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Ovulation and how to detect it

Mr Anil Gudi, Mr Amit Shah & Prof Roy Homburg

Ovulation is the release of a mature egg from the ovary which is capable of being fertilized by sperm to produce a pregnancy. In the normal course of events, ovulation will occur once a month in women of fertile age. It is the main event of the menstrual cycle, occurring about 12-14 days before the next menstruation is due.

For ovulation to occur, a highly integrated and synchronized sequence of events must unfold. The ovaries contain a vast number of immature eggs some of which, when correctly stimulated, are enveloped in a follicle which may go on to develop and have the potential to ovulate. Of the approximately 200,000 eggs present in the ovaries at the time of puberty, only 400-500 will actually be ovulated in a life time while the rest fall by the wayside and disintegrate. In each cycle, at the time of menstruation, about 15 follicles have progressed to the stage that they have the potential to be selected for ovulation in that cycle. Only one of them will ovulate and this selection process is mainly influenced by follicle stimulating hormone (FSH). The largest or dominant follicle, containing an egg and some fluid, will emerge at the beginning of the second week of the cycle at the expense of all the others that fail to thrive. This dominant follicle continues to grow and produces estrogen which peaks on about day 12 of a regular 28 day cycle and sets off an enormous surge in the level of luteinizing hormone (LH). This influences the dominant follicle (now somewhere between 18-22 mm in diameter ) to ‘burst’ releasing the egg into the pelvis where it finds its way into the nearest of the Fallopian tubes on its way to the uterus.

Following ovulation, the remains of the burst follicle become the corpus luteum (or yellow body due to its characteristic colour) which produces the hormone progesterone. The level of progesterone peaks about 7 days after ovulation and its measurement in the blood around this time is the most accurate indication that ovulation has occurred. Another method to detect ovulation is on an ultrasound examination to chart the growth of the dominant follicle which ‘disappears’ after ovulation has occurred. A urine test is available which indicates when the LH is rising suggesting that ovulation will occur about 12-24 hours afterwards. Finally, a somewhat outdated method can be used by charting the basal body temperature, the body temperature immediately on waking in the morning. Following ovulation, the basal body temperature (BBT) rises by about 0.5 degrees which is maintained until the next period and so produces a biphasic temperature curve over the cycle which is higher in the second half of the cycle following ovulation.

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