Recurrent pregnancy loss (RPL) is one of the most emotionally challenging conditions couples may face when trying to conceive. While miscarriage is relatively common, experiencing multiple pregnancy losses often raises difficult questions about the underlying causes and possible treatments.
In this webinar, Dr Abraham Zavala, Fertility Specialist at Clínica Tambre, explored the complex nature of recurrent pregnancy loss, focusing particularly on immunological factors that may contribute to the condition. The presentation also included real clinical cases to illustrate how different diagnostic approaches can help identify potential causes and guide treatment strategies.
The discussion emphasised that RPL is rarely caused by a single factor. Instead, it usually involves a combination of genetic, anatomical, immunological, and environmental elements that require a personalised evaluation.
A pregnancy loss is defined as the spontaneous end of pregnancy before the 20th week of gestation. This includes any form of pregnancy loss, regardless of how early it occurs.
A miscarriage, however, refers specifically to pregnancy loss before week 20 that has been confirmed by ultrasound imaging.
Recurrent pregnancy loss is diagnosed when a woman experiences two or more pregnancy losses.
Dr Zavala noted that this definition has changed in recent years. Previously, RPL was defined after three losses, but current clinical practice often begins investigation after two miscarriages in order to start diagnostic work earlier and improve the chances of identifying a cause.
Importantly, the number of losses does not change the definition. Two, three, or more miscarriages are all considered RPL once the threshold of two losses is reached.
Although pregnancy loss can occur at any point before week 20, the vast majority of miscarriages happen much earlier.
According to Dr Zavala, up to 99% of pregnancy losses occur during the first trimester, typically before week 12 of pregnancy.
Dr Zavala explained that the causes of recurrent pregnancy loss generally fall into two broad categories:
In other words, either the embryo itself has a genetic issue, or the environment within the uterus is not optimal for implantation and early development.
In humans, a normal set of chromosomes contains 46 chromosomes arranged in pairs.
Egg cells and sperm cells each carry 23 chromosomes, which combine during fertilization to form an embryo with 46 chromosomes.
However, this process does not always occur perfectly.
Embryos sometimes develop with an incorrect number of chromosomes. These abnormalities can occur if:
Most of the time, these genetic errors occur randomly and cannot be controlled.
Although both egg and sperm quality can influence embryo development, Dr Zavala emphasised that egg quality is currently the most important factor associated with chromosomal abnormalities.
Unfortunately, egg quality cannot be measured directly. No test equivalent to AMH or antral follicle count can evaluate egg quality.
The main indicator used in clinical practice is age, as the risk of chromosomal abnormalities increases as maternal age rises.
Even when embryos are genetically normal, implantation may still fail due to conditions affecting the uterine environment.
Several factors can influence the uterine environment, including:
Structural abnormalities of the uterus may interfere with implantation, such as:
Inflammatory conditions, including chronic endometritis, may also contribute to pregnancy loss. These infections may go unnoticed and require specialized testing to detect.
One of the central themes of Dr Zavala’s presentation was the role of the immune system in implantation and early pregnancy.
Implantation involves a complex interaction between the embryo and the mother’s immune system.
As Dr Zavala explained, the immune system is designed to protect the body from foreign material. However, an embryo contains genetic material from both parents, meaning that at least 50% of the embryo is genetically different from the mother.
Despite this, the immune system must allow implantation and support the developing pregnancy.
This delicate balance is regulated through immune mechanisms within the uterus.
Some immune-related conditions increase the risk of blood clot formation.
These conditions are known as thrombophilias.
Thrombophilias can be:
Inherited thrombophilias are genetic conditions present from birth, while acquired thrombophilias develop later in life due to immune system activity.
One important acquired thrombophilia associated with recurrent pregnancy loss is antiphospholipid syndrome (APS).
APS is caused by antibodies that increase the tendency for blood clots to form. This condition has been linked to:
Another important immune factor involves natural killer (NK) cells, a type of white blood cell.
Different types of NK cells exist within the body, including those that circulate in the bloodstream and those that reside in the uterine lining.
Uterine NK cells play a key role in the communication between the embryo and the uterine lining during implantation.
These cells interact with the embryo through receptors known as KIR receptors, while the embryo expresses HLA molecules.
Certain combinations of maternal KIR receptors and embryonic HLA types may not be optimal for implantation.
Dr Zavala noted that researchers avoid describing these combinations as incompatibilities, but some interactions may reduce the chances of successful implantation.
Several additional health conditions may also contribute to recurrent pregnancy loss.
Maintaining healthy thyroid function is essential for pregnancy.
Thyroid-stimulating hormone (TSH) levels that are considered normal in the general population may not always be ideal for women trying to conceive or already pregnant.
When TSH levels are elevated, treatment may be recommended to restore normal thyroid function.
