Fig. 12. Progesterone secretion pattern. Blood samples taken every 20 minutes over 24 hours from a normal woman were analyzed for progesterone. Her consecutive serum progesterone levels are graphed with secretory events (pulses) marked with an asterisk. By careful examination, it can be seen that her progesterone levels vary from 8 to 18 ng/mL over 24 hours. About a third of the time, progesterone levels were less than 10 ng/mL, which has been suggested by some authors as a crucial level for the diagnosis of luteal phase deficiency. This graph shows that single blood samples can be deceptive, depending on where they were obtained during the progesterone secretory profile.