A 28-year-old woman from Delhi presents to the infertility clinic with a 3-year history of primary infertility. Her husband's semen analysis is normal. Pelvic examination is unremarkable. Transvaginal ultrasound shows a normal uterus and ovaries with antral follicle count of 8 per ovary. Serum FSH on day 3 is 7.2 mIU/mL, LH 6.8 mIU/mL, and prolactin is normal. She has regular 28-day cycles. She is counseled about ovulation induction and started on clomiphene citrate 50 mg daily from day 3 to day 7. On day 12, transvaginal ultrasound shows a single dominant follicle of 18 mm with endometrial thickness of 7 mm. What is the most appropriate next step?

Explanation

## Clinical Assessment This patient has: - Normal ovulatory cycles with regular menses - Normal basal FSH (7.2 mIU/mL) - Normal antral follicle count (8 per ovary) - Adequate response to clomiphene citrate (dominant follicle 18 mm on day 12) - Suboptimal endometrial thickness (7 mm) — a known side effect of clomiphene ## Mechanism of Ovulation Induction **Key Point:** Clomiphene citrate is a selective estrogen receptor modulator (SERM) that blocks negative feedback at the hypothalamus and pituitary, increasing endogenous FSH and LH secretion. It induces monofollicular or oligo-follicular development in ovulatory women. **High-Yield:** Once a dominant follicle reaches 18–20 mm with appropriate endometrial thickness (ideally ≥8 mm, though 7 mm is borderline acceptable), exogenous hCG is administered to trigger the LH surge and final oocyte maturation. This mimics the natural LH surge. ## Rationale for hCG Trigger 1. Follicle size is adequate (18 mm = mature preovulatory follicle) 2. Timing of hCG allows ovulation 36–40 hours later, permitting timed intercourse or IUI 3. Endometrial thickness, though suboptimal, is not a contraindication to proceeding 4. Increasing clomiphene dose risks ovarian hyperstimulation and further endometrial thinning **Clinical Pearl:** Clomiphene-induced endometrial thinning is a known adverse effect due to prolonged estrogen receptor blockade in the endometrium. Despite this, pregnancy can still occur; the threshold for concern is typically <6 mm. ## Why Not the Other Options? - **Option 1 (Correct):** Standard protocol for triggering ovulation once follicle is mature - **Option 2:** Increasing dose risks hyperstimulation and worsens endometrial thinning - **Option 3:** Switching to gonadotropins is premature; clomiphene has worked adequately - **Option 4:** IUI without hCG trigger loses the benefit of synchronized ovulation timing [cite:Textbook of Reproductive Medicine, Rao et al. Ch 8]