The Resistant Endometrium: When Standard Preparations Fail

When Standard Preparations Fail to Build a Sufficient Endometrial Thickness and Appearance

Preparing the endometrium, or the lining of the uterus, is an important part of many embryo transfer cycles. In a standard medicated preparation cycle, estrogen is used to stimulate the endometrium to grow. Once the lining has developed appropriately, progesterone is introduced to prepare it for implantation and synchronize it with the developmental stage of the embryo.

For most women, this process is relatively straightforward. However, there is a smaller group of patients in whom the endometrium simply does not respond as expected. The estrogen dose may be increased, and treatment may be extended for several additional days, yet the lining remains persistently thin. This is sometimes referred to as a thin or resistant endometrium.

What Do We Mean by a “Resistant Endometrium”?

There is actually no universally accepted definition of a resistant endometrium. Endometrial thickness is measured by transvaginal ultrasound, and many fertility clinics traditionally aim for a lining of approximately 8 mm or greater before embryo transfer. However, 8 mm should not be regarded as a magical cut-off point. Pregnancies can and do occur with thinner endometria, and thickness alone cannot tell us whether an embryo will implant.

The appearance of the endometrium, its response over time, the patient’s previous treatment history and the underlying reason for a thin lining are also important. Therefore, rather than defining resistance according to a single measurement, it can be more useful clinically to think about the behavior of the endometrium.

For example, if a patient repeatedly receives an adequate dose and duration of estrogen but her endometrium remains around 5–6 mm despite attempts to optimize treatment, this represents a different clinical situation from a patient whose lining simply needs another few days of estrogen to grow into the ideal size and appearance. The question then becomes: Why is this endometrium not responding normally to estrogen?

There is rarely one single explanation. One important cause is previous damage to the endometrium. Procedures involving the uterine cavity, previous infection, intrauterine adhesions or Asherman’s syndrome can damage the basal layer from which the endometrium regenerates. For this reason, a persistently thin endometrium should not automatically be treated simply by giving progressively larger amounts of estrogen. If the lining repeatedly fails to develop, it may be appropriate to investigate the uterine cavity and exclude an underlying structural problem.

Another possible factor is blood supply. The endometrium is a highly vascular tissue. Appropriate blood flow is necessary to deliver oxygen, nutrients and hormonal signals to the developing tissue. In some women with persistently poor endometrial development, impaired uterine or endometrial blood flow may therefore contribute to the problem.

There may also be differences in the way individual patients absorb, metabolize and respond to estrogen. Oral estrogen, for example, passes through the gastrointestinal tract and liver before reaching the systemic circulation. During this process, a considerable proportion is metabolized, including conversion of estradiol into estrone and other metabolites. Transdermal estrogen, such as an estrogen patch, enters the circulation through the skin and largely avoids this first-pass metabolism. This does not mean that women with resistant endometrium have necessarily been proven to “metabolize estrogen too quickly.” We currently do not have sufficient evidence to make that conclusion. However, we do know that the pharmacology of oral and transdermal estrogen is different and that individual absorption and metabolism vary. Interestingly, studies comparing oral and transdermal estrogen have shown that blood estrogen concentrations do not necessarily predict how well the endometrium will respond.

In a patient taking oral estradiol valerate whose lining remains persistently thin, transdermal estrogen patches may be added to the oral treatment. This provides estrogen through two different routes and allows us to bypass gastrointestinal absorption and hepatic first-pass metabolism for part of the estrogen dose. Some protocols may instead use transdermal or vaginal estrogen as the predominant route. Importantly, however, simply producing a very high estrogen level in the blood does not guarantee that the endometrium will become thicker. If the tissue itself has limited proliferative capacity, repeatedly increasing estrogen may eventually provide little additional benefit.

At this point, treatment strategies aimed at factors other than estrogen exposure may be considered.

Sildenafil: Can Improving Blood Flow Help?