Polycystic ovary syndrome (PCOS) and higher body mass index (BMI) have also been associated with increased miscarriage risk.
While weight management can be challenging, addressing metabolic factors may help improve reproductive outcomes.
Vitamin D levels are also important for reproductive health.
Interestingly, even in sunny countries such as Spain, many patients still experience vitamin D deficiency because optimal sun exposure depends on specific times and durations of sunlight exposure.
Proper evaluation of recurrent pregnancy loss requires a comprehensive diagnostic approach.
Possible tests include:
Endometrial biopsy may also help identify:
These tests should not be applied universally but tailored to each patient’s clinical history.
The European Society of Human Reproduction and Embryology (ESHRE) has developed guidelines for evaluating and managing recurrent pregnancy loss.
When APS is diagnosed, treatment often includes:
If hypothyroidism is detected, levothyroxine may be prescribed to restore normal hormone levels.
Certain uterine abnormalities may require surgical correction, although not all structural issues require treatment.
Dr Zavala emphasised that psychological support is a critical part of care.
Recurrent pregnancy loss can significantly affect the emotional well-being of both partners, and counselling should be offered when appropriate.
Dr Zavala presented real-life patient cases to demonstrate how immunological factors can influence recurrent pregnancy loss and how different diagnostic and treatment approaches can help improve patient outcomes.
The first case involved a 30-year-old woman who experienced two miscarriages after natural conception.
Despite undergoing four IVF cycles and multiple embryo transfers, no pregnancies occurred.
Further investigation revealed:
After treatment with antibiotics and immunological therapy, a single chromosomally normal embryo was transferred. The pregnancy progressed successfully and remained ongoing.
The second case involved a 36-year-old woman with repeated implantation failure.
Investigations revealed:
Treatment included:
Following the first embryo transfer after treatment, the patient achieved a healthy ongoing pregnancy.
Dr Zavala concluded that recurrent pregnancy loss remains a complex condition requiring individualized evaluation.
Several important messages emerged from the presentation:
Although research continues to evolve, a better understanding of genetic, anatomical, and immunological factors is helping clinicians develop more targeted treatment strategies.
For many couples experiencing recurrent pregnancy loss, careful investigation and individualised care can ultimately lead to successful pregnancy outcomes.
Without knowing the full medical background, including previous pregnancies or additional factors, this situation would usually be classified not as recurrent pregnancy loss but as recurrent implantation failure (RIF) when two or more good-quality blastocysts have not implanted.
It is important to remember that a good-quality embryo based on morphology does not necessarily mean the embryo is chromosomally normal. Good morphology is not always synonymous with euploidy.
After two unsuccessful transfers, it may be an appropriate moment to begin more in-depth investigations. These could focus either on the embryo itself or on the environment in which the embryo is being transferred. Taking a short break can be reasonable, but it may also be useful to use this time to evaluate additional factors affecting implantation.
This question connects to an important point from the presentation. In patients who have not experienced pregnancy losses or implantation failure, most of the specialized tests discussed are not routinely recommended.
Aside from possibly performing a mock cycle to observe how the endometrium responds to medication or to a natural cycle, additional in-depth testing is generally not necessary in these cases.
It is important to understand when to perform these tests and when they are not needed.
Regarding immunological causes, they are not the most common cause of miscarriage. Many other factors should be investigated first. The most frequent cause of recurrent pregnancy loss remains embryonic genetic abnormalities, which are the leading cause of miscarriage at any age.
As for age, there is ongoing research in this area. Autoimmune conditions tend to become more common with increasing age because the immune system is exposed to more antigens over time and develops immune memory. This may lead to increased immune reactivity. However, there is currently no specific age cutoff that clearly defines this relationship.
IVIG is one of the treatments sometimes used in cases involving elevated natural killer (NK) cells or certain KIR–HLA combinations.
Before considering IVIG, it is important that these treatments are evaluated by a reproductive immunologist, as they specialize in managing immune-related fertility conditions.
IVIG works by helping regulate the immune system. However, the scientific evidence is mixed, and studies have not consistently shown that IVIG significantly improves live birth rates.
Each case must be evaluated individually because NK cells are not a single uniform group. There are different types of NK cells, and their roles vary. For example, increased cytotoxic NK cells are different from increased angiogenic NK cells. Therefore, interpretation requires careful analysis before deciding on treatment.
Endometriosis is not only a condition that causes pelvic pain; it has several implications for fertility.
Endometriosis occurs when cells from the uterine lining implant outside the uterus. The ovaries are the most common site, where the condition may form what is commonly called a “chocolate cyst”, or endometrioma.
However, endometriosis can occur in other locations. One related condition is adenomyosis, where endometrial tissue infiltrates the muscular wall of the uterus. Although often treated as a separate diagnosis, adenomyosis is essentially a form of endometriosis affecting the uterine wall.
