Progesterone
Also written as: P4, Serum progesterone
The hormone of the second half of the menstrual cycle, used mainly to confirm ovulation.
What it measures
The level of progesterone, a hormone produced mainly by the ovary after an egg is released and, in pregnancy, by the placenta. It stays low through the first half of the menstrual cycle and rises only after ovulation.
Why a doctor orders it
Most often ordered about a week after presumed ovulation to confirm that ovulation actually happened, as part of a fertility evaluation. Also measured in early pregnancy assessment and to monitor some fertility treatments.
What is generally associated with a higher result
Broadly associated with recent ovulation, pregnancy, and progesterone-containing medications. In the right part of the cycle, a raised value is the expected, healthy finding.
What is generally associated with a lower result
Broadly associated with a cycle in which ovulation did not occur, or simply with blood drawn at a point in the cycle when progesterone is naturally low. Timing against the cycle is everything for this test.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- The day of the menstrual cycle - the meaning of the test depends entirely on timing
- Cycle-to-cycle variation in when ovulation happens
- Progesterone-containing medications and hormonal contraception
- Pregnancy
Questions to take to your appointment
- Was the draw timed correctly relative to when I likely ovulated?
- If my cycles are irregular, how should the timing of this test be chosen?
- Does this result confirm whether I ovulated in that cycle?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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