Education

When a woman is born, her ovaries will already contain all of the eggs she will need in a lifetime.
Once puberty arrives, she will begin to ovulate, which involves a very intricate and detailed process involving many hormones.
The hypothalamus is an area which is very deep in the center of our brains. This area is responsible for making a hormone called Gonadotropin Releasing Hormone (GnRH).
GnRH is released in a pulsatile fashion; it will target the pituitary gland (a pea-sized structure deep in the middle our our brains), and induce the release of FSH (Follicle Stimulating Hormone).
FSH will target the ovaries and stimulate a few eggs to begin to grow and mature each month.

The eggs will mature within a fluid filled sac called a "follicle", which is often referred to as a "functional cyst" on ultrasound.
One follicle will generally win out and become the "dominant follicle".
The developing follicles will together synthesize and secrete hormones, estrogen is the predominant hormone secreted in during this "pre-ovulatory phase, also known as the pre-Peak phase or follicular phase.
Estrogen will in turn target the endometrium and build up the layer. Estrogen will also target the cervix and induce the production of cervical mucus, which is a healthy state will become stretchy, clear and/or lubricative.
We call this "the mucus build-up" in the Creighton Model System (CrMS) of charting.
The "Peak Day" is the last day of "10", "K" OR "L" and will occur in very close proximity to when the woman ovulates.
Once ovulation occurs, ie: when the egg ruptures from the fluid-filled follicle, the follicle itself will change in structure and function.
The follicle will become a new structure called the "corpus luteum" which will predominantly secrete progesterone.

The progesterone will target the cervix and cause the mucus to become thicker and or dryer and it will be more hostile to sperm.
The progesterone will also target the endometrium and cause glands to form that will secrete nutritious substances to help foster the implantation and growth if a baby is conceived.
As the corpus luteum regresses, progesterone will decrease in the blood. When progesterone levels drop, the endometrial layer will slough off, which is the woman's normal and healthy period flow.
There are many conditions that can cause low progesterone in the post-Peak phase, also called "Luteal Phase Deficiency" or "LPD".
Luteal Phase Deficiencies are found in many gynecological conditions, such as: Infertility & RPL, PCOS, endometriosis, FHA and PMS.

The Menstrual Cycle