Menstrual Cycle Overview

Overview

The menstrual cycle is a monthly process involving ovaries and endometrium from puberty to menopause.
It coordinates ovarian ovulation and uterine lining changes to enable potential fertilization and implantation.

Endometrium lines the uterus; supports embryo during pregnancy.
Two layers: functional layer sheds monthly; basal layer nourishes and regenerates functional layer.

Ovarian cycle: hormonal changes lead to ovulation, the egg’s release from an ovarian follicle.
Uterine cycle: endometrium thickens to prepare for implantation; sheds if no fertilization.

Average cycle length: 28 days with follicular (pre-ovulation) and luteal (post-ovulation) phases.
Follicular phase: before ovulation; ovarian follicles develop under hormonal stimulation.
Luteal phase: after ovulation; corpus luteum forms and secretes estrogen and progesterone.

Hypothalamus releases GnRH, triggering anterior pituitary to release FSH and LH.
FSH and LH stimulate ovarian follicles; rising estrogen thickens endometrium and creates negative feedback.
Ovulation: egg released; endometrium thickens further to support potential implantation.
Corpus luteum forms post-ovulation; secretes estrogen and progesterone to maintain lining.

No fertilization within about 14 days: corpus luteum collapses; estrogen and progesterone drop.
Hormone drop ends negative feedback; FSH rises; menstrual bleeding occurs for about 1–8 days.
Fertilization: embryo implants and releases HCG; maintains corpus luteum and high estrogen/progesterone.

Oral contraceptives and ring: keep estrogen and progesterone high to suppress ovulation and fertilization.
Plan B (morning after pill): causes endometrial shedding to prevent implantation of a fertilized egg.

Stage	Days (28-day cycle)	Key Structures	Dominant Hormones	Primary Events
Follicular (pre-ovulatory)	~Day 1–13	Hypothalamus, pituitary, ovarian follicles, endometrium	GnRH, FSH, LH, rising estrogen	Follicle growth; endometrium thickens; negative feedback limits excess FSH/LH
Ovulation	~Day 14	Mature follicle, ovary	LH surge, estrogen	Egg released; enters fallopian tube
Luteal (post-ovulatory)	~Day 14–28	Corpus luteum, endometrium	Estrogen, progesterone	Lining maintained and further thickened for implantation
No fertilization	End of luteal into next Day 1	Regressing corpus luteum, uterus	Falling estrogen, progesterone; rising FSH	Lining sheds; menstrual bleeding 1–8 days; cycle restarts
Fertilization and early pregnancy	Post-ovulation	Embryo, corpus luteum	HCG, sustained estrogen and progesterone	Implantation; corpus luteum maintained by HCG; cycle halts

Menstrual cycle: monthly ovarian and uterine changes enabling potential pregnancy.
Menarche: first menstrual period at puberty.
Menopause: cessation of menses around 50–55 years old.
Endometrium: uterine lining; functional layer sheds; basal layer regenerates.
Ovarian cycle: follicle development and ovulation.
Uterine cycle: endometrial growth and shedding.
Follicular phase: pre-ovulation phase with follicle growth.
Luteal phase: post-ovulation phase with corpus luteum activity.
GnRH: hypothalamic hormone stimulating FSH and LH release.
FSH: stimulates follicle growth in ovaries.
LH: triggers ovulation and supports corpus luteum formation.
Estrogen: thickens endometrium; mediates negative feedback on FSH and LH.
Progesterone: stabilizes and maintains the endometrium post-ovulation.
Corpus luteum: collapsed follicle secreting estrogen and progesterone.
HCG: embryonic hormone maintaining corpus luteum in early pregnancy.