Since progesterone levels vary predictably throughout the menstrual cycle, multiple (serial) measurements can be used to help recognize and manage some causes of infertility. Progesterone can be measured to determine whether or not a woman has ovulated, to determine when ovulation occurred, and to monitor the success of induced ovulation.

In early pregnancy, progesterone measurements may be used, along with human chorionic gonadotropin (hCG) testing, to help diagnose an ectopic or failing pregnancy (progesterone levels will be lower than expected), although this will not differentiate between the two conditions. Progesterone levels also may be measured throughout a high-risk pregnancy to help evaluate placenta and fetal health.

Progesterone levels may be monitored in women who have trouble maintaining a pregnancy, as low levels of the hormone can lead to miscarriage. If a woman is receiving progesterone injections to help support her early pregnancy, her progesterone levels may be monitored on a regular basis to help determine the effectiveness of that treatment.

In women who are not pregnant, progesterone levels may be used, along with other tests, to help determine the cause of abnormal uterine bleeding.

Progesterone levels are measured: 

• As part of an infertility assessment, when a woman is having trouble getting pregnant and the doctor wants to verify that she is ovulating normally 
• To determine if ovulation has occurred and when following drug therapy to induce ovulation 
• When symptoms, such as abdominal pain and spotting, suggest an ectopic pregnancy or threatened miscarriage 
• To monitor the effectiveness of treatment when a pregnant woman requires progesterone injections to help maintain her pregnancy 
• To monitor placenta and fetal health during a high-risk pregnancy 
• When a non-pregnant woman is experiencing abnormal uterine bleeding

Interpretation of progesterone test results requires knowledge of where a woman is in her menstrual cycle or pregnancy. Progesterone levels usually start to elevate when an egg is released from the ovary, rise for several days, and then either continue to rise with early pregnancy or fall to initiate menstruation.

If progesterone levels do not rise and fall on a monthly basis, a woman may not be ovulating or having menstrual periods. If levels do not rise normally during an early pregnancy, the pregnancy may be ectopic and/or may be failing. If serial measurements do not show increasing progesterone levels over time, there may be problems with the viability of the placenta and fetus.

Levels of progesterone will be naturally higher during pregnancies that involve multiples (twins, triplets, etc.) than those in which there is only one fetus.

Increased progesterone levels also are seen occasionally with luteal ovarian cysts, molar pregnancies, and with a rare form of ovarian cancer.

Increased levels are occasionally due to an overproduction of progesterone by the adrenal glands.

In late pregnancy, low levels of progesterone may be associated with toxemia.

Taking estrogen and progesterone supplements also can affect results.