Cycle tracking
Ovulation is the release of a mature oocyte (eggs) from the ovary. Usually only one oocyte is released per month. Oocytes are found in the ovaries in a very immature form and are not capable of being fertilised by a sperm. At the time of ovulation, they undergo a maturing process which culminates in their release from the ovary. The maturation of oocytes and ovulation is stimulated by two hormones secreted by the pituitary, a gland at the base of the brain, follicle stimulating hormone (FSH) and luteinising hormone (LH). It is important these two hormones are produced in appropriate amounts throughout the monthly cycle for normal ovulation to occur.

Ovulation is usually confirmed absolutely by a subsequent positive pregnancy test. However, a number of changes in blood hormone concentrations and the appearance of the ovaries in an ultrasound picture can provide strong evidence that ovulation will or has occurred.

The female sex hormone oestrogen is produced by the cells surrounding a maturing oocyte within the ovary. As the oocyte matures more oestrogen is produced, reaching a peak level about two days before ovulation. If more than one oocyte matures simultaneously, the oestrogen produced by the ovary is greatly increased. Oestrogen levels can be measured in blood tests and its effects on the body are usually obvious, particularly on the amount and consistency of mucus discharged from the vagina. As the oestrogen level increases, the amount of mucus increases. This mucus is stringy and has the appearance and consistency of raw egg white.

As the oocyte matures a cyst called a follicle develops on the ovary. This follicle, which can be seen and measured on an ultrasound picture of the ovaries, may grow to about 2cm in diameter just before ovulation. Serial ultrasound pictures are another way of detecting ovulation.

Ovulation is triggered by a surge of Luteinising Hormone (LH) from the pituitary gland. LH also stimulates the ovary to begin producing the hormone progesterone. Progesterone is only produced in significant amounts after ovulation has occurred and can be measured in the blood. Progesterone changes the consistency of the vaginal mucus so that it becomes tacky or sticky. This hormone also causes a slight increase in body temperature.

In summary, ovulation may be detected by changes in the ultrasound measurement of follicle size, vaginal mucus, a small increase in body temperature or by changes in the amounts of oestrogen, LH and progesterone in the blood. The value of body temperature charts is limited because ovulation has already occurred by the time a temperature rise is recognised.

Ovulation usually occurs regularly, once a month from puberty until the menopause, apart from times of pregnancy and breast-feeding. In some women ovulation does not occur regularly, or may not occur spontaneously at all. This may be due to an abnormality with the ovaries, the pituitary gland or some other unrelated illness such as anorexia. A number of tests are necessary to determine the cause of this situation before appropriate treatment can be given.