The first half of your menstrual cycle is called the follicular phase. Follicles are the sacs in your ovaries that contain eggs. During this part of your cycle, the follicles selected for that particular month begin to grow.
The follicular phase starts with the first day of your period and ends with ovulation.
The follicular phase is also known as the “proliferative phase” because rising estradiol (estrogen) levels cause the endometrial lining of the uterus to proliferate and thicken.
What happens during the follicular phase?
Early phase
At the beginning of your cycle, estrogen and progesterone levels are low. A part of your brain called the pituitary gland produces two hormones: follicle stimulating hormone (FSH) and luteinizing hormone (LSH).
FSH stimulates the growth of eggs in the follicles. These follicles then begin to produce estrogen in the form of estradiol. Other hormones, such as inhibin A and inhibin B, are also produced at this time.
Mid-phase
Several follicles will grow to the antral stage (which means there is a fluid-filled cavity in the follicle next to the egg). The granulosa cells surrounding the follicles multiply, leading to increased estradiol production. This increase in estrogen acts as a negative feedback on the pituitary gland, so this leads to decreasing levels of both FSH and LH.
Late phase
A dominant follicle emerges, which continues to produce rising levels of estradiol and inhibin A.
FSH causes an increase in the number receptors for LH in the ovary, which in turn leads the ovaries to make growth factors. The follicle produces these growth factors in preparation for ovulation (the release of the egg) and potential implantation of a fertilized embryo.
Dr. Jenna Friedenthal is a Fellow in Reproductive Endocrinology and Infertility at Reproductive Medicine Associates of New York. Dr. Friedenthal completed her undergraduate education at Yale University. She went on to complete her medical degree from the Albert Einstein College of Medicine with Distinction in Research. While in medical school, she received the Senior Research Fellowship award for her research accomplishments in reproductive endocrinology and immunology. Dr. Friedenthal then completed her residency training in Obstetrics and Gynecology at New York University. During her residency training, Dr. Friedenthal received the Mortimer Levitz Best Basic Science Award for her research on pre-implantation genetic testing.