One approach is sildenafil citrate, better known by its original use in erectile dysfunction. The rationale for sildenafil in resistant endometrium has nothing to do with its better-known indication. Sildenafil inhibits phosphodiesterase-5 (PDE5), increasing the activity of the nitric oxide–cGMP pathway and promoting vasodilation. The objective in fertility treatment is therefore to potentially improve blood flow to the uterus and endometrium.

A systematic review and meta-analysis involving nine studies and 1,452 patients reported that sildenafil treatment was associated with an average increase in endometrial thickness of approximately 1.2 mm compared with control treatment. Improvements in measures of uterine blood flow and clinical pregnancy rates were also reported. This is consistent with our own clinical experience with the resistant endometrium. Sildenafil appears to improve endometrial blood flow and therefore proliferation in cases where prior standard protocols would not provide desirable results. Therefore, when patients are not able to obtain an ideal endometrial preparations with standard treatments, sildenafil becomes a consideration in our protocols.

Pentoxifylline and Vitamin E

Another interesting approach involves the combination of pentoxifylline and vitamin E (tocopherol). Pentoxifylline has effects on blood rheology and microcirculation and also has anti-inflammatory and potential antifibrotic properties. Vitamin E has antioxidant activity and has also been investigated in combination with pentoxifylline in conditions involving tissue fibrosis.

This combination is particularly interesting when poor endometrial development may be associated with previous tissue injury, fibrosis or impaired microcirculation. Pentoxifylline has effects on blood flow and tissue oxygenation and has also been studied, together with vitamin E, for its potential antifibrotic effects.

One early study by Lédée-Bataille and colleagues investigated 18 women undergoing egg-donation treatment whose endometrial thickness remained below 6 mm despite estrogen treatment. The patients received pentoxifylline and vitamin E for six months. Mean endometrial thickness increased significantly from 4.9 mm to 6.2 mm, with 13 of the 18 women (72%) classified as good responders.

A later study by Acharya, Yasmin and Balen evaluated 20 women with persistently thin endometrium of less than 6 mm that had not responded adequately to conventional treatment. After treatment with pentoxifylline and vitamin E for an average of approximately eight months, mean endometrial thickness increased from 4.37 mm to 6.05 mm. Overall, 14 of 19 evaluable women (73.7%) demonstrated an improvement in endometrial thickness.

More recently, Krief and colleagues examined the treatment in a considerably larger retrospective study. Rather than looking only at endometrial thickness, the researchers used three-dimensional ultrasound to assess endometrial volume. Among women with an initial endometrial volume below 2 mL, pentoxifylline and vitamin E treatment was associated with an increase in mean endometrial volume from 1.34 ± 0.38 mL to 1.82 ± 0.63 mL, an average increase of approximately 0.47 mL. The average treatment duration was approximately 132 days.

Taken together, these studies suggest that pentoxifylline combined with vitamin E can improve endometrial development in some women with persistently poor endometrial growth. There is another important distinction though. Unlike adding estrogen or sildenafil during an embryo-transfer cycle, pentoxifylline and vitamin E have generally been studied as longer-term treatments. Some of the published protocols used them for several months before another attempt at endometrial preparation. Therefore, I tend to think of this strategy differently. Rather than trying to rescue the lining over several days, the objective is potentially to improve the underlying endometrial environment before attempting another transfer cycle.

In some cases, sildenafil, pentoxifylline and vitamin E can be combined within a single treatment cycle when there is evidence that prior attempts failed to provide an optimal endometrial preparation. However, it is also important to keep in mind that not every thin endometrium requires every treatment.

However, a resistant endometrium should not automatically lead to a long list of medications being added to the patient’s protocol. Instead, treatment should begin by asking why the lining is thin.

Has the uterine cavity been assessed?
Is there a history of uterine surgery, infection or adhesions?
Is estrogen exposure genuinely adequate?
Would another route of estrogen administration be appropriate?
Does the lining simply require longer exposure?
Could uterine blood flow be contributing?
Has the patient demonstrated the same pattern during several previous cycles?

