Ovulation cycle represents the time band of the process in which ovum or an egg is nurtured and released from a mature follicle in any of a woman’s ovaries.
All of the ova or eggs in every woman were produced during her fetal life in the womb. They are then stored in the ovaries inside primordial follicles and stay as such till she attains puberty. A typical follicle consists of an ova surrounded by a single layer of spindle-shaped cells.
There are 7 million follicles in a baby girl by the 30th week in pregnancy, about 2 million at birth and between 0.3 to 0.4 million at puberty – out of which only 450 develop into mature follicles in the ovulation cycles during the life time of a woman. The rest undergo degeneration and never mature.
The length of the ovulatory cycle, also referred to as menstrual cycle, varies from woman to woman from 21 - 35 days (with the average being 28 days). This cycle is generally divided into 3 phases, namely:
When ovulatory cycle is shorter than 21 days, it’s referred to as polymenorrhea, while it’s referred to as oligomenorrhea when longer than 35 days.
Ovulation cycle length that’s longer than 35 days is considered abnormal and such is often associated with infertility issues that include lack of ovulation. The total absence of ovulation cycle without menstrual bleeding is referred to as amenorrhea.
A normal ovary has a huge number of primordial (immature) follicles. When stimulus is received at the start of a new cycle - in this case, a raised level of Luteinizing hormone (LH) and Follicle Stimulating Hormone (FSH) produced by the anterior pituitary gland, some of the primordial follicles become larger.
The follicles compete with one another for dominance. Eventually, one of these follicles outgrows the others and progresses to maturity. This mature follicle is called a Graafian follicle. The others undergo degeneration or what is technically referred as follicular atresia.
A mature follicle is capable of secreting estrogen hormone. This hormone is produced by the Theca granulosa & Theca interna layers of the follicular wall.
Estrogen causes the proliferation of the lining of the uterine wall, making it thicker and hence preparing it for conception.
The length of this phase is roughly 14 days in an average woman of 28 days cycle length. Some conditions, however, can cause this length to vary.
The length of this phase decreases with increasing age and certain pathologies, whereas a low fat diet, use of oral contraceptives and addicted intake of drugs such as marijuana may slightly lengthen this phase.
As stated above, estrogen is produced by the dominant mature follicle. A rise in estrogen level causes further increase in luteinizing hormone secretion, called LH surge.
This LH surge is associated with the rupture of a mature follicle to set free an ovum into what is called peritoneal cavity, from where it then enters the uterine or fallopian tube.
This ovulation phase is simply about the release of one or more ovum from a mature follicle in any of the woman’s ovaries.
It is easy to predict the time of ovulation in a woman with any regular cycle length. Ovulation occurs 14 days before the menstrual bleeding or menstruation, so in a woman with a cycle length of 28 days, ovulation will occur on the 14th day.
In a woman of 35 days cycle length, ovulation will occur on the 22nd day. You can read more about the process of predicting ovulation using test kits here
After ovulation, the ruptured follicle undergoes changes and is converted into a structure named corpus luteum. Corpus luteum is a yellowish mass. It secretes the hormone progesterone, estrogen and inhibin.
Progesterone causes an increase in the blood supply and foster secretory activity on the uterine walls; making the uterus suitable for sustaining pregnancy.
The basal body temperature is also raised owing to the effect of progesterone on the hypothalamic center – the place where body temperature is controlled.
On the other hand, Inhibin hinders further secretion of Follicle Stimulating Hormone (FSH) to prevent more follicles from maturing.
This post-ovulatory phase can have two (2) outcomes depending on whether or not fertilization takes place.
If fertilization takes place, the corpus luteum is rescued from regression as a result of the production of another hormone, namely Human Chorionic Gonadotropin (HCG), by the placenta.
The rescued corpus luteum then continues the production of progesterone and estrogen until the first trimester, after which the placenta completely takes over the production of these hormones.
Hence, the corpus luteum helps in sustaining pregnancy before the placenta has acquired sufficient size and growth to sustain pregnancy on its own.
If fertilization does NOT take place, the corpus luteum starts to regress and is replaced by a body forming connective tissue called corpus albicans.
As a result, the level of estrogen and progesterone falls rapidly, resulting in insufficient amounts to sustain the proliferated uterine wall, which becomes sloughed.
Blood and cellular debris are then shed via the vaginal canal as menstrual blood. Soon after, another ovulation cycle starts over again.