Progesterone: a steroid hormone produced mainly by the corpus luteum of the ovary after ovulation, and by the placenta during pregnancy. It is the body’s main gestagen: it prepares the endometrium for implantation, sustains pregnancy in its first weeks and, together with estradiol, regulates the second half of the menstrual cycle. Its blood level varies a great deal depending on the day of the cycle, so it is always interpreted in light of the moment when it was measured.
What is progesterone for?
The main functions of progesterone are concentrated in the luteal phase of the cycle and in pregnancy:
- Maturing the endometrium: it transforms the proliferative endometrium (thickened by estrogens) into a secretory endometrium, receptive to implantation.
- Sustaining early pregnancy: it keeps the endometrium active and reduces uterine contractions until the placenta takes over around weeks 10-12.
- Slowing endometrial growth: it counterbalances the proliferative effect of estradiol.
- Raising basal body temperature: the 0.3-0.5 °C rise in the second phase of the cycle is due to it.
- Thickening cervical mucus: it makes mucus less permeable to sperm after ovulation.
- Acting on the central nervous system: it has a sedative effect and is linked to some symptoms of premenstrual syndrome.
In men it is also present at very low levels: it acts as a precursor in the synthesis of testosterone and corticosteroids, but its reproductive role is concentrated in women.
How it changes across the menstrual cycle
Progesterone does not have a single reference value: it depends almost entirely on the day of the cycle. As a reference, in ng/mL:
- Follicular phase (days 1-13): <1 ng/mL. Basal values, practically undetectable.
- Ovulation: 1-3 ng/mL. Brief rise coinciding with the LH peak.
- Luteal phase (day 21): >10 ng/mL is generally considered compatible with confirmed ovulation. Values between 5 and 10 ng/mL are ambiguous, and <5 ng/mL in the luteal phase suggests anovulation or an insufficient luteal phase.
- First trimester of pregnancy: 11-44 ng/mL, with wide individual variability.
- Second trimester: 25-83 ng/mL.
- Third trimester: 65-290 ng/mL.
The ranges are indicative and change with the laboratory and the unit used (ng/mL or nmol/L). The first thing to look at on a result is the reference range on the report itself.
The classic clinical measurement is taken on day 21 of a regular cycle or 7 days after confirmed ovulation, the moment when progesterone reaches its peak. A single blood test does not diagnose anything on its own: progesterone is pulsatile and should be interpreted alongside the ultrasound scan and the rest of the hormone work-up.
Progesterone in pregnancy
During the first weeks of gestation, progesterone is produced by the corpus luteum, which is kept active by the hCG secreted by the newly implanted embryo. Around weeks 10-12 the placenta takes over production and levels rise progressively until the end of pregnancy.
In a normally progressing pregnancy, progesterone climbs steadily. When marked drops appear on serial blood tests, or values are well below range in the first trimester, they are combined with beta-hCG and the ultrasound scan to interpret the context: the isolated number does not give the diagnosis.
Progesterone in assisted reproduction
In assisted reproduction we need to distinguish two uses of progesterone: as medication to support the luteal phase after a transfer (fresh or frozen) and as an analytical parameter measured to confirm that the endometrium is receptive before transferring.
Progesterone as medication: luteal phase support
After egg retrieval in in vitro fertilization (IVF) or in a frozen embryo transfer, the corpus luteum does not always produce enough progesterone (in substituted cycles it doesn’t exist at all). Exogenous micronized progesterone is administered to sustain the endometrium until the placenta takes over around weeks 10-12 if the pregnancy progresses.
The available routes are vaginal, oral, rectal, and subcutaneous. The choice depends on the protocol and individual tolerance; the exact regimen is tailored by the medical team. Synthetic progestins (dydrogesterone, medroxyprogesterone, and others) are derived molecules with different pharmacological properties and are not interchangeable with natural progesterone.
Blood monitoring: confirming endometrial receptivity
In every transfer (fresh or frozen, natural or substituted cycle) we monitor blood progesterone to make sure the implantation window is open. At least two blood tests are taken:
- One before starting progesterone, as a baseline reference.
- Another after 3-5 days of administration, just before transfer, to confirm that levels are adequate.
If the first test shows progesterone already elevated in the follicular phase, we rethink the cycle to avoid transferring with an endometrium that has run ahead of the embryo. If the second test shows insufficient progesterone, we adjust the protocol or reschedule the transfer.
Progesterone as a marker of premature ovulation
During ovarian stimulation, an elevated progesterone in the days leading up to trigger suggests that the endometrium may have run ahead of the embryo, which in many cases leads to freezing the embryos and transferring in a later cycle (freeze-all strategy).o que en muchos casos lleva a vitrificar los embriones y transferir en un ciclo posterior (estrategia freeze-all).
Frequently asked questions about progesterone
As a reference, a value above 10 ng/mL on day 21 of the cycle or 7 days after ovulation is compatible with confirmed ovulation and an adequate luteal phase. Between 5 and 10 ng/mL is ambiguous, and below 5 ng/mL suggests anovulation or luteal phase deficiency. The exact ranges are set by each laboratory.
It keeps the endometrium receptive, reduces uterine contractions, and sustains pregnancy in its first weeks. During the first trimester it is produced by the corpus luteum; from weeks 10-12 the placenta takes over.
In the luteal phase or during pregnancy, high progesterone is physiologically expected and is not interpreted as a problem. In the follicular phase it is worth looking into, since it can indicate a persistent luteal cyst or a cycle dating error.
Yes. While progesterone is being administered (oral or vaginal), the endometrium stays stable and the period does not appear. When it is stopped, the hormonal drop triggers bleeding in the following days.
Yes, at very low levels. It acts as a precursor in the synthesis of testosterone and corticosteroids and comes mostly from the adrenal glands and testes.
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No. Progesterone is a pulsatile hormone and its value changes a lot across the day and the cycle. An isolated number never replaces the full hormone work-up, the ultrasound scan, and the clinical history.
