Definition and criteria of a normal menstrual cycle
The menstrual cycle is a recurring physiological process in the body of a woman of reproductive age aimed at preparing for a potential pregnancy. The conventional boundaries of the reproductive period are from 14 to 45–50 years.
According to the criteria of the International Federation of Gynecology and Obstetrics (FIGO, 2018), a normal cycle is characterized by the following parameters:
- duration: from 24 to 38 days;
- fluctuations in duration within 8 days are acceptable;
- duration of menstruation: 3–7 days;
- the cycle must be ovulatory and adequately provided with hormones.
The first day of the cycle is considered the first day of menstrual bleeding. The term "menstrual cycle" encompasses three parallel processes: the ovarian, uterine, and cervical cycles.
Ovarian cycle and folliculogenesis
The ovarian cycle includes three phases: follicular, ovulation, and luteal.
Follicles are formed during the intrauterine period in the amount of 2–3 million. By the time of birth, they are represented by primordial follicles awaiting menarche. Stages of follicular development:
- The primordial follicle is surrounded by a layer of granulosa cells that synthesize estrogens.
- The primary follicle is formed under the influence of local growth factors; granulosa cells enlarge.
- The secondary follicle acquires a theca cell layer.
- The dominant follicle develops under the influence of FSH; a cavity containing follicular fluid with estrogens forms inside.
- The preovulatory follicle has a dense wall; the oocyte is surrounded by the cumulus oophorus — a cluster of cells supporting its nutrition.
Against the background of the LH surge, ovulation occurs — the release of the oocyte from the follicle around the 14th day. At the site of the ruptured follicle, the corpus luteum is formed from granulosa and theca cells, producing progesterone. If fertilization does not occur, it undergoes involution, turning into the corpus albicans.
Uterine and cervical cycles
The uterine cycle includes three phases:
- Desquamation phase (menstruation) — shedding of the functional layer of the endometrium, lasting 6–7 days.
- Proliferative phase — under the influence of estrogens, uterine glands enlarge, spiral arteries elongate, and the endometrium thickens.
- Secretory phase — under the influence of progesterone, the endometrium becomes "lush" and ready for implantation.
Endometrial shedding occurs due to a drop in hormone levels: spasm and ischemia of the spiral arteries occur, vessels rupture, which provokes the detachment of the functional layer.
The cervical cycle is reflected in changes in the cervix: during ovulation, it rises, softens, and becomes moist. Symptoms of ovulation:
- The "pupil" sign — opening of the external os with a drop of mucus;
- The "fern" sign — crystallization of cervical mucus on a glass slide under a microscope;
- Spinnbarkeit of cervical mucus — stretching by more than 10 cm without breaking.
Female Sex Hormones
Estrogens are represented by three fractions:
- Estradiol is the main estrogen of the reproductive period, regulates the cycle, stimulates the development of reproductive organs, mammary glands, and secondary sex characteristics;
- Estriol is a pregnancy hormone, actively synthesized from the 25th week, reflecting the state of the fetoplacental complex;
- Estrone predominates after menopause, partially replacing the functions of estradiol.
Progesterone is synthesized by the corpus luteum and placenta; prepares the endometrium for implantation, supports pregnancy, and suppresses myometrial contractility and ovulation.
FSH (follicle-stimulating hormone) initiates follicular growth, selection of the dominant follicle, and stimulates estrogen synthesis by granulosa cells. LH (luteinizing hormone) with a surge in the middle of the cycle induces ovulation and maintains corpus luteum function.
Prolactin stimulates lactation, inhibits ovulation, and affects the duration of the luteal phase; premenstrual breast tenderness is associated with it. AMH (anti-Müllerian hormone) is synthesized by granulosa cells of primordial follicles, reflects ovarian reserve, and reduces the sensitivity of growing follicles to FSH, protecting them from excessive depletion. Testosterone regulates libido, muscle mass, and bone density; its excess leads to hyperandrogenism (e.g., in polycystic ovary syndrome).
Five-tier system of hormonal regulation
Regulation of menstrual function is hierarchical:
- Cerebral cortex — perceives environmental influences;
- Hypothalamus — produces liberins and statins, including gonadotropin-releasing hormone (GnRH) in a circannual/pulsatile rhythm;
- Adenohypophysis — secretes FSH, LH, and prolactin;
- Ovaries — produce estrogens, progesterone, androgens, inhibin, and growth factors;
- Target organs and tissues — contain receptors for sex hormones.
The system operates on a feedback principle: an increase in estrogen and progesterone levels inhibits the secretion of GnRH in the hypothalamus and gonadotropins in the pituitary gland. A drop in sex hormone levels again stimulates the release of GnRH, and the cycle resumes.
Key points
- Normal cycle according to FIGO 2018: 24–38 days, menstruation 3–7 days, ovulatory cycle.
- The menstrual cycle combines the ovarian, uterine, and cervical cycles.
- Follicular development: primordial → primary → secondary → dominant → preovulatory.
- Estradiol regulates the cycle, estriol is the pregnancy hormone, and estrone predominates in menopause.
- Progesterone is synthesized by the corpus luteum and prepares the endometrium for implantation.
- AMH reflects ovarian reserve and protects follicles from excessive depletion.
- Symptoms of ovulation: "pupil", "fern", cervical mucus spinnbarkeit >10 cm.
- Regulation is implemented at five levels with a negative feedback mechanism.