The global total fertility rate (TFR) has been declining, reaching 2.2 live births per woman in 2024, down from 3.3 in the 1990s, and is projected to fall to 1 by 2050 [1]. This decline poses a significant challenge globally, prompting public health organizations to actively promote assisted reproductive technologies (ART) as a viable solution.

Among the various ART options, in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) are the most widely used techniques, with success rates ranging from 35% to 40%, depending on the treatment center. These success rates are influenced by multiple factors, including the physiological condition of the patient and the clinical practices employed [2]. One critical factor affecting outcomes is endometrial thickness, which plays a significant role in the implantation process and overall outcomes of IVF/ICSI treatments [2, 3]. A deeper understanding of these variables is essential for improving IVF/ICSI success rates and addressing infertility worldwide.

A thin endometrium (less than 7 mm during the mid-cycle or anovulatory phase) is associated with poor pregnancy outcomes [4], including reduced implantation rates, lower pregnancy rates, and increased miscarriage rates [5]. While an endometrial thickness of at least 8 mm is generally considered optimal for embryo transfer (ET), several studies suggest that a thickness of $\ge$7 mm also significantly improves pregnancy rates [6, 7].

Although women diagnosed with a thin endometrium often receive hormone therapy to increase endometrial thickness according to standard protocols, alternative approaches also exist. These include increased estrogen doses, low-dose anticoagulants, vitamin E supplementation, sildenafil to enhance uterine blood flow, and treatments involving granulocyte-colony stimulating factor (G-CSF) injections. However, thin endometrial conditions may persist, resulting in the cancellation of ET during IVF cycles [5, 8]. This can cause increased financial and psychological stress for couples experiencing infertility [9].

Platelet-rich plasma (PRP) has emerged as a promising therapeutic approach, with research indicating its potential to enhance endometrial thickness and improve blood supply in women with a thin endometrium [9]. A 2015 study by Chang et al. [10] found that PRP not only increased endometrial thickness but also showed a trend toward improved pregnancy rates. This effect is likely due to PRP’s ability to increase cell proliferation, promote cell regeneration, and repair damaged cellular components [8]. These mechanisms are driven by growth factors and cytokines present in PRP, including vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF), and insulin-like growth factor-1 (IGF-1) [4, 11]. Western blot analyses have shown that VEGF levels in PRP are up to 2.4 times higher than in plasma, while PDGF levels are up to 2.8 times higher, both differences being statistically significant (p = 0.030 and p = 0.001, respectively) [12].

Additionally, studies on both human [12] and murine [13] endometrial tissues have shown that PRP injection into the uterine cavity increases the expression of Homeobox A10 (HOXA10), a key marker of endometrial receptivity for embryo implantation. These studies also reported elevated Ki67 expression, indicating enhanced proliferation of glands and stroma in the endometrial tissue. Such histopathological findings further support PRP’s potential to improve endometrial receptivity. Moreover, the PRP preparation process is straightforward, and neither the plasma nor platelet components have been found to cause adverse effects [9, 14]. Due to its safety and efficacy, PRP has been widely applied across various medical fields, including the treatment of hair loss (alopecia), the reduction of inflammation and joint pain, and cosmetic procedures for skin rejuvenation [8].

In reproductive medicine, several studies have explored the application and optimization of PRP for infertility treatment, investigating its effect on both the endometrium and ovaries. However, results remain inconsistent, and a standardized protocol to optimize ART outcomes has yet to be established. For example, a 2022 study by Dogra et al. [8] measured endometrial thickness every 48 hours, administering PRP as needed until the thickness reached 7 mm. Their findings showed a significant increase in endometrial thickness following PRP administration in women with a history of cycle cancellation, with an average increase of 1.07 mm in fresh IVF cycles and 0.83 mm in frozen embryo transfer (FET) cycles [8]. In contrast, a 2020 study by Aghajanzadeh et al. [9] found no significant changes in endometrial thickness, implantation rate, clinical pregnancy rate, ongoing pregnancy rate, or miscarriage rate among women with recurrent implantation failure (RIF) submitted who received PRP 48 hours before ET. However, no prior studies have conducted a direct comparative analysis of the effects of a single PRP injection versus repeated PRP injections on endometrial thickness.

