Higher Biology - Physiology and Health
Human Menstrual Cycle
In human females, the cycle takes roughly 28 days. The first day of menstruation regarded as day one.
In the first half of the cycle (the follicular phase), the follicle stimulating hormone (FSH) secreted by the pituitary gland, stimulates the production of a follicle in the ovary which produces oestrogen.
Oestrogen stimulates the lining of the uterus (the endometrium) to proliferate in preparation for a pregnancy.
Approximately half-way through the cycle, oestrogen levels peak which stimulates a surge of luteinising hormone (LH) from the pituitary. The surge of LH triggers the release of the egg at ovulation.
In the second half of the cycle (the luteal phase), the remains of the ovulatory follicle develop into the corpus luteum which secretes progesterone.
Progesterone maintains the uterine lining and promotes further vascularisation of the endometrium in preparation for the implantation of a blastocyst if the egg has been fertilised.
In the normal cycle (non-pregnant), increasing levels of progesterone from the corpus luteum cause LH levels to decrease via negative feedback on the pituitary. This decrease in LH leads to the degeneration of the corpus luteum which in turn results in a decrease in progesterone levels. As progesterone levels decrease, the endometrium begins to breakdown and subsequently menstruation begins. Then the cycle starts again.
If a pregnancy is established, the corpus luteum is rescued from degeneration by a hormone called human chorionic gonadotrophin (hCG) which is produced by cells of the early embryo – hCG is what is measured in pregnancy tests. The corpus luteum continues to produce progesterone to support the endometrium and prevent menstruation.