Menstrual Cycle Phases

The menstrual cycle is the beginning of the reproductive phase of a woman’s life, beginning when a girl hits puberty and ending with the onset of menopause. There are different phases that occur during this cycle, and understanding these phases can give a woman insight into when it is best to attempt to get pregnant, as well as why their bodies react the way they do during this time.

The follicular phase of the menstrual cycle occurs before the release of the egg, starting the first day of bleeding and lasting 13 to 14 days. As the uterine lining is being shed the Follicle Stimulating Hormone levels get increased to cause between 3 and 30 egg-containing ovarian follicles to develop. By the end of this phase, FSH levels will decrease and only one dominate follicle will start producing estrogen, which will cause the other follicles to break down.  FSH is a pituitary gland excreted hormone that is present in men (for sperm production), and in women to stimulate egg production.

A strong surge in luteinizing hormone signals the end of the follicular phase and the beginning of the ovulation phase of the menstrual cycle. LH is the hormone responsible for causing the egg to rupture from the follicle and be released (usually within 16–32 hours). The released egg has approximately 12 hours to be fertilized by sperm in order to cause pregnancy, and typically this is more likely to happen if there is already sperm in the reproductive tract when the egg is released.

The release of the egg causes the follicle to close and form a corpus luteum, which helps prepare the uterus in case the egg was able to be fertilized. This is the Luteal Phase of the menstruation cycle, and unless fertilization occurred, will last approximately 14 days. The corpus releases progesterone (another hormone) that will cause the uterine lining (endometrium) to thicken (which is also helped by the hormone estrogen) and fill with nutrients and fluids in order to receive the fertilized egg. The release of the progesterone also thickens cervical fluids to impede bacteria and sperm from being able to enter the uterus. During this phase the body temperature is usually increased, (which is how doctors can try to determine if ovulation is occurring in women who may be having fertility problems).

If fertilization did occur, the egg will be implanted in the uterus within 9 days, which causes the placenta to start growing and causes the release of human chorionic gonadotropin (HCG – which is what pregnancy tests are used to detect). This hormone helps the corpus luteum to continue to produce progesterone until the fetus is able to start producing its own hormones. If there was no fertilization, after 14 days the corpus luteum breaks down along with the endometrium and is expelled resulting in the onset of vaginal bleeding signaling the starting over of the follicular phase.


Merck (2007) Menstrual Cycle: Biology of the Female Reproduction System  Z6EFP

Storck, S (2010) FSH EW56B