This study aims to evaluate the potential effects of PRP in enhancing endometrial thickness in women with a thin endometrium. Specifically, we compare endometrial thickness in patients undergoing FET across three groups: those receiving a single PRP injection with standard estrogen therapy, those receiving repeated PRP injections with standard estrogen, and those receiving standard estrogen therapy alone. As a secondary objective, we compare chemical pregnancy rates, clinical pregnancy rates, ongoing pregnancy rates, abortion rates, and cycle cancellation rates among the groups.

This study investigated the effects of intrauterine instillation of autologous PRP on endometrial thickness and pregnancy outcomes in infertile women undergoing FET cycles. Our findings indicate that a single PRP instillation resulted in higher endometrial thickness compared to the control group; however, this difference was not statistically significant. In contrast, repeated PRP instillations showed limited benefit in increasing endometrial thickness and were associated with a higher cycle cancellation rate.

The observed increase in endometrial thickness in the PRP-1 group aligns with previous studies [16, 17, 18, 19], which suggest that PRP promotes angiogenesis and tissue regeneration through growth factors such as VEGF, PDGF, and IGF-1 [4, 11, 12]. Similarly, a 2024 meta-analysis [20] and a recent 2025 Turkish study [21] showed that PRP can increase endometrial thickness. However, the lack of statistical significance in our study, compared to previous reports, may be explained by the control group starting with a higher endometrial thickness after 5 days of standard HRT (D8; Control vs. PRP-1). Despite this, following the full effect of PRP instillation, the PRP-1 group demonstrated a greater increase in thickness compared to the control group (1.52 $\pm$ 1.1 mm vs. 1.43 $\pm$ 0.88 mm). This may be one of the confounding factors in our results. Despite starting with the same baseline endometrial thickness, their responses to standard HRT differed. Another possible reason is the variation in definitions of endometrial thickness across studies; for instance, the meta-analysis included patients with endometrial thickness 8 mm and combined protocols that included single and repeated PRP injections, which likely contributed to the high heterogeneity observed (I² = 84%).

Table 2 shows that the difference in mean endometrial thickness among the three groups reached only marginal statistical significance. Although the 1.52 mm gain observed in the PRP-1 group did not translate into higher pregnancy or live-birth rates, it may still increase the chance of proceeding to ET in women with a history of repeated cycle cancellations.

Although the PRP-1 group exhibited improved endometrial thickness, no significant differences were observed among the groups in terms of clinical pregnancy rates, chemical pregnancy rates, ongoing pregnancy rates, or miscarriage rates. These results are consistent with previous studies. Allahveisi et al. (2020) [22] reported no significant differences between PRP and control groups in chemical pregnancy rates (28% vs. 36%) and clinical pregnancy rates (28% vs. 24%). Similarly, Aghajanzadeh et al. (2020) [9] found no significant differences in chemical pregnancy, clinical pregnancy, and ongoing pregnancy rates. One potential explanation for these findings is the baseline characteristics of our study population, which included a high proportion of patients with unfavorable prognostic factors, such as refractory thin endometrium and RIF. The absence of significant differences in pregnancy rates highlights the need for further investigation into the effects of PRP on additional implantation-related factors and pregnancy outcomes, including the potential impact on embryonic chromosomal abnormalities. Notably, this study did not include embryos screened by Preimplantation Genetic Testing for Aneuploidy (PGT-A).

In contrast, the PRP-2 group did not show improvements in endometrial thickness and had a significantly higher cycle cancellation rate, which differs from the findings by Nazari et al. (2019) [15]. Although the exact causal relationship of repeated PRP treatment is not fully understood, it may be influenced by various factors, including the accumulation of excessive endometrial fluid inside the uterine cavity following repeated PRP administration [23, 24]. This may lead to endometrial dysfunction, potentially disrupting the endometrial microenvironment, similar to conditions like hydrosalpinx or uterine isthmocele. Excess intrauterine fluid may also increase uterine peristalsis, potentially leading to embryo expulsion [25]. In future research, we recommend more precise diagnostic evaluations to detect intrauterine fluid or hydrosalpinx fluid (e.g. 3D-ultrasonography, office hysteroscopy) in cases where PRP was instilled into the uterine cavity and the cycle was cancelled, in order to confirm this hypothesis.

