Endometrial thickness: How thin is too thin?

The optimal preparation of the uterine lining to maximize implantation and optimize obstetric outcomes has evolved since the 1984 report, published in Nature, of successful exogenous hormone endometrial preparation in a young woman with premature ovarian failure, who subsequently delivered a healthy infant because of a donor oocyte in vitro fertilization (IVF) cycle (1Lutjen P. Trounson A. Leeton J. Findlay J. Wood C. Renou P. The establishment and maintenance of pregnancy using in vitro fertilization and embryo donation in a patient with primary ovarian failure.Nature. 1984; 307: 174-175Crossref PubMed Scopus (539) Google Scholar). Early reports emphasized the importance of stair-step introduction of exogenous estradiol and subsequent progesterone supplementation (2Navot D. Laufer N. Kopolovic J. Rabinowitz R. Birkenfeld A. Lewin A. et al.Artificially induced endometrial cycles and establishment of pregnancies in the absence of ovaries.N Engl J Med. 1986; 314: 806-811Crossref PubMed Scopus (259) Google Scholar, 3Rosenwaks Z. Donor eggs: their application in modern reproductive technologies.Fertil Steril. 1987; 47: 895-909Abstract Full Text PDF PubMed Scopus (181) Google Scholar, 4Sauer M.V. Paulson R.J. Human oocyte and preembryo donation: an evolving method for the treatment of infertility.Am J Obstet Gynecol. 1990; 163: 1421-1424Abstract Full Text PDF PubMed Scopus (44) Google Scholar, 5Sauer M.V. Paulson R.J. Lobo R.A. Reversing the natural decline in human fertility: an extended clinical trial of oocyte donation to women of advanced reproductive age. JAMA 1992;268:1275–1279. Published correction appears in.JAMA. 1993; 269: 476PubMed Google Scholar). Over the past 40 years, studies have noted that the mode of administration (oral, transdermal, intramuscular, and vaginal) and the exact duration of estrogen supplementation may not matter as long as an adequate endometrial response, lining thickness, and trilaminar architecture are achieved (6Ghobara T. Gelbaya T.A. Ayeleke R.O. Cycle regimens for frozen-thawed embryo transfer.Cochrane Database Syst Rev. 2017; 7: CD003414PubMed Google Scholar, 7Greco E. Litwicka K. Arrivi C. Varricchio M. Caragia A. Greco A. et al.The endometrial preparation for frozen-thawed euploid blastocyst transfer: a prospective randomized trial comparing clinical results from natural modified cycle and exogenous hormone stimulation with GnRH agonist.J Assist Reprod Genet. 2016; 33: 873-884Crossref PubMed Scopus (39) Google Scholar, 8Mackens S. Santos-Ribeiro S. van de Vijver A. Racca A. Van Landuyt L. Tournaye H. et al.Frozen embryo transfer: a review on the optimal endometrial preparation and timing.Hum Reprod. 2017; 32: 2234-2242Crossref PubMed Scopus (139) Google Scholar, 9Glujovsky D. Pesce R. Sueldo C. Quinteiro Retamar A.M. Hart R.J. Ciapponi A. Endometrial preparation for women undergoing embryo transfer with frozen embryos or embryos derived from donor oocytes.Cochrane Database Syst Rev. 2020; 10: CD006359PubMed Google Scholar). In fact, emerging data have suggested potential obstetric benefits of natural cycle (NC) frozen embryo transfer (FET; NC-FET), wherein the corpus luteum remains functional and contributes to the support of early pregnancy (10Lee J.C. Badell M.L. Kawwass J.F. The impact of endometrial preparation for frozen embryo transfer on maternal and neonatal outcomes: a review.Reprod Biol Endocrinol. 2022; 20: 1-12Crossref PubMed Scopus (4) Google Scholar, 11Wu H. Zhou P. Lin X. Wang S. Zhang S. Endometrial preparation for frozen-thawed embryo transfer cycles: a systematic review and network meta-analysis.J Assist Reprod Genet. 