The follicular phase starts on the first day of your cycle (i.e. the first day of your period), and ends at ovulation. During the follicular phase, your temperatures are in their lower, pre-ovulatory range. After ovulation, your temperatures shift to their higher, post-ovulatory range, and you enter the luteal phase, which lasts until the end of your cycle.
The follicular phase is named as such because of the increasing amounts of follicle-stimulating hormone (FSH) produced by your body during this phase. FSH serves to develop the follicles in the ovary, one of which will rupture and release an egg at ovulation several weeks later. During the second half of the follicular phase, estrogen levels rise, causing your body to build up the endometrium, or uterine lining, in which the egg will implant if fertilization occurs.
Although the length of the luteal phase is usually the same (or will vary by only a day or two) from cycle to cycle, the length of your follicular phase is less predictable. This is because a lot of women don’t always ovulate on the same day each cycle. So, if you want to predict when your next period will arrive, you should do so based not on the overall lengths of your previous cycles, but on the lengths of previous luteal phases.
So for example, if your luteal phase is usually about 15 days, and in your current cycle you confirm ovulation with a sustained BBT shift on day 13, you can expect to start your period around what would be day 29 of your cycle (making that day 1 of your next cycle, of course).
This woman's follicular phase is from day 1 to day 13 of this particular cycle.