Each month between puberty and the menopause, a woman’s body goes through the menstrual cycle in preparation for conception and pregnancy. A mature egg is released, and the lining of the uterus (the endometrium) becomes thicker, ready for a fertilized egg to implant. If the egg is not fertilized, it passes out of the body during menstruation. The menstrual cycle lasts 28 days on average but may not be the same length every month and varies from woman to woman. The cycle is regulated by a complex interaction between four sex hormones. Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are produced in the pituitary gland, and oestrogen and progesterone are secreted by the ovaries.
Female reproductive system
Once a month, FSH causes an egg to mature and LH triggers its release. Just before ovulation, oestrogen levels peak. A rise in progesterone causes the lining of the uterus to thicken.
Inside the ovary
The cycle begins with an egg developing inside a follicle in the ovary. The mature egg is released into the fallopian tube, leaving the empty follicle, called the corpus luteum, in the ovary.
Hormones cause the endometrium to double in thickness, to about 6 mm (1/4 in). If fertilization does not occur, some of the endometrial tissue is shed as menstrual blood, together with the unfertilized egg.
By absorbing nutrients from cells inside the follicle, an immature egg is able to develop and grow to maturity.
After ovulation, the thickened endometrium tissue looks spongy and is ready to receive the fertilized egg.