Updated by the Progyny Editorial Team — September 2025.
The first half of your menstrual cycle is called the follicular phase. Follicles are sacs in the ovaries that contain eggs. During this time, some follicles are selected to grow.
The follicular phase begins on the first day of your period and ends with ovulation. It is also called the proliferative phase, because rising estradiol (a type of estrogen) levels cause the lining of the uterus to thicken.
What happens during the follicular phase?
Early phase
At the start of your cycle, estrogen and progesterone levels are low. A part of the brain called the pituitary gland releases two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
FSH stimulates the growth of eggs in the follicles. These follicles then begin to produce estradiol. Other hormones, such as inhibin A and inhibin B, are also made at this time.
Mid phase
Several follicles grow to the antral stage, which means a fluid-filled cavity forms next to the egg. The granulosa cells that surround the follicles multiply, leading to more estradiol production.
As estradiol levels rise, they signal the pituitary gland to lower production of both FSH and LH.
Late phase
One follicle becomes dominant and keeps producing higher levels of estradiol and inhibin A.
FSH increases the number of receptors for LH in the ovary. In turn, this helps the ovary make growth factors. The dominant follicle produces these growth factors in preparation for ovulation — the release of the egg — and potential implantation if fertilization occurs.
Key takeaway
The follicular phase is the first half of your menstrual cycle. During this time, follicles grow, hormone levels rise, and one dominant follicle prepares to release an egg. This stage sets the foundation for ovulation and possible pregnancy.
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Disclaimer: The information provided by Progyny is for educational purposes only and is not medical advice. Always consult a qualified healthcare provider for medical guidance.