Medical Abbreviations

Menstrual cycle is a series of natural events that occurs within a healthy woman’s body as it prepares the uterus for the possibility of pregnancy each month.In this process uterus grows and sheds a lining which could support the development of a fertilised egg.
Normal duration of menstrual cycle is 28 days. It’s duration can vary from 21-35 days. Thelarche (development of breast tissues) is considered generally the representation of gonadarche. In general, menarche (onset of menstrual cycle) occurs 24 months after thelarche.

Menstrual cycle comprises of ovarian cycle and uterine cycle. Ovarian cycle is regular cyclical growth of follicles and also release of ovum. Uterine cycle is regular cyclical growth of endometrium and it’s shedding in case of failed pregnancy.

Ovarian cycle consists of 2 phases:

• Follicular Phase- duration varies

• Luteal Phase- has fixed duration of 14 days

Uterine cycle consists of:

• Proliferative endometrium

• Secretory endometrium

Ovarian and endometrial cycles are described in the figure below:-

Ovarian follicle comprises of theca cells on outside and granulosa cells on inside. Combined action of FSH, LH, theca cells and granulosa cells synthesize estrogen. Progesterone is synthesized by corpus luteum (followed by placenta in later phase of pregnancy).

Life span of endometrium in non pregnant state is fixed i.e., 10-12 days. If pregnancy doesn’t occur during this time, luteolysis (apoptosis) occurs causing progesterone withdrawal leading to endometrial shed off.

Changes during menstrual cycle according to changes in hormone level and HPO axis (hypothalamo- pituitary- ovarian axis) :

• FSH secreted from gonadotrophs acts on primordial follicles which promote growth of follicles.

• Granulosa cells of follicles start secreting estradiol in low amounts.

• Granulosa cells also secrete inhibin-B which causes negative feedback on pituitary leading to decline in levels on FSH.

• Dominant follicle which has maximum number of FSH receptor continues to grow despite of falling FSH levels but other smaller follicles die due to lack of enough FSH receptors on them.

• Dominant follicle secretes estrogen in high amount causing estradiol peak.

• Estradiol in high amount causes positive feedback on pituitary causing LH surge and LH peak.

• LH surge triggers ovulation.

• Ovulation causes formation of corpus luteum.

• Corpus luteum synthesizes progesterone in high amounts leading to progesterone peak.

• Corpus luteum also synthesizes inhibin-A which causes negative feedback on pituitary (similar to inhibin-B) and LH level starts to decline.

• Corpus luteum also secrete estrogen in some amount causing estradiol peak again.

• After luteolysis, progesterone withdrawal occurs.

• Luteolysis causes positive feedback on pituitary leading to increase in FSH level again for the next cycle.

The above explained concept is now represented in diagrammatic form for better understanding.