The first phase of the menstrual cycle is the follicular phase. It begins on the first day of your period and ends when you ovulate, typically lasting about 14 days in a 28-day cycle. During this phase, your body is doing two things at once: shedding the old uterine lining (your period) and preparing a new egg for release.
What Happens During the Follicular Phase
The follicular phase gets its name from the follicles in your ovaries, which are small fluid-filled sacs that each contain an egg. At the start of this phase, your brain’s pituitary gland releases follicle-stimulating hormone (FSH), which signals your ovaries to begin developing anywhere from 11 to 20 follicles at once. Each one holds a single egg, but only one will fully mature.
Over the course of several days, one follicle outpaces the others and becomes what’s called the dominant follicle. This winning follicle starts pumping out estrogen, which does two important things. First, it tells your uterus to start building a fresh, thickened lining in case a fertilized egg needs to implant. Second, the rising estrogen signals your brain to dial back FSH production. Without that hormonal support, the remaining follicles wither and get reabsorbed by the body. By the end of the follicular phase, only the dominant follicle remains, ready to release its egg at ovulation.
The Menstruation Window
Your period is actually a sub-phase within the follicular phase, typically occupying the first three to seven days. It starts when progesterone levels drop sharply after the previous cycle’s egg goes unfertilized. That progesterone withdrawal is the direct trigger for menstruation.
Without progesterone’s support, the specialized blood vessels in your uterine lining constrict tightly. This cuts off blood supply to the upper layers of tissue, which then begins to break down. Your body releases enzymes that dissolve the structural framework of the lining, while inflammatory signals draw immune cells into the area to help clear the tissue. The result is the bleeding you experience as your period.
Cramping during menstruation comes from prostaglandins, natural chemicals produced in the uterine lining that cause the uterus to contract. Prostaglandin levels are highest on the first day of your period, which is why cramps tend to be worst at the start and ease up as bleeding continues and the lining sheds.
How Estrogen Shifts Throughout the Phase
Estrogen levels change dramatically from the beginning of the follicular phase to the end. Right after your period starts, estrogen sits at its lowest point in the entire cycle. As the dominant follicle grows and produces more estrogen over the next week or so, levels climb steadily. By the time the follicular phase ends, estrogen can be more than 20 times higher than it was at the start.
This estrogen surge is what ultimately triggers ovulation. When estrogen reaches a critical threshold, your pituitary gland responds with a sudden burst of luteinizing hormone (LH), which causes the dominant follicle to rupture and release its mature egg. That moment marks the end of the follicular phase and the beginning of ovulation.
How Long the Follicular Phase Lasts
In a textbook 28-day cycle, the follicular phase lasts about 14 days. But it’s also the most variable part of the cycle. If your periods are sometimes early or late, the follicular phase is almost always the reason. Stress, illness, significant weight changes, and intense exercise can all delay follicle development, pushing ovulation later and making your cycle longer overall. The second half of the cycle (the luteal phase) tends to stay much more consistent at around 12 to 14 days.
Counting Day One
Day one of the follicular phase, and of your entire cycle, is the first day of true menstrual bleeding. Light spotting before your period picks up doesn’t count. If you’re tracking your cycle, mark the day you first see a full flow of blood as day one. Any spotting that shows up between periods is considered irregular bleeding and isn’t the start of a new cycle.
This distinction matters if you’re tracking fertility, using cycle-based birth control, or monitoring your cycle length. Getting day one right keeps all the other timing anchored correctly.