Lots of infertile women are confused about what progesterone does. They know that progesterone is a key reproductive hormone; that it is present in birth control pills; and that it is used to induce a period when this is overdue.
Therefore, when their doctor prescribes the same progesterone when they are pregnant, explaining that it is being used to support the pregnancy, they are very confused. How could a hormone which is being used to bring on a period possibly also be used for supporting a pregnancy ? It just does not make any sense.
Actually, it does ! Progesterone is the hormone which the corpus luteum in the ovary produces to help during the implantation of the embryo . It creates secretory changes in the uterus lining, so that it is ready to receive the embryo. This is why it's role in supporting the pregnancy is quite straightforward. Even the name, pro ( = for, towards, helping) -gest ( = pregnancy) -erone ( = steroid hormone) suggests this role - it supports the pregnancy !
So how is this used to induce a period ? Remember that it's not the progesterone per se which induces the period. In fact, if you take progesterone for long periods of time in high enough doses, the period will get indefinitely postponed. When your doctor induces a period for you with progesterone, he gives it only for a short amount of time - for about 3-5 days . The period starts 3-8 days after you stop the last tablet of progesterone , because the uterine lining has lost the support the progesterone was providing. This is why this progesterone induced bleeding is called a " withdrawal bleed" - it's the withdrawal of the progesterone which causes the onset of the period ( just like it does in a normal menstrual cycle , when the corpus luteum dies and stops producing progeserone because there is no embryo to produce HCG to rescue it !)