These questions help determine the next step. In some patients, extending estrogen treatment or adding a transdermal estrogen patch may be sufficient. In others, particularly where vascular resistance is suspected, sildenafil may be considered. For patients with a longstanding history of very poor endometrial proliferation, especially where previous endometrial injury or fibrosis may be relevant, a longer period of treatment with agents such as pentoxifylline and vitamin E may be considered before another embryo-transfer attempt.

Other regenerative approaches, including platelet-rich plasma (PRP), G-CSF and regenerative strategies, have also been investigated for refractory thin endometrium.

Platelet-Rich Plasma (PRP)

Platelet-rich plasma (PRP) has received increasing attention as another potential treatment for resistant thin endometrium. PRP is prepared from the patient’s own blood and contains a concentrated mixture of platelets and platelet-derived growth factors that may support tissue repair, cell proliferation and the formation of new blood vessels within the endometrium. It is most commonly administered directly into the uterine cavity before embryo transfer. A 2024 meta-analysis of eight randomized controlled trials involving 678 women with thin endometrium found that PRP treatment was associated with an average increase in endometrial thickness of approximately 1.23 mm compared with control treatment. The analysis also reported higher implantation, clinical pregnancy and live-birth rates in the PRP groups. We have been offering endometrial PRP treatment in our IVF unit since 2012. Based on our clinical experience, its relatively short treatment period and its potential to improve endometrial development in selected resistant cases have made PRP one of the more frequently requested interventions for patients with a persistently thin or poorly responsive endometrium.

Granulocyte Colony-Stimulating Factor (G-CSF)

Granulocyte colony-stimulating factor (G-CSF) is a naturally occurring cytokine best known for its role in stimulating the production and function of certain white blood cells. It has also been investigated in reproductive medicine because of its potential effects on endometrial development, vascularization and the implantation environment. For resistant thin endometrium, G-CSF is generally administered by infusion directly into the uterine cavity. A systematic review and meta-analysis involving 11 studies and 683 women reported that intrauterine G-CSF was associated with an average increase in endometrial thickness of approximately 1.79 mm, as well as higher implantation and clinical pregnancy rates and a lower cycle-cancellation rate. However, the studies included different patient populations and treatment protocols, and subsequent reviews have not always produced equally consistent findings. G-CSF can therefore be considered as a possible treatment in selected cases of refractory thin endometrium, but the evidence is not sufficiently consistent to regard it as a standard treatment for every patient with a thin lining.

Stem Cells and Regenerative Approaches

A particularly interesting area of research involves regenerative treatment of the endometrium itself. This may be especially relevant when poor endometrial growth is associated with damage to the basal layer of the endometrium, fibrosis or intrauterine adhesions, where simply providing additional estrogen may not be sufficient to restore normal growth. Different types of mesenchymal stem cells (MSCs) have been investigated, including cells derived from bone marrow, adipose tissue, umbilical cord, menstrual blood and endometrial tissue. Rather than simply providing another hormonal stimulus, these approaches aim to promote tissue repair and regeneration, potentially through effects on angiogenesis, inflammation, fibrosis and growth-factor signaling. A 2026 systematic review and meta-analysis included 18 clinical studies involving 323 women with refractory thin endometrium or Asherman’s syndrome and reported an average increase in endometrial thickness of approximately 2.35 mm following MSC treatment. In the randomized studies included in the analysis, MSC treatment was also associated with higher clinical pregnancy and live-birth rates. These findings are promising, particularly for women with significant endometrial damage who have failed conventional treatments. However, the studies remain relatively small and differ substantially in the source of stem cells, preparation methods and routes of administration. Larger and better-standardized randomized studies are therefore needed to establish which patients benefit most and which regenerative approach is optimal.

Dr. Ahmet Ozyigit,
MD, MSc, PgDip,FAAMM, ABAARM

Elite Research and Surgical Hospital

References

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