Ovulation is one part of the female menstrual cycle whereby a mature ovarian follicle (part of the ovary) discharges an egg (also known as an ovum, oocyte, or female gamete). It is during this process that the egg travels down the fallopian tube where it may be met by a sperm and become fertilized.
Ovulation is controlled by a part of the brain called the hypothalamus, which sends signals that instruct the anterior lobe and pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
The process usually occurs between the 10th and 19th day into the menstrual cycle, and this is the time where humans are most fertile.
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What are the phases of ovulation?
The entire ovulation phase is defined by a period of elevated hormones during the menstrual cycle. The process itself can be informally divided into three phases:
- Periovulatory (follicular phase): a layer of cells around the ovum begins to mucify (become more mucous-like) and expand, and the uterus lining begins to thicken.
- Ovulatory (ovulation phase): enzymes are secreted and form a hole (or stigma) that the ovum and its network of cells use to exit the follicle and eventually enter the fallopian tube. This is the period of fertility and usually lasts from 24 to 48 hours.
- Postovulatory (luteal phase): a hormone called LH or luteinizing hormone is secreted. A fertilized egg will be implanted into the womb, while an unfertilized egg slowly stops producing hormones. In addition, the lining of the uterus begins to break down and prepares to exit the body during menses.
When does ovulation occur?
A woman's menstrual cycle lasts between 28 and 32 days on average. The beginning of each cycle is considered to be the first day of her menstrual period (menses). Ovulation itself generally occurs between day 10 and day 19 of the menstrual cycle, or 12 to 16 days before the next period is due.
Young women will begin to menstruate - a time referred to as menarche - between the ages of 9-15, and during this time will ovulate and be able to become pregnant.
Ovulation typically stops after menopause around the age of 51. Ovulation still occurs during the time leading up to menopause, however, which is referred to as peri-menopause.
How can ovulation be detected?
Ovulation may be detected by the effect that it can have on a woman's body temperature.
There are several indications that a woman is ovulating. During ovulation, the cervical mucus increases in volume and becomes thicker due to increased estrogen levels. It is often said that the cervical mucus resembles egg whites at a woman's most fertile point.
Ovulation may also lead to a 0.4 to 1.0 degree increase in body temperature. This is driven by the hormone progesterone that is secreted when an egg is released. Women are generally most fertile for two to three days before the temperature achieves its maximum.
A few women are actually capable of sensing ovulation in their mid-sections. It is described as being mildly achy or a pang of pain. This condition is called Mittelschmerz - from the German "middle pain" - and it may last between a few minutes and a few hours.
Finally, ovulation predictor kits are available from drug stores that are able to detect the increase in luteinizing hormone in the urine just before ovulation.
On the next page, we look at ovulation calendars, ovulation disorders and available treatment options for ovulatory dysfunction.