Progesterone

Progesterone is a fundamental steroid hormone secreted by the corpus luteum after ovulation and plays a critical role in preparing the endometrium for pregnancy. Monitoring its levels provides an important biochemical indicator for evaluating luteal phase adequacy.

Measuring progesterone levels objectively reveals hormone fluctuations that vary according to the timing of the cycle, thereby confirming luteal phase function. This test offers a reliable diagnostic parameter in the assessment of reproductive health.

Symptoms of progesterone deficiency offer clinical guidance in identifying endometrial development disorders that may affect implantation success. These findings significantly contribute to planning treatment strategies in infertility investigations.

Progesterone treatment protocols include dose and formulation options aimed at optimizing endometrial receptivity in assisted reproductive techniques. The application offers a controlled approach that enhances patient-specific treatment compliance by monitoring hormonal response.

What Is Progesterone?

Progesterone is a hormone secreted mainly by the ovaries after ovulation in women and prepares the uterine lining for pregnancy. It is also produced by the placenta during pregnancy. Progesterone levels increase in the second half of the menstrual cycle and support the implantation of the embryo into the uterus. Low levels may indicate menstrual irregularities or infertility.

Why Does IVF Treatment Lead to Progesterone Deficiency?

The goal of IVF treatment is to obtain a much higher number of eggs than in a normal cycle. To achieve this, medication protocols called “controlled ovarian stimulation” (COS) are used. However, due to the nature of the treatment, this process inadvertently disrupts the body’s own progesterone production mechanism.

This condition is known as “luteal phase defect” and is an expected result of the treatment. The reasons are as follows:

• Growth of a large number of follicles (egg sacs)
• Secretion of supraphysiological hormone levels from the growing follicles
• The body perceiving these excessively high hormone levels as a “danger”
• The main control centers in the brain (hypothalamus and pituitary) activating a “brake mechanism”
• This brake mechanism suppressing LH, the hormone that keeps the corpus luteum alive
• The corpus luteum losing its function rapidly and stopping progesterone production when LH support is cut off
• GnRH agonist or antagonist medications used to prevent premature ovulation also suppressing LH release
• Physical damage to some of the cells inside the follicles during the egg retrieval (OPU) procedure

When all these factors come together, the body’s own progesterone production becomes insufficient to support a healthy pregnancy after egg retrieval. The uterine lining loses support and may begin to shed before the embryo can implant.

For this reason, external progesterone supplementation in IVF treatments is not optional but an absolute necessity for treatment success. This support fills the hormonal gap created by the treatment.

How Does the Choice of Trigger Shot Affect Progesterone Support?

The “trigger shot” used to bring the eggs to their final maturation stage is one of the most critical steps of the treatment and directly determines the type and intensity of progesterone support to be given afterward. There are two main types of trigger shots:

• hCG (Human Chorionic Gonadotropin) Trigger: This injection mimics the body’s natural LH hormone. However, the duration of hCG in the body (half-life) is much longer than that of LH (lasting several days). This prolonged effect provides strong “life support” to the corpus luteum formed after egg retrieval and helps them produce progesterone. This partially alleviates luteal phase deficiency. However, as the effect of hCG decreases over time and the body’s own LH remains suppressed, external progesterone support is still required. Its biggest disadvantage is the significant increase in OHSS (Ovarian Hyperstimulation Syndrome) risk, especially in women who develop a high number of eggs.
• GnRH Agonist Trigger: This injection does not provide hormones externally; instead, it stimulates the patient’s own pituitary gland, causing a sudden and short-term release of LH and FSH (a “flare”). This brief natural surge is sufficient to mature the eggs. However, immediately afterward, the pituitary rapidly becomes “exhausted” and fully suppressed. As a result, no LH signal remains to support the corpus luteum. This leads to a very rapid and profound progesterone deficiency. Its greatest advantage is that it almost completely eliminates the risk of OHSS; therefore, it is known as the “safe trigger” for high-risk patients. The cost of this safety is the need for a much more intensive and careful progesterone support protocol.

In summary, while the hCG trigger provides some support, the GnRH agonist trigger provides almost none, and this completely changes the progesterone replacement strategy.

What Are the Different Methods of Progesterone Support?

