Menstrual Cycle Phases
The menstrual cycle is divided by phases which are regulated by hormones. The three phases of the cycle are follicular, ovulatory, and luteal. The follicular phase is prior to egg release, the ovulatory phase is egg release, and the luteal phase comes after the egg is released.
The follicular phase begins on the first day of your menstrual period, or on day 1 of your cycle. During this time, egg follicles develop within the ovaries. At the start of this phase, your endometrium (uterine lining) has thickened with nutrients and fluids in readiness to sustain an embryo. If the egg fails to be fertilized, estrogen and progesterone levels will be low. In response to the low levels of these hormones, the endometrium is shed and menstruation begins.
At this point, the pituitary gland steps up its manufacture of follicle stimulating hormone (FSH) just a bit. This encourages 3-30 egg follicles to grow. Every follicle contains one egg. Later on, the levels of FSH decrease so that growth continues in just one follicle known as the dominant follicle. This follicle will start to make estrogen and the other follicles will then disintegrate.
The follicular phase of the menstrual cycle lasts around 2 weeks. But of the three phases of a woman's cycle, this is the one whose length varies the most. With the approach of menopause, the follicular phase becomes shorter. The follicular phase is ended when the body produces a surge of luteinizing hormone (LH). This surge brings on the release of the egg (ovulation).
During the ovulatory phase LH stimulates the remaining, dominant follicle to bulge out of the ovary's surface until the follicle ruptures, allowing the release of the egg. FSH levels rise a bit at this time, though experts aren't sure why this is so. The ovulatory phase is brief, lasting only 16-32 hours. This phase ends with the release of the egg.
Next up is the luteal phase which lasts around 2 weeks unless interrupted by fertilization of the egg (pregnancy). The luteal phase is ended by menstruation or pregnancy.
During the luteal phase, the ruptured follicle closes back up and forms something called the corpus luteum which will begin to manufacture ever-increasing amounts of progesterone. The progesterone that is produced helps to prepare the uterus to receive a fertilized egg, should fertilization occur. This hormone causes a thickening of the endometrium as it fills up with nourishment for a potential embryo.
Progesterone also helps thicken cervical mucus so that bacteria and sperm can't enter into the uterus. In addition, the hormone causes a slight spike in body temperature during the luteal phase. The body temperature then remains in this elevated state until the menstrual period arrives.
Estrogen levels are also high during the majority of the luteal phase. Estrogen helps thicken the endometrium and causes the breasts' milk ducts to widen. This is the reason for the breast tenderness experienced by some women each month.
Should the egg fail to be fertilized, the corpus luteum will begin to degenerate after 2 weeks and menstruation will begin.