What is progesterone?
Progesterone is a sex hormone in females that is important for the regulation of menstrual cycles and maintaining early stages of pregnancy. After ovulation, when an egg is released from the ovary, the remnants of the ovarian follicle called corpus luteum starts to produce large amounts of progesterone. Progesterone prepares the endometrium (the lining of the uterus) and makes the environment in the uterus suitable for the fertilised egg to implant.
Progesterone level in the blood peaks approximately at day 21 in the menstrual cycle. If no fertilised egg implants, progesterone level will start to decrease and a new menstrual cycle will begin. If a fertilised egg is implanted, progesterone will continue to be produced to stimulate the blood vessels of the endometrium to grow and glands to produce nutrients to nourish the fertilised egg to grow into an embryo.
In short, it is useful to analyse progesterone levels in the blood to:
Investigate cause of infertility
Detect anovulatory cycles
Monitor early pregnancy
Monitor progesterone replacement therapy
Why is this analysis important?
Women usually release one egg during every menstrual cycle. Having normal menstruation is usually, but not always a sign that a woman is also ovulating normally. It is also possible to experience so-called “anovulatory cycles”, which means that the woman goes through a menstrual cycle without releasing any eggs. In a normal menstrual cycle, menstrual bleeding is caused by a drop in progesterone level prior to menstruation. In an anovulatory cycle, bleeding is instead caused by insufficient oestrogen to support the endometrium.
Anovulatory bleeding is often more irregular than normal menstrual bleeding, but other than that, it can be difficult to tell apart from normal menstrual bleeding. Testing progesterone levels in the blood on day 21 in the menstrual cycle (if your normal menstrual cycle is 28 days) can help determine if you have had an ovulation in the present cycle. Having frequent anovulatory cycles can be a cause of infertility. Failure to ovulate is a reason for infertility in about 20% of cases.
When a fertilised egg is implanted in the uterus, progesterone levels will remain high during the entire pregnancy. About 30% of women will experience abdominal pain or bleeding during early pregnancy and this is often associated with a lot of anxiety. Ultrasound can help determine whether a pregnancy is viable or not, but the results can sometimes be inconclusive, especially in the very beginning of a pregnancy when it is not possible to see heartbeat from the baby. Studies have shown that testing progesterone in the blood can help determine the viability of a pregnancy.
The reference range for progesterone levels in the blood can be different depending on the laboratory and technique used. Doctors usually also take into account a number of factors, such as the day in the menstrual cycle and pregnancy status, when evaluating progesterone values.
High progesterone levels in the blood may be associated with:
Treatment with progesterone replacement therapy
Certain progesterone producing tumours (rare)
Low progesterone levels in the blood may be associated with:
Test taken during the follicular phase of the menstrual cycle
Very high level of exercise
To detect anovulatory cycles, the progesterone test should be taken 7 days before the expected start of the next menstrual bleeding. This means that if you normally have a 28-day menstrual cycle, the test should be taken on day 21, that is 21 days after the first day of menstrual bleeding. If you normally have a 32-day cycle, then you should take the test on day 25 and so on.
To determine the viability of a pregnancy, progesterone levels need to be interpreted together with ultrasound and hCG results.
Use of contraceptives or hormone replacement therapy (HRT) containing progesterone or progestin can affect the results of the progesterone test. Discuss with your doctor before ordering a test.
Tests that include this marker
Jorine Verhaegen et al. Accuracy of single progesterone test to predict early pregnancy outcome in women with pain or bleeding: meta-analysis of cohort studies. British Medical Journal. 2012;345:e6077Jerome F. Strauss, III et al. Yen & Jaffe's Reproductive Endocrinology. Seventh Edition. 2014