Over the years, various administration methods have been developed to make progesterone support both effective and comfortable for patients. Each has its own advantages and disadvantages:

Intramuscular (IM) Injection (Progesterone Injection)

This traditional method, considered the “gold standard” for many years especially in the USA, involves injecting oil-based progesterone into the muscle from the hip. It is quickly absorbed into the blood and provides high, stable progesterone levels. It is often preferred in frozen embryo transfer (FET) cycles where the body does not produce any progesterone.

However, it is the most challenging method in terms of patient comfort. Possible issues include:

• Painful injections
• Swelling at injection site
• Redness or firmness
• Rare sterile abscesses
• Difficulty of administration (cannot be self-administered)

Vaginal Progesterone (Gel, Capsule, or Suppository)

This is currently the most commonly used method worldwide. It may be in the form of soft capsules containing micronized progesterone, suppositories, or a sustained-release gel applied once daily.

Its greatest characteristic is the “first uterine pass effect.” When administered vaginally, progesterone is absorbed directly into the uterine tissue and vessels without significantly entering the bloodstream. This allows very high concentrations to reach the endometrium, the primary site of action. This is why vaginal administration reflects less in blood tests (serum progesterone).

Advantages and disadvantages:

• Ease of use (self-administered)
• Painless application
• Direct effect on uterine tissue
• Vaginal discharge (most common side effect)
• Local irritation or itching (less common)

Subcutaneous (SC) Injection (Progesterone Injection)

This is one of the newer and more innovative options in progesterone therapy. It is a water-soluble form of progesterone and is administered under the skin, usually in the abdomen, with a very small needle similar to an insulin injection.

This method aims to combine the advantages of IM injections (reliable blood levels) with those of vaginal administration (ease of use).

• Provides reliable blood levels like IM injections
• Easy and painless application similar to an insulin injection
• Self-administration possible
• High patient satisfaction
• Does not cause the painful reactions seen with IM injections

Oral Progesterone (Dydrogesterone Tablet)

Oral use is the most comfortable method for patients. However, when natural micronized progesterone is taken orally, it is almost completely broken down in the liver and the amount reaching the uterus remains very low. Therefore, natural progesterone pills are considered ineffective in IVF treatment.

However, dydrogesterone has a different structure (a “retro-progesterone”). Thanks to this special molecular structure, it is not broken down in the liver when taken orally and is absorbed at a high rate. It binds strongly to progesterone receptors in the uterus, creating the necessary effect.

• Easiest use (tablet)
• Highest patient compliance and satisfaction
• Headache (possible side effect)
• Nausea (possible side effect)
• Breast tenderness (possible side effect)

What Does Scientific Evidence Say About Which Progesterone Method Is Better?

Clinical decisions are based on the results of large scientific studies (meta-analyses) involving thousands of patients. These studies guide us on which method is better under which condition.

• Intramuscular (IM) vs. Vaginal Progesterone:

Years of research have generally shown no significant difference in live birth rates between IM injections and vaginal progesterone when looking at both fresh and frozen transfers together. The high local effect provided by vaginal administration in the uterus is as successful as the high systemic effect provided by IM injections. Patient satisfaction overwhelmingly favors the vaginal route.

• Frozen Embryo Transfer (FET) Exception:

The situation changes somewhat in “programmed” FET cycles. In these cycles, the body has no corpus luteum and pregnancy relies entirely on externally administered hormones. In this “full replacement” scenario, some large and important studies have shown that using only vaginal progesterone may be associated with higher miscarriage rates compared to IM or combined IM+vaginal regimens. Therefore, in programmed FET cycles, choosing IM or SC injections that provide stable blood levels may be a safer approach.

• Oral (Dydrogesterone) vs. Vaginal Progesterone:

The results of this comparison vary depending on whether the transfer is fresh or frozen.

• Fresh Transfers: The evidence is very interesting. Two large international trials (LOTUS I and II) and the combined meta-analysis of their data found that oral dydrogesterone may be associated with statistically significantly higher live birth rates compared to vaginal progesterone in fresh transfers. This makes dydrogesterone not only “convenient” but also “potentially more effective” for fresh transfers.
• Frozen Transfers (FET): Current scientific evidence (2023 meta-analysis) shows no significant difference between oral dydrogesterone and vaginal progesterone in FET cycles in terms of live birth, pregnancy, or miscarriage rates. Both methods appear equally effective in this scenario.

When Should Progesterone Support Begin and How Long Should It Continue?

