Menstrual Cycle

Learn about your menstrual cycle and how to predict your fertile days.

A cycle has three phases.

Follicular Phase - Occurs from menstruation until ovulation and is responsible for the development of the follicles and the thickening of the uterus in preparation for ovulation.

Ovulation - At this time, the dominant follicle ruptures and an egg is released.

Luteal Phase - The endometrium continues to thicken in preparation for implantation in case fertilization has occured. If fertilization does not occur, the uteral lining is shed and a new cycle begins.

The menstrual cycle is controlled by hormones.

A hormone is a natural chemical produced in one part of the body and released into the bloodstream in order to trigger or regulate a function in another part of the body.

There are six hormones involved in the functioning of a woman's reproductive system. Gonadotropin-Releasing Hormone (GnRH), Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), Estrogen, Progesterone, and Testosterone. It is the proper interraction of these six hormones, that produce a healthy menstrual cycle.

Follicular Phase - Days 1-13 in a 28 Day Cycle, From menstruation until ovulation

A cycle begins on the first day of blood flow.
During this phase, the hypothalamus (located in your brain) produces gonadotropin-releasing hormone (GnRH).
This causes a chemical reaction in the pituitary gland which stimulates the production of follicle-stimulating hormone (FSH) and later lutenizing-hormone (LH).
FSH stimulates the development and maturation of follicles in the ovaries. The developing follicles begin to release estrogen.
On about the seventh day, one follicle will become dominant. This follicle will contain the egg that will be released that month. The remaining follicles, lose nourishment and cease growing. The dominant follicle continues to produce estrogen as it matures.
Estrogen will continue to rise and peak a couple of days before ovulation. The rise in estrogen causes the lining of the uterus, the endometrium to become thicker in preparation for the implantation of a fertilized egg.
When estrogen levels peak, about two days before ovulation, it signals the release of LH. It will also cause a change in your cervical mucus.
LH begins to rise about 36 hours before ovulation and peaks about 12 to 24 hours before ovulation. LH is what is detected by Ovulation Predictor Kits (OPKs).
Your most fertile time is the 2 days prior to ovulation to the day of ovulation. This is because a man's sperm may survive in fertile cervical mucus for up to 72 hours whereas a woman's egg will survive for only 12-24 hours without being fertilized. If you wish to become pregnant, now is the time to begin having unprotected sex.

Ovulation - Day 14 in a 28 Day Cycle

LH travels through the bloodstream to the ovary and causes the ovary to produce enzymes that will rupture the dominant follicle and thereby release the egg.
The egg then travels through the fallopian tube where it may be fertilized. This is ovulation.

Luteal Phase - Days 15-28 in a 28 Day Cycle, From ovulation until new cycle

After ovulation, LH and Estrogen levels drop. The ruptured follicle becomes the corpus luteum and continues to produce some estrogen and progesterone which is the dominant hormone of the last phase of the cycle. The rise in progesterone will cause a body temperature shift which can be detected if you are maintaining a Basal Body Temperature (BBT) chart. This rise in temperature is the only at-home way to confirm ovulation. Although an LH surge is generally followed by ovulation, it does not guarantee that it will occur.
The endometrium continues to thicken as a result of the progesterone and estrogen. If an egg is fertilized and implanted (approximately day 23 or 24 in a 28 day cycle), the pregnancy hormone (hcG) is produced. This causes the corpus luteum's life to be extended so that it could countinue to produce the progesterone and estrogen needed to nourish the implanted egg until this hormone production can be taken over by the placenta. The hormone hcG is what is detected by Home Pregnancy Tests (HPTs).
If no pregnancy occurs, the corpus luteum dies, progesterone and estrogen levels fall, and the endometrial lining is shed through menstruation.

Additional Information

Many women do not have a 28 day cycle. Normal cycle lengths range from 20 to 40 days
Most women do not have the same cycle length every month. As a matter of fact it is unusal for a woman to experience a cycle length variation of less that four days. A regular menstrual cycle can vary in length from cycle to cycle, up to seven days.
Not everyone ovulates mid-cycle which is why it is important to learn to read your own cycle when trying to conceive.
Although your cycle length may vary from month to month, the luteal phase (the last phase) will not vary by more than a day. It is the follicular phase length that may fluctuate. So if you have a luteal phase of 14 days, then it should be 14-15 days every month whether your cycle that month is 28 days or 31 days long.