As you can see, the monthly cycle is governed by the reproductive hormones, the main ones being oestrogen, progesterone, follicle stimulating hormone (FSH) and luteinising hormone (LH).
What are hormones?
Hormones are chemical messengers and the word conies from a Greek word meaning ‘urge on’. Carried in the bloodstream, they trigger activity in different organs and body parts. The reproductive hormones control the monthly cycle and help to maintain pregnancy.
At the beginning of each menstrual cycle, the oestrogen and progesterone levels are low and the follicle stimulating hormone (FSH) is produced by the pituitary gland in the brain which controls the whole endocrine (hormone) system.
This begins the process of ovulation by stimulating the ovaries to produce the hormone, oestrogen.
Oestrogen is not just one hormone but several grouped together. But for the sake of clarity I will use the term oestrogen to include all of them. Oestrogen is the key hormone responsible for a woman maturing from childhood to adulthood. It causes the breasts to develop and produces the characteristic feminine shape.
The lining of the womb (uterus) starts to thicken each month as it prepares to receive a fertilised egg.
The oestrogen level continues to rise until the middle of the month when the pituitary gland produces luteinising hormone (LH) which triggers ovulation. The egg (ovum) is then released from a follicle in the ovary and passes down the fallopian tube.
After ovulation, the ovaries produce progesterone, which prevents any further ovulation taking place in that cycle. If fertilisation does not occur, the lining of the womb breaks down and menstruation takes place. At the same time there is a dramatic and rapid fall in the levels of oestrogen and progesterone and, with this drop in hormone levels, the cycle starts all over again.
When fertilisation occurs the egg implants itself into the thick and nourishing wall of the womb, where it begins to develop.
Fertilisation usually takes place in the fallopian tube. Once this has happened, the empty follicle, which released the egg, forms the corpus luteum which produces progesterone.
In each menstrual cycle, a group of about 20 follicles containing the developing eggs grows on the surface of the ovary. Generally only the biggest follicle continues to develop, which is why humans usually only have one baby at a time.
Progesterone is an important hormone in fertility because it maintains the womb lining during the second half of the cycle, in readiness for a fertilised egg. It is also responsible for maintaining the pregnancy.