Obstetric Physiology Flashcards
To outline the basic physiology underlying obstetrics.
What controls the menstrual cycle?
The cycle is controlled by the ‘hypothalamic-pituitary-ovarian (HPO) axis’. Pulsatile production of gonadotrophin-releasing hormones by the hypothalamus stimulates the pituitary to produce the gonadotrophins: follicle stimulating hormone (FSH) and luteinizing hormone (LH). These stimulate the ovary to produce oestrogen and progesterone. The ovarian hormones modulate the production of gonadotrophins by feeding back on the hypothalamus and pituitary.
Describe the proliferative phase of the menstrual cycle.
Day 1 of the cycle is the first day of menstruation. Cycle lengths vary greatly (eg 20-45 days in adolescence); only 12% are 28 days. Cycles soon after menarche and before the menopause are most likely to be irregular and anovulatory. In the first 4 days of the cycle, FSH levels are high, stimulating the development of a primary follicle in the ovary. The follicle produces oestrogen, which stimulates the development of a glandular ‘proliferative’ endometrium and of cervical mucus which is receptive to sperm. The mucus becomes clear and stringy (like raw egg white) and if allowed to dry on a slide produces ‘ferning patterns’ due to its high salt content. Oestrogen also controls FSH and LH output by positive and negative feedback.
Describe the secretory phase of the menstrual cycle.
14 days before the onset of menstruation (on the 16th day of the cycle of a 30-day cycle) the oestrogen level becomes high enough to stimulate a surge of LH. This stimulates ovulation. Having released the ovum, the primary follicle then forms a corpus luteum and starts to produce progesterone. Under this influence, the endometrial lining is prepared for implantation: glands become convoluted (‘secretory phase’). The cervical mucus becomes viscid and hostile to sperm and no longer ferns. If the ovum is not fertilized the corpus luteum breaks down, so hormone levels fall. This causes the spiral arteries in the uterine endothelial lining to constrict and the lining sloughs—hence menstruation.
What is menstruation?
Menstruation
is the loss of blood and uterine epithelial slough; it lasts 2-7 days and is usually heaviest at the beginning. Normal loss is 20-80mL (median 28mL).
What is the climacteric?
The ovaries fail to develop follicles. Without hormonal feedback from the ovary, gonadotrophin levels rise. Periods cease (menopause), usually at ˜50 years of age.
How could you postpone menstruation (eg. on holiday)?
Try norethisterone 5mg/8h PO from 3 days before the period is due until bleeding is acceptable, or take 2 packets of combined contraceptive Pills without a break.
