A blood test to check a luteal progesterone level for luteal phase assessment is not considered accurate testing
Why is progesterone important?
Progesterone is the hormone that supports the lining of the uterine cavity which is called the “endometrium”, in order to help with the implantation of an embryo. In addition, if pregnancy occurs, progesterone helps maintain the pregnancy. Progesterone is made in, and released by, a temporary ovarian cyst called the “corpus luteum” that remains after ovulation of an egg from a follicle in the ovary. With ovulation, luteinizing hormone (LH) stimulates progesterone release from the corpus luteum. With pregnancy, the pregnancy hormone called human Chorionic Gonadotropin (hCG) continues to stimulate the corpus luteum to release progesterone until the placenta then takes over the duty of progesterone production and release after 9 weeks of pregnancy.
Why is checking a progesterone level through blood testing (serum progesterone level) not recommended during the luteal phase (after ovulation but prior to period or pregnancy)?
Traditionally, progesterone levels have been checked on day 21 of a 28 day cycle or a week after ovulation in order to assess the “luteal phase”. If the progesterone level is greater than 3 ng/ml, then ovulation has occurred and if the progesterone level was over 10 ng/ml, it was considered to be an adequate progesterone level and an indicator of a good luteal phase. However, it has been shown that progesterone is very pulsatile and can fluctuate 8-fold in as little time as 90 minutes; therefore, a single progesterone level is an inaccurate test to assess the adequacy of the luteal phase.
If testing for serum progesterone level is inaccurate and not recommended, how can the luteal phase be assessed?
Tests such as basal body temperature charts, endometrial biopsies in the luteal phase, and serum progesterone levels in the luteal phase all are considered inaccurate when it comes to assessing the adequacy of progesterone effect in the luteal phase. The time interval from LH surge until a period (luteal length) of 11-14 days is considered normal whereas that of an 8 day interval or less is considered a short luteal phase. There can be month-to-month variability in terms of luteal phase length among fertile and subfertile couples. Measuring the time length of the luteal phase (assuming that that an ovulation predictor kit is accurate in a given woman) is a noninvasive and relatively low-cost method that may be the best way to assess the luteal phase.
For more information on the clinical relevance of the luteal phase, please visit the following link within ReproductiveFacts.org: