Menstrual cycle: definition and phases
The menstrual cycle refers to cyclically recurring changes in the female body, predominantly in the reproductive system, the external manifestation of which is bleeding from the genital tract (menstruation). The main changes occur simultaneously in the ovary and endometrium: in the ovary, they ensure the maturation of the oocyte and ovulation; in the endometrium, they prepare for embryo implantation.
Two phases are distinguished in the ovary:
- Follicular phase — growth of the follicle (folliculogenesis) and maturation of the oocyte, culminating in ovulation.
- Luteal phase — formation of the corpus luteum at the site of the ruptured follicle, which is a transient hormonally active structure.
Follicles at different stages of development are simultaneously present in the ovary: primordial, primary, secondary, tertiary (Graafian follicle, dominant follicle), and atretic follicles.
Ovulation and the corpus luteum
Ovulation is the rupture of the mature dominant follicle and the release of the oocyte into the abdominal cavity, and then into the uterine tube. It is accompanied by minor bleeding from the disrupted capillaries. Granulosa cells undergo luteinization: cytoplasmic volume increases, lipid inclusions appear, and the corpus luteum is formed.
The corpus luteum functions for about 14 days regardless of the cycle length. In the absence of pregnancy, it regresses; upon fertilization, it continues to develop, maintaining pregnancy through the production of progesterone.
Changes in the endometrium
Three phases are distinguished in the endometrium:
- Proliferative phase — under the influence of estrogens, the height of epithelial cells increases, the epithelium becomes multi-layered, the glands lengthen and coil, and the stroma becomes edematous and loose. The thickness of the endometrium reaches approximately 8 mm.
- Secretory phase — after ovulation, under the action of progesterone from the corpus luteum, proliferation is inhibited. Glycogen-containing vacuoles appear in cells, and secretions containing glycogen, glycoproteins, and glycosaminoglycans appear in the glands. Compact and spongy layers are distinguished within the functional layer. On days 20–21 of the cycle (6–7 days after ovulation), the implantation window is formed.
- Menstrual phase — in the absence of fertilization, involution of the corpus luteum occurs, and the levels of estrogens and progesterone drop. Spasm of the spiral arteries, ischemia, and necrosis of the functional layer occur, lysosomal proteases are released, vessels dilate, and tissue is shed. Prostaglandins play an important role. Regeneration from the basal layer is completed by days 4–5 of the cycle.
Five levels of regulation of the menstrual cycle
- Cerebral cortex and limbic system (hippocampus, amygdala) — secrete endogenous opioid peptides (enkephalins, endorphins) and neurotransmitters: norepinephrine, dopamine, GABA, acetylcholine, serotonin, melatonin.
- Hypothalamus — secretes liberins and statins; the gonadotropin-releasing hormone (GnRH) plays a key role.
- Adenohypophysis — synthesizes FSH, LH, prolactin, ACTH, GH, TSH.
- Peripheral endocrine organs — ovaries (primary role), adrenal glands, thyroid gland.
- Target organs — uterus, endometrium, fallopian tubes, vagina, mammary glands.
The levels are connected by feedback loops: long (sex hormones → hypothalamic-pituitary system), short (pituitary gland ↔ hypothalamus), and ultrashort (hypothalamus ↔ its own neurons).
Planes of the lesser pelvis
Four classic planes are distinguished:
- Plane of the pelvic inlet — separates the greater and lesser pelvis; boundaries: upper inner edge of the pubic symphysis, linea terminalis, tip of the sacral promontory.
- Plane of greatest pelvic dimensions (wide part) — anteriorly, the midpoint of the inner surface of the pubic symphysis; laterally, the midpoint of the plates closing the acetabula; posteriorly, the joint between the II and III sacral vertebrae.
- Plane of least pelvic dimensions (narrow part) — anteriorly, the lower edge of the pubic symphysis; laterally, the ischial spines; posteriorly, the sacrococcygeal joint.
- Plane of the pelvic outlet — consists of two planes intersecting along the line of the ischial tuberosities: anteriorly, the lower edge of the pubic symphysis; laterally, the inner surfaces of the ischial tuberosities; posteriorly, the tip of the coccyx.
Obstetric formulas
- Naegele's formula — calculation of the estimated due date (EDD): 3 months are subtracted from the first day of the last menstrual period and 7 days are added. Based on a 28-day cycle with ovulation on days 14–15 and an average pregnancy duration of 280 days (40 weeks).
- Lebedev's formula and Yakubova's formula — calculation of the estimated fetal weight using two parameters: abdominal circumference and fundal height.
Fetal lie in the uterine cavity
- Fetal attitude — the mutual disposition of the fetal parts. The flexed attitude is physiological: the head is flexed, the chin is pressed against the chest, and the arms and legs are flexed at the joints (fetal position).
- Fetal lie — the relation of the longitudinal axis of the fetus to the longitudinal axis of the uterus: longitudinal, transverse, oblique. In transverse lie, the large parts of the fetus are located equally above or below the line of the iliac crests; in oblique lie, one part is higher and the other is lower.
- Fetal presentation — the relation of the presenting part of the fetus to the pelvic inlet. In longitudinal lie, cephalic (normal) and breech presentations are distinguished.
- Fetal position — the relation of the fetal back to the lateral wall of the uterus: first (back on the left), second (back on the right).
- Fetal variety — the relation of the back to the anterior (anterior variety) or posterior (posterior variety) wall of the uterus.
Key points
- The menstrual cycle includes the follicular and luteal phases in the ovary; the proliferative, secretory, and menstrual phases in the endometrium.
- The corpus luteum functions for about 14 days and produces progesterone, which prepares the endometrium for implantation.
- The implantation window occurs on days 20–21 of the cycle.
- Regulation of the cycle occurs at five levels: cortex — hypothalamus — pituitary — ovaries — target organs, connected by long, short, and ultrashort loops.
- The lesser pelvis has four planes: the inlet, the wide part, the narrow part, and the outlet.
- Naegele's formula allows determining the EDD, while Lebedev's and Yakubova's formulas determine the estimated fetal weight.
- Fetal status is described by attitude, lie, presentation, position, and variety.
- The physiological status is a longitudinal lie with cephalic presentation and flexed attitude.