A 32-year-old woman from Mumbai attends the fertility clinic with a history of infertility for 2 years. She has regular 28-day menstrual cycles with normal flow. Basal body temperature charting shows a biphasic pattern with a temperature rise of 0.5°C in the luteal phase. Serum progesterone measured on day 21 of her cycle is 8 ng/mL (normal luteal phase: >10 ng/mL). Transvaginal ultrasound on day 12 shows a dominant follicle of 18 mm with adequate endometrial thickness. What is the most likely diagnosis?

Explanation

## Diagnosis: Luteal Phase Defect (LPD) **Key Point:** Luteal phase defect is characterized by inadequate progesterone secretion from the corpus luteum, despite evidence of ovulation. Serum progesterone <10 ng/mL on day 21 (7 days post-ovulation) is diagnostic. ### Pathophysiology of the Luteal Phase After ovulation, the corpus luteum secretes progesterone to prepare and maintain the endometrium for implantation. The corpus luteum is maintained by: 1. **LH stimulation** — LH from the anterior pituitary drives progesterone synthesis 2. **hCG support** — After implantation, human chorionic gonadotropin (hCG) from the trophoblast sustains the corpus luteum In luteal phase defect, the corpus luteum produces insufficient progesterone, leading to: - Inadequate endometrial secretory changes - Poor receptivity for implantation - Early miscarriage if conception occurs - Infertility ### Clinical and Investigative Findings in This Case | Feature | Finding | Interpretation | |---------|---------|----------------| | **Menstrual cycle** | Regular 28 days | Normal cycle length | | **BBT charting** | Biphasic with 0.5°C rise | Evidence of ovulation | | **Day 12 ultrasound** | 18 mm dominant follicle | Normal follicular development | | **Day 21 progesterone** | 8 ng/mL (low) | **Inadequate luteal phase** | | **Endometrial thickness** | Adequate | Adequate structural support | **High-Yield:** The diagnosis of LPD requires: 1. Evidence of ovulation (biphasic BBT, LH surge, or ultrasound-documented ovulation) 2. Low serum progesterone in the mid-luteal phase (day 21 of 28-day cycle, or 7 days post-ovulation) 3. Infertility or recurrent miscarriage ### Mechanism of LPD LPD may result from: - **Inadequate LH secretion** — Insufficient pituitary LH stimulation of the corpus luteum - **Corpus luteum dysfunction** — Poor response of the corpus luteum to LH - **Shortened luteal phase** — Early luteolysis (regression of the corpus luteum) - **Hyperprolactinemia** — Suppresses LH pulsatility, reducing corpus luteum support **Clinical Pearl:** Progesterone supplementation (micronized progesterone 200–400 mg daily, or intramuscular progesterone 50–100 mg) in the luteal phase can support the endometrium and improve pregnancy rates in LPD. **Mnemonic for Luteal Phase Defect Causes:** **HIC** — **H**yperprolactinemia, **I**nadequate LH, **C**orpus luteum dysfunction [cite:Harrison 21e Ch 405; Endocrinology Board Review]