2021; 38: 1913-1926Crossref PubMed Scopus (16) Google Scholar). Therefore, it seems that when the uterine lining responds well and mimics the natural form, there may be many equally viable options for reaching that point; however, several questions remain, “what about when the uterine lining is thin? Is there an absolute threshold at which pregnancy is not possible? Does architecture matter more than thickness? What interventions can improve the likelihood of implantation and good perinatal outcome? How does the etiology of the poor response impact the best next step? Do medical or surgical interventions exist that will improve the uterine lining?” Although several studies have not shown a relationship between lining thickness at the time of intrauterine insemination trigger and the likelihood of pregnancy, many studies have noted a decreasing likelihood of implantation and ongoing pregnancy with decreasing endometrial thickness (EMT) at the time of both fresh embryo transfers and FETs (12Quaas A.M. Gavrizi S.Z. Peck J.D. Diamond M. Legro R. Robinson R. et al.Endometrial thickness after ovarian stimulation with gonadotropin, clomiphene, or letrozole for unexplained infertility, and association with treatment outcomes.Fertil Steril. 2021; 115: 213-220Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar, 13Weiss N.S. van Vliet M.N. Limpens J. Hompes P.G. Lambalk C.B. Mochtar M.H. et al.Endometrial thickness in women undergoing IUI with ovarian stimulation. How thick is too thin? A systematic review and meta-analysis.Hum Reprod. 2017; 32: 1009-1018Crossref PubMed Scopus (48) Google Scholar, 14Danhof N.A. van Eekelen R. Repping S. Mol B.W. van der Veen F. van Wely M. et al.Endometrial thickness as a biomarker for ongoing pregnancy in IUI for unexplained subfertility: a secondary analysis.Hum Reprod Open. 2020; 2020: hoz024Crossref PubMed Google Scholar, 15Liu K.E. Hartman M. Hartman A. Luo Z.C. Mahutte N. The impact of a thin endometrial lining on fresh and frozen-thaw IVF outcomes: an analysis of over 40,000 embryo transfers.Hum Reprod. 2018; 33: 1883-1888Crossref PubMed Scopus (141) Google Scholar, 16Mahutte N. Hartman M. Meng L. Lanes A. Luo Z.C. Liu K.E. Optimal endometrial thickness in fresh and frozen-thaw in vitro fertilization cycles: an analysis of live birth rates from 96,000 autologous embryo transfers.Fertil Steril. 2022; 117: 792-800Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar, 17Patel J.A. Patel A.J. Banker J.M. Shah S.I. Banker M. Effect of endometrial thickness and duration of estrogen supplementation on in vitro fertilization-intracytoplasmic sperm injection outcomes in fresh ovum/embryo donation cycles.J Hum Reprod Sci. 2021; 14: 167-174Crossref PubMed Scopus (2) Google Scholar, 18Liao S. Wang R. Hu C. Pan W. Pan W. Yu D. et al.Analysis of endometrial thickness patterns and pregnancy outcomes considering 12,991 fresh IVF cycles.BMC Med Inform Decis Mak. 2021; 21: 1-13Crossref PubMed Scopus (2) Google Scholar, 19Yuan X. Saravelos S.H. Wang Q. Xu Y. Li T.C. Zhou C. Endometrial thickness as a predictor of pregnancy outcomes in 10787 fresh IVF-ICSI cycles.Reprod Biomed Online. 2016; 33: 197-205Abstract Full Text Full Text PDF PubMed Scopus (90) Google Scholar, 20Simeonov M. Sapir O. Lande Y. Ben-Haroush A. Oron G. Shlush E. et al.The entire range of trigger-day endometrial thickness in fresh IVF cycles is independently correlated with live birth rate.Reprod Biomed Online. 2020; 41: 239-247Abstract Full Text Full Text PDF PubMed Scopus (10) Google Scholar, 21Gao G. Cui X. Li S. Ding P. Zhang S. Zhang Y. Endometrial thickness and IVF cycle outcomes: a meta-analysis.Reprod Biomed Online. 2020; 40: 124-133Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, 22Onogi S. Ezoe K. Nishihara S. Fukuda J. Kobayashi T. Kato K. Endometrial thickness on the day of the LH surge: an effective predictor of pregnancy outcomes after modified natural cycle-frozen blastocyst transfer.Hum Reprod Open. 2020; 2020hoaa060Crossref PubMed Google Scholar). Having said that, although the outcomes seem to worsen as the EMT decreases to <7 or 8 mm, there is no EMT at which the chance of pregnancy and subsequent live birth has been reported to be zero. Rather, there appears to be an optimal thickness range that maximizes pregnancy and obstetric outcomes, with outlying values reflecting a lower but not futile chance of success. Additionally, the etiology and architecture of a thin lining may contribute to the likelihood of improvement using a new protocol and a threshold that is deemed acceptable before transfer. For example, among patients with a known history of Asherman syndrome, data have suggested that strict cutoffs for the lining thickness should be used with caution because of continued potential for success despite a thin lining, albeit potentially lower than the chances of pregnancy if the lining were thicker (23Movilla P. Wang J. Chen T. Morales B. Wang J. Williams A. et al.Endometrial thickness measurements among Asherman syndrome patients prior to embryo transfer.Hum Reprod. 