Correct Answer: A. Progesterone
Progesterone is the dominant hormone acting on the post-ovulatory endometrium during the luteal phase. After ovulation, the corpus luteum forms from the remnants of the Graafian follicle and secretes progesterone in increasing amounts over 8–10 days. This hormone transforms the proliferative endometrium (built up by estrogen in the follicular phase) into a secretory endometrium—a process called the secretory transformation. Progesterone induces glandular tortuosity, increased vascularity, edema of the stroma, and accumulation of glycogen in the glands, preparing the endometrium for implantation. The endometrial changes are so characteristic that histological dating of the endometrium is based on progesterone-induced morphology. In clinical practice, the presence of secretory changes on endometrial biopsy confirms ovulation and adequate corpus luteum function. This is the hallmark of the luteal phase and is essential for establishing and maintaining early pregnancy.
Why the other options are wrong
B. Follicular stimulating hormone — FSH is a pituitary gonadotropin that acts on the follicle during the follicular phase to promote follicular growth and estrogen secretion. Its levels drop sharply after the LH surge and remain low during the luteal phase. FSH does not act on the endometrium; it has no direct endometrial effects. This is a distractor that confuses the hormonal axis with endometrial action. C. Oestrogen — Estrogen dominates the follicular phase and causes proliferative endometrial changes (glandular hyperplasia, increased vascularity). While estrogen levels rise slightly in the early luteal phase, progesterone is the dominant hormone post-ovulation. Estrogen alone cannot produce secretory transformation; progesterone is required. This option traps students who conflate 'endometrial hormone' with 'estrogen' without distinguishing phases. D. Luteinizing hormone — LH triggers the ovulatory surge and is critical for ovulation itself, but it does not act directly on the endometrium. LH stimulates the corpus luteum to produce progesterone, but LH is not the hormone acting on post-ovulatory endometrium—progesterone is. This is a common trap: confusing the hormone that triggers ovulation with the hormone that acts on the endometrium afterward.
High-Yield Facts
- Secretory endometrium is induced by progesterone in the luteal phase and is the hallmark of ovulation and corpus luteum function.
- Endometrial dating is based on progesterone-induced morphological changes (glandular tortuosity, stromal edema, glycogen accumulation) and is used to confirm ovulation.
- Progesterone levels peak at 7–8 days post-ovulation (mid-luteal phase) and decline if pregnancy does not occur, triggering menstruation.
- Estrogen acts in the follicular phase to produce proliferative endometrium; progesterone acts post-ovulation to produce secretory endometrium—these are sequential, not concurrent.
- Corpus luteum is the sole source of progesterone in the luteal phase and requires LH stimulation to maintain function.
Mnemonics
PEPS (Post-ovulation Endometrial Progesterone Secretion) Post-ovulation → Progesterone → Endometrial Secretory changes. Use this when asked about post-ovulatory endometrial changes. Follicular = Estrogen (Proliferation); Luteal = Progesterone (Secretion) Follicular phase: FSH ↑ → Estrogen ↑ → Proliferative endometrium. Luteal phase: LH ↑ → Corpus luteum → Progesterone ↑ → Secretory endometrium. Helps distinguish which hormone acts when.
NBE Trap
NBE pairs "post-ovulatory" with "LH" to trap students who confuse the hormone triggering ovulation with the hormone acting on the endometrium afterward. Similarly, pairing "endometrium" with "estrogen" exploits the misconception that estrogen is the only endometrial hormone.
Clinical Pearl
In Indian clinical practice, endometrial biopsy showing secretory changes confirms ovulation and adequate luteal function—critical in infertility workup. Absence of secretory changes suggests anovulation or luteal phase defect, guiding treatment with progesterone supplementation or ovulation induction agents like clomiphene citrate.
_Reference: DC Dutta's Textbook of Obstetrics (4th ed.), Ch. 2 (Menstrual Cycle); OP Ghai's Essentials of Obstetrics (3rd ed.), Ch. 1 (Menstrual Cycle and Ovulation)_