Furthermore, the PRP-2 group exhibited a significantly higher cycle cancellation rate (77.78%) compared to the PRP-1 and control groups. The underlying mechanism for this increased cancellation rate remains unclear but may be associated with excessive exposure to PRP, growth factors, or cytokines beyond an optimal therapeutic range.

To our knowledge, this is the first RCT study to compare the effects of single PRP instillation, repeated PRP instillation, and standard hormone replacement therapy on endometrial thickness and ART outcomes. Additionally, this study examined both baseline serum platelet concentrations and platelet concentrations in PRP preparations to ensure that each sample contained more than 1 million platelets per preparation, to maximizing PRP quality [8]. Another key finding is that repeated PRP instillations do not appear to provide additional benefits for endometrial thickness and may, in fact, contribute to higher cycle cancellation rates. Therefore, a single PRP instillation should be considered as an alternative treatment option for patients with thin endometrium who fail to achieve the target thickness after standard hormonal treatment.

This study has some limitations. First, the inclusion of non-next generation sequencing (NGS) embryos may have influenced ART success rates, particularly given the average participant age of 40 years. However, pregnancy rates were not the primary focus of this study, and the absence of NGS did not impact the primary outcome of interest (endometrial thickness). Second, the study population included patients with inherently poor prognoses, such as those with refractory thin endometrium or RIF. While a single PRP instillation increased endometrial thickness, the difference was not statistically significant in this study. Third, our study excluded women with intrauterine adhesions, who may represent one of the targets that could benefit from PRP treatment. A recent study [26] and meta-analysis [27] have shown that PRP can increase other aspects of reproductive function, such as increase menstrual blood flow, improving menopausal hormonal profile, or increasing the Anti-Mullerian Hormone (AMH) level.

Finally, although the data were sufficient for analysis, the small sample size warrants cautious the interpretation of the results. Therefore, future RCT studies should include larger sample sizes to increase statistical power and aim to elucidate the comparative efficacy of single versus repeated PRP injections. Furthermore, future studies should include euploid embryos to better assess PRP’s effects on clinical pregnancy rates. Additionally, investigating endometrial receptivity through molecular markers (e.g., HOXA10, leukemia inhibitory factor [LIF]) assessing microbiome changes, before and after PRP administration, may provide valuable insights, particularly for patients with thin endometrium or RIF. Future research should also explore other reproductive aspects like anti-adhesive efficacy of PRP in postoperative intrauterine surgery, to decrease risk of adhesion recurrence or prevent adhesion formation, which represents an important area for further research.

The study found that a single intrauterine PRP instillation prior to a FET cycle improved endometrial thickness, although the difference was not statistically significant. However, repeated PRP instillations did not provide additional benefits in endometrial thickness or ART outcomes and were associated with a higher cycle cancellation rate. These findings suggest that a single PRP injection may be sufficient to treat and improve thin endometrium in infertile women, potentially eliminating the need for repeated or multiple injections. Additionally, PRP instillation is a safe and straightforward method for enhancing endometrial thickness, but it does not appear to significantly improve ART outcomes. Further research is warranted to explore the long-term effects of PRP on pregnancy outcomes and to refine its clinical application in assisted reproductive technology.

All data is available upon request as a supplementary file. Please contact the corresponding authors for further information.

SPO, NP and SPE designed the project. SPO and NP performed the research and wrote the manuscript. SPO, NP, PL, SPE, IT, JP, PH and PT collected the data. SPO and NP analyzed the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

According to the guidelines of the Declaration of Helsinki, this study was approved by the Siriraj Institutional Review Board (Protocol No. 853/2566, IRB4; (COA No. Si 987/2023). The study has been registered on https://clinicaltrials.gov/ registration number: NCT06234540. All participants provided written informed consent.

We extend our gratitude to all participants and to the staff of the Siriraj Infertility Clinic. The authors would like to thank Dr. Saowalak Hunnangkul from Siriraj Research Data Management Unit (SI-RDMU) for guidance, support and assistance with computerized statistical analysis.

Faculty of Medicine, Siriraj Hospital, Mahidol University (R016233020).

The authors declare no conflicts of interest.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/CEOG39260.