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Progesterone is a female hormone produced by the ovaries, adrenal glands, and the placenta during pregnancy. Progesterone helps prepare the lining of the uterus to receive the egg if a sperm fertilizes it. If fertilization doesn't happen, progesterone levels drop and menstrual bleeding begins. 
During pregnancy, the placenta produces high levels of progesterone, starting near the end of the first trimester and continuing until the baby is born. Levels of progesterone are about 10 times higher in a pregnant woman than they in a non-pregnant female. Moreover, some types of cancer cause abnormal progesterone levels in both men and women. 
Why Are Progesterone Levels Measured?
A progesterone test is done to help find the cause of infertility, monitor the success of medications for infertility or the effect of treatment with progesterone, determine whether ovulation is occurring, and assess the risk of miscarriage. Progesterone tests also help diagnose problems with the adrenal glands and some types of cancer.
If your doctor found it necessary for you to have a progesterone test, you should stop taking medications, including birth control pills, that contain estrogen or progesterone or both, for up to 4 weeks before having the test.
You should also tell your healthcare professional if you have had a test that used a radioactive substance within the last seven days. Some tests, such as thyroid or bone scans, use a radioactive tracer, and that can interfere with the test results. Let your healthcare professional know the first day of your last menstrual period and describe your menstrual bleeding pattern: whether it's light, does it begins with spotting, when it's the heaviest, and anything else they may ask.
You should also talk to your health professional about any concerns you have regarding the need for the test, its risks, or what the results might indicate.