The timing and duration of progesterone support are as critical for treatment success as the dosage.

• Fresh Embryo Transfer Cycles:

Progesterone support should begin anytime from the evening of the egg retrieval (OPU) day to no later than the 3rd day after OPU (when the embryo is on day 3). This timing ensures synchrony between the embryo’s arrival in the uterus and the “implantation window” of the uterine lining.

Support should continue at least until the day of the pregnancy test (usually 12–14 days after transfer). If the test is positive, common clinical practice is to continue support until the 8th–12th week of pregnancy (until the placenta takes over). Interestingly, scientific studies have not found strong evidence that continuing support after a positive test increases live birth rates compared to stopping at the test date. However, due to cautious clinical approaches and the psychological comfort for patients (“knowing the baby is being supported”), continuation is often preferred.

• Frozen Embryo Transfer (FET) Cycles:

Here the protocol differs completely depending on whether the cycle is “programmed” or “natural.”

Programmed (Artificial) FET Cycles: In this method, the patient’s own ovulation is suppressed with medication and the uterine lining is prepared with external estrogen. Since there is no progesterone source (corpus luteum) in the body, external progesterone supplementation is vital. Timing is exact: progesterone is started when the uterine lining is ready, and the transfer day is calculated based on the embryo’s age and progesterone exposure duration (e.g., a day-5 blastocyst is transferred on day 6 of progesterone support).

Natural Cycle FET: In this method, the patient’s own ovulation is monitored, and the corpus luteum produces progesterone after ovulation. For years, it was thought that no extra support was needed in these cycles. However, a very recent and important meta-analysis published in 2022 showed that providing external progesterone support even in natural FET cycles significantly increases live birth rates. This finding has changed clinical practice, and progesterone support is now routinely recommended in natural cycles as well.

Is It Beneficial to Add Other Hormones to Progesterone Support?

Various hormones have been investigated to further strengthen progesterone support. However, current scientific evidence does not support their routine use. Considered additional treatments include:

• Estrogen (Estradiol)
• hCG (Low Dose)
• GnRH Agonists

As for why these additions are not recommended: it has been proven that adding estrogen to progesterone in fresh transfer cycles does not provide any additional pregnancy benefit. Low-dose hCG injections increase progesterone but significantly raise the risk of OHSS and do not offer a clear advantage in live birth rates, so they are strictly not recommended routinely. Adding GnRH agonists during the luteal phase also lacks strong evidence and is not advised.

Is a Progesterone Blood Test Necessary During Treatment?

The answer varies depending on the type of treatment.

Fresh Transfer Cycles: Measuring progesterone levels is generally not very useful. The level measured in the blood is a mix of the body’s own fluctuating progesterone and the externally administered medication (which may not reflect well in the blood if given vaginally). Therefore, the result may be confusing and not reliable for clinical decisions.

Programmed FET Cycles: Measuring progesterone levels is highly valuable. Since the body produces no progesterone in these cycles, the blood level directly shows how well the externally administered medication is absorbed and whether it is sufficient.

Research shows there is an “optimal middle range” (therapeutic window) for progesterone levels. Too low levels (around %P < 10–15 { ng/mL}%) reduce implantation chances. Interestingly, some studies show that excessively high levels (%P > 30–35 { ng/mL}%) may also impair uterine receptivity and negatively affect success.

If progesterone levels measured on transfer day in a programmed FET cycle are below the desired range, a “rescue protocol” may be considered (such as adding a subcutaneous or intramuscular injection to the existing dose). This may increase pregnancy chances in patients with absorption problems. However, sometimes low progesterone may signal not just an absorption issue but also an underlying issue with the uterine lining or the embryo.

How Is Personalized Progesterone Support Planned?

As seen, progesterone support is not a “one-size-fits-all” treatment. A personalized, evidence-based approach is followed to maximize success.

The following steps are considered when planning:

• Determining the type of treatment cycle (Fresh transfer, Programmed FET, Natural cycle FET)
• Selecting the most appropriate progesterone administration route (based on scientific evidence, cycle type, and patient comfort preferences)
• Applying the correct timing and dosage (following standard, evidence-based protocols)
• In frozen embryo transfers, monitoring blood levels to confirm adequate dosage
• Discussing all options with the patient and making a shared decision

Frequently Asked Questions