[Ginie]

The first half of your cycle - this is when you should be taking PM. Not 21 days, not 15 days, just take it from day 1 of your cycle and stop 1-2 days before ovulation. PM has high estrogenic properties. What do you think will happen if you load your body with estrogens (even from plant source) after ovulation? You're going to mess your hormones. Instead of progesterone (the main hormone of the second half of your cycle) it will be loaded with estrogen. Btw, PM also postpones ovulation because there should be a drop in estrogen right before it but there isn't. Ovulation gets postponed further and further and in the end it either doen't happen or it prolongs the cycle (because there should be 14 days after ovulation anyway to complete the cycle).
The process of ovulation is started and controlled by a drop in the hormone estrogen (mostly etradiol) to a low level. When the drop in estrogen levels occurs the hypothalamus (a part of the brain) is signaled to increase its secretion of gonadotropin-releasing hormone (GnRH) thus sending a message to the pituitary gland to increase its secretion of follicle-stimulating hormone (FSH). The increase in FSH causes the growth of ten to 20 of the ovarian follicles.


If you only use PM up til 1-2 days before the ovulation (use tests or use BBT charts to find out when you ovulate) then you won't mess your hormones and will be fine. Here's a scary study done on mice.
http://www.springerlink.com/content/f6221184631n82rl/
This is not to scare you but let you know that PM is very powerful and should be taken with causion.