Hi
Progesterone has a variety of uses, and is taken in different amounts at different times in your cycle for a different number of days depending on what it is the doctor is wanting to treat.
Natural progesterone is secreted by the corpus luteum following release of the egg by the follicle, and is the main hormone in action for the luteal phase (in combination with oestrogen). It's presence prevents the uterus lining from disintegrating (which is what causes a period). The corpus luteum stops producing progesterone around 10-14 dpo if the woman hasn't conceived that cycle, and it is this sudden drop in progesterone levels that trigers the uterus to shed its lining and have a period.
If progesterone is prescribed by a doctor, it can be for several reasons:
1) to cause a period if someone hasn't had their period for a few months but isn't pregnant - the woman takes it for 5-10 days and then the sudden drop when she stops taking it mimics the drop in a normal cycle from the corpus luteum dying and hopefully her period will start
2) to cause regular periods in someone with irregular cycles - for the same reason as above, but this time the progesterone is given immediately following ovulation in order to ensure a woman's period arrives 14 or so days later.
3) to treat recurrent early pregnancy loss due to shortened luteal phase - this is where a woman's corpus luteum keeps dying off too soon, and the embryo isn't given a proper chance to implant fully before the uterus sheds its lining. By topping up her natural levels, the utuerus lining is preserved until a time that the embryo is established to secrete hCG and keep the corpus luteum churning out enough progesterone to keep the pregnancy safe.
4) Reduce heavy periods - a woman might be prescribed progesterone to take from the beginning of her cycle in order to prevent too much lining building up in the uterus, resulting in heavy periods.
That's probably all the main uses I can think of, though I'm sure there are more!
