Progesterone is a hormone which is produced by the ovary in large amounts following ovulation. Before ovulation it is almost undetectable in blood tests. This is the reason why the best way to detect whether ovulation has occurred is to examine the level of progesterone in the blood about seven days following the expected date of ovulation. In a normal, regularly cycling woman this would be on day 21 of the cycle when the progesterone level peaks. The job of progesterone is to prepare the lining of the uterus (endometrium) for the implantation of a potential pregnancy. Should a pregnancy not occur, progesterone levels fall rapidly immediately before the onset of menstruation and only rise again following the next ovulation. Should a conception occur, high levels of progesterone continue to be produced under the influence of luteinizing hormone (LH) in order to maintain the growth of the pregnancy in the uterus. After about 7-8 weeks into the pregnancy, the placenta takes over the production of progesterone from the ovary so that high levels are maintained until the time of delivery.
During infertility treatment by IVF, it is essential to supplement the progesterone production in order to maintain a pregnancy. This is usually done by administering progesterone vaginal suppositories on a daily basis and maintaining them until production by the placenta has taken over. Failure to do so will almost inevitably result in an early miscarriage. The vaginal route is used as progesterone pills taken by mouth are relatively ineffective and progesterone injections can be quite painful. As it is LH that naturally encourages progesterone production, its substitute, hCG (human chorionic gonadotrophin) is sometimes used for providing progesterone support following the placement of the embryo in the uterus after IVF treatment but this sometimes may encourage ovarian hyperstimulation syndrome and so it is rarely used for this purpose.