2020; 35: 2746-2754Crossref PubMed Scopus (4) Google Scholar). As with most clinical scenarios in the world of fertility optimization, treating patients on a case-by-case basis that incorporates their individual contributing factors and endometrial lining potential is likely ideal. For patients in whom the lining thickness is <6 mm, it is reasonable to attempt to investigate the potential causes and try to optimize the lining thickness before embryo transfer. If there appears to be no absolute EMT below which pregnancy is impossible, the next logical question is “what are the outcomes of the pregnancies and infants conceived during cycles with thin endometrium?” Multiple studies have shown that singleton pregnancies conceived using IVF are at an increased risk of obstetric and neonatal complications compared with spontaneously conceived pregnancies, although the reason for this has not been definitively elucidated (24Pandey S. Shetty A. Hamilton M. Bhattacharya S. Maheshwari A. Obstetric and perinatal outcomes in singleton pregnancies resulting from IVF/ICSI: a systematic review and meta-analysis.Hum Reprod Update. 2012; 18: 485-503Crossref PubMed Scopus (475) Google Scholar, 25Qin J.B. Sheng X.Q. Wu D. Gao S. You Y. Yang T. et al.Worldwide prevalence of adverse pregnancy outcomes among singleton pregnancies after in vitro fertilization/intracytoplasmic sperm injection: a systematic review and meta-analysis.Arch Gynecol Obstet. 2017; 295: 285-301Crossref PubMed Scopus (71) Google Scholar). The placenta, a highly specialized temporary organ, is perhaps the most crucial element for good perinatal outcomes, and placentation is likely influenced by the underlying endometrium. In a study conducted by Miwa et al. (26Miwa I. Tamura H. Takasaki A. Yamagata Y. Shimamura K. Sugino N. Pathophysiologic features of “thin” endometrium.Fertil Steril. 2009; 91: 998-1004Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar), it was found that vascular endothelial growth factor was underexpressed in a thin endometrium (<8 mm), leading to poor vascularization of the endometrium and impaired placentation. Apart from pathophysiology, several studies have shown that pregnancies conceived with a thin endometrium using assisted reproductive technology (ART) are at an increased risk of pregnancy loss, hypertensive disorders of pregnancy, preterm delivery, placenta previa, and cesarean section (27Zhang T. Li Z. Ren X. Huang B. Zhu G. Yang W. et al.Endometrial thickness as a predictor of the reproductive outcomes in fresh and frozen embryo transfer cycles: a retrospective cohort study of 1,512 IVF cycles with morphologically good-quality blastocyst.Medicine (Baltimore). 2018; 97: e9689Crossref PubMed Scopus (69) Google Scholar, 28Liu X. Wang J. Fu X. Li J. Zhang M. Yan J. et al.Thin endometrium is associated with the risk of hypertensive disorders of pregnancy in fresh IVF/ICSI embryo transfer cycles: a retrospective cohort study of 9,266 singleton births.Reprod Biol Endocrinol. 2021; 19: 1-9Crossref PubMed Scopus (7) Google Scholar, 29Jing S. Li X. Zhang S. Gong F. Lu G. Lin G. The risk of placenta previa and cesarean section associated with a thin endometrial thickness: a retrospective study of 5251 singleton births during frozen embryo transfer in China.Arch Gynecol Obstet. 2019; 300: 1227-1237Crossref PubMed Scopus (12) Google Scholar, 30Mouhayar Y. Franasiak J.M. Sharara F.I. Obstetrical complications of thin endometrium in assisted reproductive technologies: a systematic review.J Assist Reprod Genet. 