Normal Menstrual Cycle

The follicular phase begins with the first day of bleeding (CD1). It is called the follicular phase because  this is when the ovarian follicles, fluid filled structures containing one egg each, grow. Follicular growth is  driven by the appropriately named follicle stimulating hormone (FSH) which is produced by the pituitary gland. 

In a typical menstrual cycle, communication between the brain and ovary leads to the development of a single “dominant follicle”, which will ultimately release its egg at ovulation. As the dominant follicle grows under the influence of FSH, it starts to produce increasing levels of estrogen.  

Estrogen has two basic functions in the menstrual cycle. One is to develop the uterine lining, which increases in thickness during the follicular phase. The second is to trigger the surge of another hormone, luteinizing hormone (LH), which is released from the pituitary gland when estrogen levels are at their highest, prior to ovulation. This LH “surge” leads to ovulation of the egg (oocyte) from the dominant follicle. LH is also the hormone that is picked up by home ovulation predictor kits.  

The most fertile period is from the three days leading up to ovulation through the day of ovulation. This is the also called the “fertile window.”  

After ovulation, the luteal phase begins. It is called the luteal phase because the ruptured follicle that released the egg turns into a new endocrine organ called the corpus luteum. The corpus luteum produces  the hormone progesterone. Progesterone causes the uterine lining to mature and become supportive for  implantation of an embryo, and is also necessary to sustain an early pregnancy once implantation has occurred.  

If pregnancy occurs, the uterine lining is maintained and progesterone levels stay elevated as the embryo  develops. 

If pregnancy does not occur, there is a drop in progesterone levels, which results in shedding of the endometrial lining and transitioning into the next menstrual cycle.