Progesterone is an important hormone that can effect your ability to conceive and also your ability to maintain a healthy pregnancy.
The role of progesterone when trying to get pregnant is to thicken the endometrium (uterine lining) to a desired thickness which is essential for implantation of the embryo in the uterus (womb) and therefore to achieve a successful pregnancy.
Low progesterone levels can potentially cause irregular menses which can then affect the timing of your intercourse. Progesterone levels are usually low in the early part of your menstrual cycle and tends to rise after you have ovulated (release of the egg from your ovary into the Fallopian tube during your fertile period).
If your progesterone levels remain low after your fertile period, this may indicate that you have not ovulated and therefore you may experience some difficulty in getting pregnant in addition to having irregular menstrual cycles.
Once you are pregnant, progesterone is needed to maintain the endometrial thickness and to support your pregnancy so much so that low progesterone levels are associated with abnormal vaginal spotting / bleeding during early pregnancy and also increase risk of miscarriage.