2019; 36: 607-611Crossref PubMed Scopus (25) Google Scholar). Additionally, infants conceived in these pregnancies are at risk of intrauterine growth restriction, being small for gestational age, and a lower birth weight (28Liu X. Wang J. Fu X. Li J. Zhang M. Yan J. et al.Thin endometrium is associated with the risk of hypertensive disorders of pregnancy in fresh IVF/ICSI embryo transfer cycles: a retrospective cohort study of 9,266 singleton births.Reprod Biol Endocrinol. 2021; 19: 1-9Crossref PubMed Scopus (7) Google Scholar, 31Moffat R. Beutler S. Schötzau A. De Geyter M. De Geyter C. Endometrial thickness influences neonatal birth weight in pregnancies with obstetric complications achieved after fresh IVF-ICSI cycles.Arch Gynecol Obstet. 2017; 296: 115-122Crossref PubMed Scopus (23) Google Scholar, 32Du M. Zhang J. Liu M. Guan Y. Wang X. Endometrial thickness is a risk factor for singleton low birth weight from single blastocyst transfer: a retrospective cohort study.Front Endocrinol (Lausanne). 2021; 12730512Crossref Scopus (2) Google Scholar, 33Guo Z. Xu X. Zhang L. Zhang L. Yan L. Ma J. Endometrial thickness is associated with incidence of small-for-gestational-age infants in fresh in vitro fertilization-intracytoplasmic sperm injection and embryo transfer cycles.Fertil Steril. 2020; 113: 745-752Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar). It can be argued that the reason for these findings is multifactorial (i.e., not just a problem with the endometrial lining) because studies conducted on obstetric and neonatal outcomes in intrauterine insemination (IUI) cycles with a thin endometrium failed to find a negative association between the two (34Huang J. Lin J. Lu X. Gao H. Song N. Cai R. et al.Association between endometrial thickness and neonatal outcomes in intrauterine insemination cycles: a retrospective analysis of 1,016 live-born singletons.Reprod Biol Endocrinol. 2020; 18: 1-9Crossref PubMed Scopus (2) Google Scholar). In addition, as previously mentioned, emerging evidence has suggested that programmed FET cycles have higher rates of maternal and neonatal complications than NC-FETs (irrespective of the endometrial lining) (10Lee J.C. Badell M.L. Kawwass J.F. The impact of endometrial preparation for frozen embryo transfer on maternal and neonatal outcomes: a review.Reprod Biol Endocrinol. 2022; 20: 1-12Crossref PubMed Scopus (4) Google Scholar). Although a thin endometrial lining is not the only factor influencing obstetric and neonatal outcomes in IVF, there is a large body of evidence that suggests that EMT plays a significant role; interventions for increasing EMT (with the belief of maximizing IVF outcomes and minimizing obstetrical or neonatal complications) should be attempted when a thin endometrium is encountered. When confronted with a thin endometrium before starting progesterone for embryo transfer, 1 option is to extend the course and, perhaps, increase the dose of estrogen administered. The route of estrogen administration can also be altered, considering the multiple options, including oral, vaginal, transdermal, or intramuscular formulations. Because the duration of estrogen administration makes little difference, extending by a week should not have any detrimental effect, and many women will achieve a more optimal thickness by this simple intervention. Another option is to end this attempt at endometrial preparation by inducing a period and starting over, often with changes in the dose and route of estrogen administration from the start. In a study that we conducted at the University of Iowa on the natural history of a thin endometrium (Jacobs et al., unpublished data), we found that the incidence of inadequate EMT (<6 mm) leading to cycle cancellation was low (29 cases over a 6-year study period or approximately 1% of all FET cycles), and all cases were able to achieve adequate EMT within 4 FET attempts using the interventions detailed above. We also found that the predictive factors for the development of a thin endometrial lining were a thinner lining in a fresh stimulation cycle and a lower body weight, which may help clinicians in deciding on the appropriate endometrial preparation protocol for individuals at risk. Natural cycle preparation for embryo transfer, which allows for normal endogenous estrogen production, is an option that is gaining favor among ovulatory women, although whether