The Role of Progesterone in Reproduction

Progesterone is one of the most important hormones in pregnancy. It plays a fundamental role in the menstrual cycle, pregnancy and other physiological processes, being especially important for the preparation and maintenance of the uterus for pregnancy and for regulating various aspects of the reproductive cycle.

What Is the Hormone Progesterone?

Progesterone is a steroid hormone synthesized mainly by the corpus luteum, a temporary structure that forms in the ovary after ovulation. It is also known as the “pregnancy hormone” because of its key role in gestation.

What Does Progesterone Do?

Progesterone performs several vital functions in the body, and its importance becomes evident at different stages of a woman’s life:

In the woman’s menstrual cycle: preparing the uterus for implantation

During the menstrual cycle, progesterone is responsible for preparing the uterine lining (endometrium) for implantation of the fertilized embryo. If fertilization does not occur, progesterone levels fall, triggering menstruation and the start of a new cycle.

In pregnancy: maintaining early pregnancy

During pregnancy, progesterone is essential for maintaining the uterine lining and preventing it from shedding. During this period, the corpus luteum produces increasing levels of progesterone. This causes thickening of the endometrium and increased blood flow, providing an ideal environment for the embryo to attach and be nourished.

Progesterone also helps prevent early uterine contractions that could result in premature birth or harm the developing embryo. It also suppresses the maternal immune response against the fetus, thereby preventing possible rejection.

As pregnancy progresses, the placenta becomes the main source of progesterone.

Suppression of Ovulation

In addition to its functions during pregnancy, progesterone also regulates the reproductive cycle by inhibiting the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. This prevents the maturation and release of new eggs, which is crucial during pregnancy to avoid competition between multiple embryos.

Serum Progesterone: What Are Normal Progesterone Levels?

Serum progesterone refers to the amount of progesterone present in the bloodstream. Measuring serum progesterone levels is important in evaluating a woman’s reproductive health, especially if she is trying to conceive.

Normal progesterone levels for achieving pregnancy vary at different stages of the menstrual cycle and pregnancy:

• Early follicular phase: progesterone values should be below 0.1–0.7 ng/mL.
• Late follicular phase: when we want to ensure there has not been early excessive production that could impede implantation, levels should be below 1.2–1.5 ng/mL.
• Early to mid-luteal phase: progesterone values should be above 8.8–10.5 ng/mL.

It is important to note that exact values may vary depending on the laboratory and the measurement units used. Multiple studies have confirmed that low progesterone levels at the time of implantation or in the early stages of pregnancy can lead to implantation failure or miscarriage.

Progesterone Medications in IVF

Because of this, progesterone is used in assisted reproduction treatments, for example in in vitro fertilization (IVF). It is mainly employed to support embryo implantation in the early stages of pregnancy, even up to three or four months in patients with a history of recurrent miscarriage. In addition, in cases at risk of preterm birth, its use may be extended almost throughout the entire pregnancy.

In natural or spontaneous cycles, progesterone levels on day 21 should be between 12.5–18.8 ng/mL.

Would You Like to Know Your Progesterone Level?

If you’re trying to get pregnant or have concerns about your progesterone levels, at our Madrid clinic you can receive a precise and personalized evaluation.

Don’t hesitate to contact our professionals — they will answer all your questions without any obligation.

References

• American College of Obstetricians and Gynecologists. (2018). Progesterone. ACOG Practice Bulletin. Obstetrics and Gynecology, 131(5), e148–e157.
• American Family Physician. (2019). Progesterone. American Family Physician, 99(11), 923–929.
• Mayo Clinic. (2023). Progesterone. Mayo Clinic.
• National Institutes of Health. (2023). Progesterone. NIH.
• UpToDate. (2023). Progesterone. UpToDate.
• American Pregnancy Association. (2023). Progesterone. American Pregnancy Association.
• Mayo Clinic. (2023). Progesterone supplements. Mayo Clinic.
• National Institutes of Health. (2023). Progesterone supplements. NIH.
• UpToDate. (2023). Progesterone supplements. UpToDate.