Your menstrual cycle is controlled by hormone signals in the brain. Each month, your ovary will release an egg (known as ovulation) after the egg has matured. The egg enters into the fallopian tube and makes its way down to the uterus with hopes that a sperm will fertilize it. Your monthly menstrual cycle begins on the first day of your period and lasts until the first day of your next period.﻿﻿

The Two Parts of Your Menstrual Cycle

The first part of your menstrual cycle is called the follicular phase. This phase starts on the first day of your last period and continues until the day you ovulate. This phase of your menstrual cycle may be different for every woman (lasting anywhere from 7 days to 40 days). The second part of your menstrual cycle is called the luteal phase. This phase begins on the day of ovulation and lasts until your next period starts. The luteal phase usually has a more precise timeline—for most women, this part of your menstrual cycle typically lasts around 12-16 days.﻿﻿

Breaking Down Your Menstrual Cycle

Now that you understand the two phases of your menstrual cycle, let’s take a look at what is normally going on inside your body each month. This is where you can see how your hormones are controlling what is happening.

Hormones and the Follicular Phase

The Starting Place

Everything begins with when the hypothalamus (a structure in your brain) produces the gonadotropin-releasing hormone (GnRH).

Once produced, the GnRH will then trigger your pituitary gland to release the follicle stimulating hormone (FSH). This hormone's job is to stimulate the growth and development of your eggs. ﻿ ﻿

The Follicle Stimulating Hormone

The FSH then makes the journey through your bloodstream to your ovaries.

Once the follicle stimulating hormone arrives, it triggers the growth of a follicle (a small sac that contains an egg) to develop into a mature egg. ﻿ ﻿

The Role of the Follicle

As the follicle develops and matures, it produces the hormone, estrogen.

After about 10 days, the estrogen levels reach high numbers.

The estrogen level peaks approximately one day before ovulation (in a 28-day cycle, this is is typically day 13).

This estrogen peak tells the brain that the egg has matured and triggers the pituitary gland to release a surge of luteinizing hormone (LH).

The LH Surge

This LH surge acts as a cue to the ovarian follicle.

About 24-36 hours after the LH surge, a mature egg breaks out of the ovary and is released into the fallopian tube—this is ovulation.

The egg leaves behind the corpus luteum (the empty follicle). ﻿ ﻿

Hormones and the Luteal Phase

The Corpus Luteum

The cells of the corpus luteum release the hormone progesterone. This hormone helps to thicken and prepare your uterine lining for the implantation of a fertilized egg.

The corpus luteum will continue to make progesterone during the luteal phase of your menstrual cycle—which is around 12-16 days.

After ovulation, if your egg is fertilized, the corpus luteum will continue to make progesterone to help sustain your pregnancy until the placenta is ready to do this job.

The Shrinking of the Corpus Luteum

If, after ovulation, the egg is not fertilized, the corpus luteum will begin to shrink.

It stops making progesterone—which leads to no more hormones being made to support the uterine lining.

The lining will begin to shed off.

This leads the start of your monthly period.

The low levels of estrogen and progesterone will then signal the hypothalamus to start the whole menstrual cycle process over again.﻿﻿