the endometrial lining is improved among women with a thin endometrium has not been fully studied. A review of IVF stimulation cycles, with attention to EMT, may suggest that the use of gonadotropin stimulation of endogenous estrogen production is useful. Sometimes, in our experience, just a minimal stimulation cycle using low doses of gonadotropins can reduce the cost to the patient while still achieving adequate EMT to proceed with the transfer. The infusion of platelet-rich plasma (PRP), an autologous concentrate of platelets prepared from fresh whole blood, has been used to treat a variety of diseases in the fields of dermatology, orthopedics, and plastic surgery (among others) (35Roohaninasab M. Goodarzi A. Ghassemi M. Sadeghzadeh-Bazargan A. Behrangi E. Najar Nobari N. Systematic review of platelet-rich plasma in treating alopecia: focusing on efficacy, safety, and therapeutic durability.Dermatol Ther. 2021; 34e14768Crossref Scopus (15) Google Scholar, 36Cole B.J. Karas V. Hussey K. Pilz K. Fortier L.A. Hyaluronic acid versus platelet-rich plasma: a prospective, double-blind randomized controlled trial comparing clinical outcomes and effects on intra-articular biology for the treatment of knee osteoarthritis. Am J Sports Med 2017;45:339–346. Published correction appears in.Am J Sports Med. 2017; 45: NP10PubMed Google Scholar, 37Davey M.S. Davey M.G. Hurley E.T. Cassidy J. Mullett H. McInerney N. et al.Platelet-rich plasma in non-operative management of mild to moderate carpal tunnel syndrome—a systematic review & meta-analysis of short-term outcomes.J Orthop. 2021; 25: 155-161Crossref PubMed Scopus (1) Google Scholar, 38Chamata E.S. Bartlett E.L. Weir D. Rohrich R.J. Platelet-rich plasma: evolving role in plastic surgery.Plast Reconstr Surg. 2021; 147: 219-230Crossref PubMed Scopus (9) Google Scholar). Its role as a treatment modality in reproductive medicine is less established but, nonetheless, intriguing. In principle, the cytokines and growth factors found in platelet α-granules allow PRP to promote tissue regeneration at the cellular level (39Mouanness M. Ali-Bynom S. Jackman J. Seckin S. Merhi Z. Use of intra-uterine injection of platelet-rich plasma (PRP) for endometrial receptivity and thickness: a literature review of the mechanisms of action.Reprod Sci. 2021; 28: 1659-1670PubMed Google Scholar). It could then be surmised that if the root cause of refractory endometrium is endometrial injury or damage secondary to a procedure or disease process (i.e., dilation and curettage, endometritis, intrauterine adhesions, etc.), then the infusion of PRP to the endometrium may promote endometrial regeneration, leading to increased EMT (and, thus, endometrial receptivity). Several studies have shown that the intrauterine infusion of PRP improves EMT; however, its impact on clinical pregnancy and live birth is debatable (40Chang Y. Li J. Wei L.N. Pang J. Chen J. Liang X. Autologous platelet-rich plasma infusion improves clinical pregnancy rate in frozen embryo transfer cycles for women with thin endometrium.Medicine (Baltimore). 2019; 98e14062Google Scholar, 41Eftekhar M. Neghab N. Naghshineh E. Khani P. Can autologous platelet rich plasma expand endometrial thickness and improve pregnancy rate during frozen-thawed embryo transfer cycle? A randomized clinical trial. Taiwan J Obstet Gynecol 2018;57:810–3. Published correction appears in Taiwan.J Obstet Gynecol. 2021; 60: 973Google Scholar, 42Molina A. Sánchez J. Sánchez W. Vielma V. Platelet-rich plasma as an adjuvant in the endometrial preparation of patients with refractory endometrium.JBRA Assist Reprod. 2018; 22: 42-48PubMed Google Scholar, 43Kusumi M. Ihana T. Kurosawa T. Ohashi Y. Tsutsumi O. Intrauterine administration of platelet-rich plasma improves embryo implantation by increasing the endometrial thickness in women with repeated implantation failure: a single-arm self-controlled trial.Reprod Med Biol. 2020; 19: 350-356Crossref PubMed Scopus (6) Google Scholar, 44Zadehmodarres S. Salehpour S. Saharkhiz N. Nazari L. Treatment of thin endometrium with autologous platelet-rich plasma: a pilot study.JBRA Assist Reprod. 