, **I**nfection, **S**uboptimal formulation, **S**tructure (endometrial pathology), **E**strogen/progestin imbalance, **D**rug interactions. [cite:Park 26e Ch 3]