While most women have some understanding of their menstrual cycles, many are surprised to learn that they don’t know much about the ovulation cycle itself–or that there’s even a difference!
What’s the Difference?
Primarily used in reference to the uterine lining, or endometrium, and its cycle of
- building up (proliferative phase)
- preparing hormonally for pregnancy (secretory phase), and
- shedding (menstrual phase).
Refers to the development of follicles to the point of release from the ovary and eventual development of the corpus luteum.
Naturally, these two cycles are interrelated, working together in well-timed concert (in the best of circumstances) to produce female fertility. Here, we will detail the ovulation cycle.
In the Beginning
It all begins when a female is still growing in her own mother’s womb, specifically, some time between weeks 14 and 20 of the pregnancy. That’s when all the oocytes (eggs) that a female will ever have in her lifetime, estimated at around seven million, are created in her ovaries. From that point on, her oocytes decrease, so much so that at the female’s birth, her eggs have dwindled to only about two million.
Ovulation, then, is not the manufacture of oocyte cells, but the maturing and release of them, contrary to the male’s continual production of sperm.
From their beginning, oocytes are surrounded by follicle cells which will nourish the eggs for up to 50 years or more (until menopause). Most follicles will not mature and ovulate, but will die and be reabsorbed into the ovary through a process called atresia. It is important to know that atresia progresses always, regardless of pregnancy, use of hormonal contraceptives, or even amenorrhea (lack of menstruation)–a female is constantly losing eggs. By the time she has her first period, there are approximately 300,000 left in her ovaries.
Count first day of menstrual period as Cycle Day (CD) 1.
The Hormone Relay System
Maturation and release of oocytes is controlled by the following hormones:
- Gonadotropin-releasing hormone (GnRH)
- Follicle stimulating hormone (FSH)
- Luteinizing hormone (LH)
The Great Mystery
A resting follicle which contains an egg, a group of primordial follicles will begin to grow for reasons still unknown. This growth must begin in the month prior to an egg’s intended ovulation. The follicles grow through the stages of primary follicle, secondary follicle, tertiary follicle, and finally to be called a preovulatory Graafian follicle, measuring about 1.5 centimeters in diameter.
The number of growing follicles in each month’s group is inversely related to a woman’s age; that is, the younger she is, the more primordial follicles will begin to mature, etc.
More Hormones: Estrogen
Estrogen is secreted by the growing follicles themselves, causing a reduction in the amount of FSH production. This appears to sort out the strongest eggs from the growing group, so that by the end of Week 1, there is usually just one follicle left–the dominant follicle, approximately 8 to 10 millimeters in size. It is literally a small cyst.
This completes the follicular phase of ovulation and begins the luteal phase. We are now at approximately CD 14:
More Hormones: Progesterone
High levels of estrogen in the blood signal the release of a surge of LH–this same surge is the one detected by home ovulation predictor kits (OPK’s). LH causes the follicle cells to start making use of progesterone, which is being secreted by the ovary.
Generally, ovulation takes place approximately 36 hours after the start of the LH surge, 20 hours from the time of peak LH levels in the blood.
After the follicle opens and releases its oocyte, it becomes a new structure, the corpus luteum. The corpus luteum is responsible for producing high levels of progesterone, necessary to sustain a pregnancy.
Progesterone levels tend to peak in the bloodstream on or around CD 21, and without being signalled by hCG (hormone emitted by a pregnancy), it will fall rapidly and another follicular phase begins. If, on the other hand, conception occurs, the corpus luteum will continue producing progesterone until the placenta takes over.
The luteal phase tends to be around 11 to 16 days in length.
The entire process of ovulation takes around 24 to 35 days to complete.