2017; 21: 54-56Crossref PubMed Scopus (94) Google Scholar, 45Agarwal M. Mettler L. Jain S. Meshram S. Günther V. Alkatout I. Management of a thin endometrium by hysteroscopic instillation of platelet-rich plasma into the endomyometrial junction: a pilot study.J Clin Med. 2020; 9: 2795Crossref Scopus (11) Google Scholar). One such prospective cohort study, by Chang et al. (40Chang Y. Li J. Wei L.N. Pang J. Chen J. Liang X. Autologous platelet-rich plasma infusion improves clinical pregnancy rate in frozen embryo transfer cycles for women with thin endometrium.Medicine (Baltimore). 2019; 98e14062Google Scholar), included 64 patients with cycle cancellation because of a thin endometrium (<7 mm) in a hormone replacement therapy (HRT) cycle. These patients were given the option of experimental intrauterine infusion of PRP with the next cycle. Thirty-four patients elected to receive PRP (in addition to HRT), and 30 patients underwent a standard HRT cycle. The intrauterine infusion of PRP was performed on the 10th day and again on the day of progesterone initiation. The PRP group had a significantly thicker endometrium than the control group (7.62 ± 0.22 mm vs. 6.52 ± 0.31 mm, respectively; P<.05). The clinical pregnancy rate was also significantly higher in the PRP group (44.12% vs. 20%, respectively; P<.05). It should be worth noting that this study was limited by its small sample size and the lack of randomization. In comparison, in a study by Eftekhar et al. (41Eftekhar M. Neghab N. Naghshineh E. Khani P. Can autologous platelet rich plasma expand endometrial thickness and improve pregnancy rate during frozen-thawed embryo transfer cycle? A randomized clinical trial. Taiwan J Obstet Gynecol 2018;57:810–3. Published correction appears in Taiwan.J Obstet Gynecol. 2021; 60: 973Google Scholar), 83 women with a thin endometrium (<7 mm) were randomized during HRT cycles to receive either HRT with PRP infusion (on the 13th day of the cycle) (n = 40) or standard HRT (n = 43). Forty-eight hours after PRP infusion, the PRP group had a significantly thicker endometrium than the control group (8.67 ± 0.64 mm vs. 8.04 ± 0.27 mm, respectively; P = .001). However, there was no difference in ongoing clinical pregnancies per cycle (27% in the PRP group vs. 14% in the control group, P = .127). Most (low-quality) studies do find an improvement in EMT with PRP infusion, but whether that impacts live birth rates (LBRs) is to be determined. What is more is that the cost of PRP therapy for other medical conditions (arthritis and alopecia) can range from $500 to $2,500, and it is unlikely that PRP used in the IVF setting would be any more affordable. Taken all together, although a reasonable amount of data exists, drawing final conclusions about the potential benefits of PRP for patients with refractory endometrium from the current literature is difficult because of poor study design (the lack of randomization and blinding), the lack of treatment standardization (differences in preparation and infusion protocols), and the inclusion of multiple infertility diseases in trials (recurrent implantation failure, recurrent pregnancy loss, and refractory endometrium) (39Mouanness M. Ali-Bynom S. Jackman J. Seckin S. Merhi Z. Use of intra-uterine injection of platelet-rich plasma (PRP) for endometrial receptivity and thickness: a literature review of the mechanisms of action.Reprod Sci. 2021; 28: 1659-1670PubMed Google Scholar, 46Urman B. Boza A. Balaban B. Platelet-rich plasma another add-on treatment getting out of hand? How can clinicians preserve the best interest of their patients?.Hum Reprod. 