[TibetanPrincess]

Hello there,

Follicular phase is the first half of the beginning of your cycle. Estrogen starts to climb. So it is recommended to take PM as it modulates estrone/estradoil levels to be balanced.
Luteal is the last half of your cycle. Progesterone starts to climb. PC is often used around this time to spark developement when one's natural levels are low to balance out hormones.
The first day you start your menses is the start of the cycle. These two hormones are at their lowest levels.
If you were to continue to take PM all month long, you would delay your menses. We don't want that.
Hope that helps