Both endometriosis and adenomyosis can influence fertility in several ways:
They may affect the number of follicles and egg quality in the ovaries.
They may alter the uterine lining and distort the uterine cavity.
They may affect blood flow to the endometrium.
They may influence uterine contractions, making the uterus more sensitive to hormonal stimulation.
They may create a pro-inflammatory environment.
In this case, because euploid embryos were transferred and implantation still failed, the underlying issue may not be related to embryo genetics. A detailed evaluation of the uterine cavity and the impact of endometriosis or adenomyosis would be particularly important.
Hashimoto’s disease is the most common cause of hypothyroidism, so thyroid function must be carefully monitored before embryo transfer. Thyroid-stimulating hormone (TSH) levels should be within an optimal range, usually controlled with levothyroxine.
Regarding a cold before embryo transfer, there is no definitive answer. However, having a cold may indicate that the immune system is temporarily dysregulated, whether upregulated or downregulated. Because of this, many clinicians may prefer to postpone the transfer until the patient has fully recovered, especially if fever or infection is present.
Mast cell activation affects different types of immune cells than those typically evaluated in reproductive immunology, such as NK cells.
However, histamine overload may influence uterine blood flow, so monitoring this condition is important.
Many fertility clinics are led primarily by gynecologists. Patients with complex immune-related conditions may benefit from consulting a clinic that includes a reproductive immunology unit, where multidisciplinary care can be provided.
PCOS presents with different phenotypes, and it is important to understand that PCOS is not always associated with high BMI.
Diagnosis typically requires at least two of the following three criteria:
Irregular menstrual cycles
Hyperandrogenism (elevated male-type hormones)
Polycystic ovaries on ultrasound
Features such as obesity or insulin resistance are not required for diagnosis.
PCOS is sometimes associated with insulin resistance and hormonal imbalances that may influence egg quality. Patients with PCOS often have a high ovarian reserve, but this may sometimes be associated with variable egg quality.
Treatment protocols should be individualized and may include:
FSH or LH stimulation protocols
Combined FSH/LH protocols
Medications such as metformin or inositol
Regarding lupus anticoagulant, this antibody is associated with antiphospholipid syndrome (APS), a condition that increases the risk of blood clot formation.
The main antibodies involved in APS are:
Lupus anticoagulant
Anticardiolipin antibodies
Beta-2 glycoprotein I antibodies
When APS is diagnosed, treatment typically includes blood-thinning medications such as heparin and low-dose aspirin during treatment and pregnancy.
Treatment for KIR–HLA combinations depends on the type of fertility treatment.
For example, when egg donation is used, it may be possible to select a donor whose HLA type is more compatible with the patient’s KIR receptors.
In the case presented, the patient underwent IVF using her own eggs and her partner’s sperm, so donor selection was not an option.
Instead, the patient was referred to a reproductive immunology unit, where the immunologist recommended treatment with G-CSF (granulocyte colony-stimulating factor) to address the specific KIR–HLA combination.
It would first be important to determine how the NK cells were measured.
NK cells can be assessed through:
Blood testing
Endometrial biopsy using immunohistochemistry markers such as CD56 and CD138
However, these methods do not always accurately reflect the true activity of NK cells in the uterus.
NK cell levels in the uterine lining also vary throughout the menstrual cycle, particularly around the implantation window. Ideally, testing should occur around five days after progesterone exposure, which corresponds to the implantation period.
Without this information, it is difficult to interpret the NK cell results or recommend specific treatment.
First, it is important to acknowledge that miscarriage is a very difficult experience.
Although chromosomal abnormalities are the most common cause of miscarriage, they are not the only cause. Even when a euploid embryo is transferred, other factors may influence the outcome.
Medications such as Clexane and aspirin are prescribed for specific conditions and may not address every possible cause.
After a miscarriage following a euploid transfer, further evaluation may include examining the uterine cavity for abnormalities such as:
Polyps
Fibroids
Other structural issues
More detailed uterine assessment before another transfer may help identify potential contributing factors.
This situation can sometimes occur.
During natural conception, the body goes through multiple cycles of follicular selection, which naturally selects the best egg over time.
In IVF, this selection occurs during a single stimulation cycle. If follicles do not respond well to stimulation medication, the outcome may appear less favorable than natural conception.
This may indicate a poor ovarian response, meaning that follicles are present but do not respond optimally to medication.
In such cases, it is important to review the ovarian stimulation protocol and consider adjustments to improve ovarian response.
This treatment protocol is sometimes used in cases involving immune factors.
Typically, prednisolone may be started before embryo transfer and later switched to dexamethasone, or the protocol may alternate between the two depending on the clinical situation.
These corticosteroid medications are sometimes used when there is evidence of increased NK cell activity or other immune-related conditions, but the exact protocol should always be determined by the treating specialist.
