The Menstrual Cycle

The menstrual cycle is on average 28 days, although this is only a guide and often varies. There are four stages to the menstrual cycle: menstruation, the follicular phase, ovulation and the luteal phase.

MenstruationIllustration of Menstrual phase lasts from day 1-5 showing uterus shedding its inner lining and menstrual fluid flowing out of vagina - Menstrupedia

The thickened endometrium (lining of the uterus) leaves the body through the vagina; the menstrual flow contains blood, endometrial cells and mucus. This usually takes 3 – 7 days to complete.

 

Follicular phaseIllustration of Follicular phase lasts from day 1-13 showing an egg cell maturing in a follicle in one of the ovaries and endometrium begins to develop in the inner surface of the uterus - Menstrupedia

This phase commences on the first day of menstruation and finishes with ovulation. The pituitary gland releases the follicle stimulating hormone (FSH) which stimulates the ovary to create several follicles containing immature eggs; one of these follicles will mature into an ovum. The endometrium thickens in preparation for pregnancy. The level of oestrogen also rises due to the developing follicle.

OvulationIllustration of Ovulation phase day 14 showing an egg being released from the ovary and enters the fallopian tube. Fimbriae of the fallopian tube is labeled - Menstrupedia

Two weeks before the period is due the mature egg is released from the ovary; stimulated by the rise in oestrogen, the hypothalamus releases gonadotrophin-releasing hormone (GnRH) which prompts the pituitary gland to produce FSH and luteinising hormone (LH). High levels of LH then trigger ovulation; the egg ruptures the follicle, leaves the ovary and travels down the fallopian tube towards the uterus.

Luteal phaseIllustration of Luteal phase lasts from day 15-28 showing a fully developed endometrium in the uterus. If the egg cell is not fertilized, this phase leads to the menstrual phase of the next cycle - Menstrupedia

The ruptured follicle develops into a corpus luteum. The corpus luteum then begins to release progesterone and small amounts of oestrogen. These hormones maintain the thickened endometrium in the hope that a fertilised egg will implant. If an egg does not implant, then the corpus luteum dies and progesterone levels drop which triggers menstruation.

 

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