The menstrual cycle

What is it:

The time between the first day of a woman's period and the day before her next period. The average length of the cycle is 28 days and the normal range is from 24 - 35 days.

The menstrual cycle (~28 days):

Day 1 - Oestrogen and progesterone levels drop so the endometrium sheds and then begins the menstrual bleed. The hormone level dropping causes negative feedback and LH and FSH increase. These are made in the pituitary gland and the oestrogen and progesterone are made in the ovaries. Day 2/3/4 - The rise in FSH means that one of the immature follicles in the ovaries begins to mature as this stimulates them. The follicles produce oestrogen which starts to thicken the endometrium. Day 7 - This rise in oestrogen and FSH cause also cause a rise in LH so the follicle begins to fully develop so that a mature egg can be released. Day 14 - Ovulation happens around here and this is where LH is at its peak but then comes straight back down again. Day 15 - LH and FSH have decreased so the follicle, after the egg has been released, becomes a corpus luteum and then progesterone increases so the thickness of the endometrium is maintained. Day 16 - 28 - If fertilisation doesn't occur, the corpus luteum will deteriorate so progesterone and oestrogen will both drop causing a rise in FSH and the cycle will start again.

Simplified:

Menstrual bleed -> FSH increases and 10 - 12 follicles start to mature -> Oestrogen increases -> Endometrium starts to build up -> LH increases -> OVULATION -> Corpus luteum formed from an empty follicle -> Progest. and oestr. increase, LH and FSH decrease -> No fertilisation -> Progrest. and oestr. decrease -> Cycle begins again!

The different cycles:

OVARIAN CYCLE This is controlled by the pituitary hormones. UTERINE CYCLE Controlled by the ovarian hormones which is produced in the ovaries.

The hormones:

PITUITARY HORMONES These are LH (luteinising hormone) triggers ovulation and the development of a corpus luteum. FSH (follicle stimulating hormones) stimulates the growth of follicles in the ovary before the release of an egg. OVARIAN HORMONES Oestrogen causes the eggs in the ovaries to mature and the endometrium lining to thicken. Progesterone is produced to thicken the endometrium lining to prepare for implantation of a fertilised egg.