2019; 34: 2099-2103PubMed Google Scholar). At this juncture, the intrauterine infusion of PRP for refractory endometrium is at best at the experimental stage, and clinicians should exercise caution (and provide appropriate counseling) if offering it as a therapeutic treatment option. Investigators have also hypothesized that granulocyte colony-stimulating factor (G-CSF) may be used to improve the endometrial lining and its receptivity. Granulocyte colony-stimulating factor is known to be widely expressed in the female reproductive system, including the endometrium, and its expression is under the regulation of reproductive hormones. Epithelial cells are a major source of biologically active G-CSF in the human endometrium. In a mouse study using a model of endometrial injury, G-CSF was shown to improve EMT and endometrial regeneration. Therefore, an evaluation of whether the addition of this growth factor improves EMT or the implantation rate makes biological sense. A number of studies have been performed using the intrauterine perfusion of G-CSF in women with a thin endometrium (47Gleicher N. Kim A. Michaeli T. Lee H.J. Shohat-Tal A. Lazzaroni E. et al.A pilot cohort study of granulocyte colony-stimulating factor in the treatment of unresponsive thin endometrium resistant to standard therapies.Hum Reprod. 2013; 28: 172-177Crossref PubMed Scopus (112) Google Scholar, 48Tehraninejad E. Tanha F.D. Asadi E. Kamali K. Aziminikoo E. Rezayof E. G-CSF intrauterine for thin endometrium, and pregnancy outcome.J Family Reprod Health. 2015; 9: 107-112PubMed Google Scholar, 49Sarvi F. Arabahmadi M. Alleyassin A. Aghahosseini M. Ghasemi M. Effect of increased endometrial thickness and implantation rate by granulocyte colony-stimulating factor on unresponsive thin endometrium in fresh in vitro fertilization cycles: a randomized clinical trial.Obstet Gynecol Int. 2017; 20173596079Crossref PubMed Scopus (21) Google Scholar). Unfortunately, the quality of these studies is poor, with no large, prospective, randomized trials performed specifically in women with a thin endometrium. In addition, a 2020 Cochrane review that included 13 trials (4 of which were trials in which women with a thin endometrium were evaluated) found minimal evidence to suggest that the infusion of G-CSF improves IVF outcomes or EMT in women with a thin endometrium (50Kamath M.S. Kirubakaran R. Sunkara S.K. Granulocyte-colony stimulating factor administration for subfertile women undergoing assisted reproduction.Cochrane Database Syst Rev. 2020; 1: CD013226PubMed Google Scholar). In addition, in reproductive medicine, G-CSF is most commonly administered either subcutaneously or intrauterine. Of the 13 trials included in the Cochrane review, only 4 trials collected data on adverse outcomes (none of them reported major adverse events), leaving the safety of G-CSF administration (particularly intrauterine infusion) in question. Another adjuvant proposed to improve EMT and IVF outcomes is sildenafil, a vasodilating medication used most frequently for erectile dysfunction. Theoretically, this medication, in conjunction with estrogen, would have beneficial effects on uterine and endometrial blood flow, perhaps leading to increased EMT. Studies have found mixed results of the effectiveness of sildenafil in increasing EMT in patients using ART (51Li X. Luan T. Zhao C. Zhang M. Dong L. 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Additionally, a 2018 Cochrane review of 15 trials found that although vasodilators did increase EMT (with a range of mean difference of 0.80 higher [95% confidence interval {CI}, 0.18–1.42] to 3.57 higher [95% CI, 3.01–4.13]) in women undergoing IVF, they did so at a price (increased adverse events, including headaches and tachycardia) (54Gutarra-Vilchez R.B. Cosp X.B. Glujovsky D. Viteri-García A. Runzer-Colmenares F.M. Martinez-Zapata M.J. Vasodilators for women undergoing fertility treatment.Cochrane Database Syst Rev. 2018; 10: CD010001PubMed Google Scholar). Interestingly, 1 retrospective study comparing the LBRs between adjuvant vaginal sildenafil and no adjuvant in the first HRT-FET cycle in patients with adequate EMT (>7 mm) found higher LBRs and lower miscarriage rates in the adjuvant vaginal sildenafil group despite no differences in the EMTs between the 2 groups, suggesting that sildenafil is beneficial because of improvement in endometrial blood flow (rather than an increase in EMT) (55Tao Y. Wang N. Adjuvant vaginal use of sildenafil citrate in a hormone replacement cycle improved live birth rates among 10,069 women during first frozen embryo transfers.Drug Des Devel Ther. 2020; 14: 5289-5297Crossref PubMed Scopus (2) Google Scholar). Whatever the mechanism, the use of vasodilators