The median length of the menstrual cycle is 28 days (range 21–40 days). The first day of menses is day 1, which marks the beginning of the follicular phase. Ovulation usually occurs on day 14. After ovulation, the luteal phase starts and lasts until the beginning of the next cycle.
The hypothalamus secretes gonadotropin-releasing hormone, which stimulates the anterior pituitary to secrete gonadotropins, follicle-stimulating hormone (FSH), and luteinizing hormone (LH).
In the follicular phase, FSH levels increase and cause recruitment of a small group of follicles for continued growth. Between days 5 and 7, one of these becomes the dominant follicle, which later ruptures to release the oocyte. The dominant follicle develops increasing amounts of estradiol and inhibin, providing a negative feedback on the secretion of gonadotropin-releasing hormone and FSH.
The dominant follicle continues to grow and synthesizes estradiol, progesterone, and androgen. Estradiol stops the menstrual flow from the previous cycle, thickens the endometrial lining, and produces thin, watery cervical mucus. FSH regulates aromatase enzymes that induce conversion of androgens to estrogens in the follicle.
The pituitary releases a midcycle LH surge that stimulates the final stages of follicular maturation and ovulation. Ovulation occurs 24 to 36 hours after the estradiol peak and 10 to 16 hours after the LH peak.
The LH surge is the most clinically useful predictor of approaching ovulation. Conception is most successful when intercourse takes place from 2 days before ovulation to the day of ovulation.
After ovulation, the remaining luteinized follicles become the corpus luteum, which synthesizes androgen, estrogen, and progesterone (Fig. 30–1).
If pregnancy occurs, human chorionic gonadotropin prevents regression of the corpus luteum and stimulates continued production of estrogen and progesterone. If pregnancy does not occur, the corpus luteum degenerates, progesterone declines, and menstruation occurs.
Menstrual cycle events, idealized 28-day cycle. (Hatcher RA, Trussell J, Nelson AL, et al. Contraceptive Technology. 21st ed. Ardent New York: Median, Inc.; 2015.)
(FSH, follicle-stimulating hormone; HCG, human chorionic gonadotropin; LH, luteinizing hormone.)
LH: 15 mIU/mL = 15 IU/L; 50 to 100 mIU/mL = 50 to 100 IU/L.
FSH: 10 to 12 mIU/mL = 10 to 12 IU/L; 25 mIU/mL = 25 IU/L.
Estrogen: 40 pg/mL = ~150 pmol/L; 250 to 400 pg/mL = ~920 to 1470 pmol/L; 125 to 250 pg/mL = ~460 to 920 pmol/L.
Progesterone: 1 ng/mL = 3 nmol/L; 10 to 15 ng/mL = ~30 to 50 nmol/L.
Temperatures: 99°F = 37.2°C; 98°F = 36.7°C; 97°F = 36.1°C.
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