Day 21 progesterone test for ovulation

Progesterone is a hormone that helps prepare and maintain the lining of the womb for pregnancy. Lab Tests Online UK explains that in the first half of the cycle oestrogen makes the lining grow, and a surge in luteinising hormone mid-cycle leads to release of an egg. In the second half, the corpus luteum forms at the site of the released egg and produces progesterone.

Because progesterone is made after an egg is released, a clear rise in the second half of the cycle is evidence that ovulation has happened. This is why the NHS lists a progesterone blood test among the tests used to check whether you are ovulating when investigating infertility.

The name "day 21" assumes a 28-day cycle with day 1 as the first day of bleeding. Lab Tests Online UK says progesterone concentrations normally peak about 7 days before the onset of menses, so the sample should be taken at day 21 or 7 days before the expected period. The NHS says that timing of the test is based on how regular your periods are.

If your cycle is longer, the right day is later. If your cycle is irregular or prolonged, Lab Tests Online UK says progesterone can be measured weekly until the period begins. If your periods are irregular, the NHS says you will be offered a test of gonadotrophins, the hormones that stimulate the ovaries to produce eggs.

Interpretation depends on exactly where you were in your cycle when the sample was taken. Lab Tests Online UK says progesterone concentrations usually start to rise when an egg is released, continue to rise for several days, and then either keep rising in early pregnancy or fall to start the period. If concentrations do not rise and fall monthly, a woman may not be ovulating or having periods.

The sources used here do not give numerical cut-offs, and laboratories report their own units and ranges. Ask the person who ordered the test what the laboratory considers consistent with ovulation, and how the timing was taken into account. A low result on the wrong day is common and does not necessarily mean you did not ovulate; a repeat on a better-timed day is often offered.

A progesterone result tells you about one cycle. It does not show whether the eggs are of good quality, whether the fallopian tubes are open, or whether a partner's sperm is healthy. The NHS lists a wider set of tests for women that include chlamydia testing and ultrasound scans of the womb, ovaries and fallopian tubes, and for men a semen analysis.

Lab Tests Online UK also states that progesterone measurement does not have an established role in males.

The test is also not meant for early pregnancy symptoms. Lab Tests Online UK notes that NICE recommends ultrasound scanning together with human chorionic gonadotrophin (hCG) testing to diagnose early pregnancy problems rather than measuring progesterone.

No special preparation is needed, according to Lab Tests Online UK. The main practical work is tracking your cycle: note the first day of bleeding for several months so that the sampling day can be chosen accurately. Tell the person arranging the test if you are taking hormonal contraception or hormone supplements, because these can cause inaccurate results.

If you are using ovulation tests at home, you can mention the dates of a positive result. The sources used here do not specify how to combine home tests with the blood test, so the clinician will advise.

The NHS says that after an initial examination you may be referred to a specialist infertility team for further tests, and that progesterone testing is among the first blood tests for women. Private providers also offer a day 21 progesterone test on its own. Whichever route you use, ask for the result to be interpreted by someone who knows your cycle length and dates, as a stand-alone number with no timing context is hard to interpret.