The short answer: It depends when they are measured.
1) Early Pregnancy
Because serum progesterone is a reflection of progesterone production by the corpus luteum, which is stimulated by a viable pregnancy, it is helpful as a screening tool to rule out an abnormal pregnancy like a miscarriage or tubal pregnancy. These levels are valid in those who use vaginal progesterone but erroneous in those taking oral or intramuscular progesterone supplementation. Viable intrauterine pregnancies can be diagnosed with 97.5% sensitivity if the serum progesterone levels are greater than 25 ng/mL a few days after being late on your period. Conversely, finding serum progesterone levels of less than 5 ng/mL can aid in the diagnosis of a nonviable pregnancy with 100% sensitivity. In the event that the serum progesterone level is 5-25 ng/mL, further testing using ultrasound, additional hormonal assays, or serial examinations is warranted to establish the viability of the pregnancy. Clearly, in this situation, measurement of progesterone level early in pregnancy is a useful indicator of fetal health. This does NOT mean, however, that extra progesterone intake has any therapeutic effect on pregnancy outcome. In other words, progesterone will not affect the health of the embryo.
2) After Ovulation (luteal phase)
The luteal phase is the latter half of the menstrual cycle during which the corpus luteum produces progesterone to keep the uterine lining receptive to a fertilized egg. When the corpus luteum stops producing progesterone, it disintegrates and your period begins. The length of the luteal phase should be longer than 10 days to sustain a pregnancy. A short luteal phase cannot sustain a pregnancy because the uterine lining begins to break down, bringing on the menstrual bleeding and causing an early miscarriage.
The three main causes of luteal phase defect include poor follicle production, premature demise of the corpus luteum, and failure of the uterine lining to respond to normal levels of progesterone. The latter can only be diagnosed by endometrial biopsy.
Since progesterone production is a concern with some luteal phase defects, many doctors will frequently check your blood progesterone level on cycle day 21. There are some drawbacks to this approach. The level of progesterone in your blood fluctuates from hour to hour throughout the day. You may appear to have a normal blood progesterone level at one point in the day but have a lower level if your blood was drawn at a different time of day. Also, many women with unresponsive uterine linings have normal progesterone levels. The best test would be a biopsy of the uterine lining, but these too fluctuate in normal women, and the test quite uncomfortable.
Because clomiphene is such an affordable and effective treatment of a potential LPD, one has to wonder whether progesterone levels are really worth the expense and anxiety in the treatment of general infertility. In IVF, the situation is completely different. Routine supplementation of progesterone is needed because the natural production of progesterone is suppressed.