The role of age and AMH on cumulative live birth rates over multiple frozen-thawed embryo transfer cycles: a study based on low prognosis patients of POSEIDON 3 and 4 groups.

BACKGROUND: Among the POSEIDON criteria, group 3 and group 4 have an expected low prognosis. For those patients with inadequate ovary reserve, embryo accumulated from consecutive oocyte retrieval cycles for multiple frozen-thawed embryo transfers (FET) has become more common. It is necessary to inform them of the pregnancy outcomes after single or multiple FET cycles before the treatment. However few studies about cumulative live birth rate (CLBR) for those with low prognosis have been reported. METHODS: This retrospective study included 4712 patients undergoing frozen embryo transfer cycles from July 2015 to August 2020. Patients were stratified as POSEIDON group 3, group 4, control 1 group (< 35 years) and control 2 group (≥ 35 years). The primary outcome is CLBRs up to six FET cycles and the secondary outcomes were LBRs per transfer cycle. Optimistic approach was used for the analysis of CLBRs and the depiction of cumulative incidence curves. RESULTS: Under optimistic model analyses, control 1 group exhibited the highest CLBR (93.98%, 95%CI 91.63-95.67%) within 6 FET cycles, followed by the CLBR from women in POSEIDON group 3(92.51%, 95%CI 77.1-97.55)was slightly lower than that in control 1 group. The CLBR of POSEIDON group 4(55% ,95%CI 39.34-70.66%)was the lowest and significantly lower than that of control 2 group(88.7%, 95%CI 80.68-96.72%). Further, patients in POSEIDON group 4 reached a CLBR plateau after 5 FET cycles. CONCLUSIONS: The patients of POSEIDON group 3 may not be considered as traditional "low prognosis" in clinical practice as extending the number of FET cycles up to 6 can archive considerably CLBR as control women. While for the POSEIDON group 4, a simple repeat of the FET cycle is not recommended after four failed FET cycles, some strategies such as PGT-A may be beneficial.

Predictors of success after in vitro fertilization.

The last few decades have witnessed a rise in the global uptake of in vitro fertilization (IVF) treatment. To ensure optimal use of this technology, it is important for patients and clinicians to have access to tools that can provide accurate estimates of treatment success and understand the contribution of key clinical and laboratory parameters that influence the chance of conception after IVF treatment. The focus of this review was to identify key predictors of IVF treatment success and assess their impact in terms of live birth rates. We have identified 11 predictors that consistently feature in currently available prediction models, including age, duration of infertility, ethnicity, body mass index, antral follicle count, previous pregnancy history, cause of infertility, sperm parameters, number of oocytes collected, morphology of transferred embryos, and day of embryo transfer.

Whither cryopreservation? The landscape gets colder.

The increase in utilization and changing legal landscape has made the field of embryo and gamete cryopreservation fraught with potential future challenges and liabilities. Clinics should be aware of the current state of the science, potential legal ramifications of what is currently routine practice, and long-term ethical implications of our work.

Perinatal outcomes of singleton live births after preimplantation genetic testing during single frozen-thawed blastocyst transfer cycles: a propensity score-matched study.

OBJECTIVE: To determine whether singleton pregnancy achieved after preimplantation genetic testing (PGT) is associated with a higher risk of adverse perinatal outcomes than in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI) singleton pregnancy. DESIGN: A retrospective cohort study. SETTING: A university-affiliated fertility center. PATIENT(S): This cohort study included singleton live births resulting from PGT (n = 232) and IVF/ICSI singleton pregnancies (n = 2,829) with single frozen-thawed blastocyst transfer. Multiple baseline covariates were used for propensity score matching, yielding 214 PGT singleton pregnancies matched to 617 IVF/ICSI singleton pregnancies. INTERVENTION(S): Trophectoderm biopsy. MAIN OUTCOME MEASURE(S): The primary outcome was gestational hypertension, and various clinical perinatal secondary outcomes related to maternal and neonatal health were measured. RESULT(S): Compared with IVF/ICSI singleton pregnancy, PGT singleton pregnancy was associated with a significantly higher risk of gestational hypertension (adjusted odds ratio, 2.58; 95% confidence interval, 1.32, 5.05). In the matched sample, the risk of gestational hypertension remained higher with PGT singleton pregnancy (odds ratio, 2.33; 95% confidence interval, 1.04, 5.22) than with IVF/ICSI singleton pregnancy. No statistical differences were noted in any other measured outcomes between the groups. CONCLUSION(S): The perinatal outcomes of PGT and IVF/ICSI singleton pregnancies were similar except for the observed potentially higher risk of gestational hypertension with PGT singleton pregnancy. However, because the data on PGT singleton pregnancies are limited, this conclusion warrants further investigation.

The impact of different cycle regimens on birthweight of singletons in frozen-thawed embryo transfer cycles of ovulatory women.

OBJECTIVE: To investigate whether different endometrial preparation regimens affect neonatal outcomes after frozen-thawed embryo transfer (FET). DESIGN: Retrospective cohort study. SETTING: Tertiary care academic medical center. PATIENTS: A total of 3,639 patients with live-born singletons were categorized into three groups on the basis of the type of endometrial preparation regimens. Of these, 1,225, 2,136, and 278 live-born singletons were conceived through natural cycle FET, artificial cycle FET, and stimulated cycle FET, respectively. INTERVENTION(S): None. MAIN OUTCOME MEASURES: The main outcomes were the measures of birthweight including the absolute mean birthweight, Z-score, low birthweight, high birthweight (HBW), small for gestational age, and large for gestational age (LGA). RESULTS: After controlling for a variety of covariates, singletons from the artificial cycle FET group had a higher mean birthweight and Z-score than those from the natural cycle FET group and stimulated cycle FET group. The risk of LGA infants significantly increased in the artificial cycle group (14.0%) than that in the natural cycle group (10.3%) and stimulated cycle group (7.6%). The risk of hypertensive disorders of pregnancy in the artificial cycle group (4.4%) was significantly higher than that in the natural cycle group (2.5%). The stimulated cycle FET singletons had a higher risk of low birthweight than the natural cycle FET singletons. The other perinatal outcomes, including the incidence of preterm birth, small for gestational age, and gestational diabetes mellitus, were comparable between the groups before or after adjustment for confounders. CONCLUSIONS: Singletons from artificial cycle FET were associated with a higher risk of LGA infants, and natural cycle FET may be a better regimen for ovulatory women. Our results indicate a link between the absence of the corpus luteum and adverse perinatal outcomes, and further studies are needed to detect the underlying mechanism.

The association between season, day length, and temperature on clinical outcomes after cryopreserved embryo transfer.

OBJECTIVE: To investigate whether there is an association between season, temperature, and day length at oocyte retrieval and/or embryo transfer (ET) and clinical outcomes in frozen ET cycles. DESIGN: Retrospective cohort study. SETTING: Large academically affiliated research hospital. PATIENT(S): A total of 3,004 frozen ET cycles from 1,937 different women with oocyte retrieval and transfer between 2012 and 2017. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Implantation, clinical pregnancy, spontaneous abortion, and live birth. RESULT(S): Frozen ETs with oocyte retrieval dates in summer had 45% greater odds of clinical pregnancy (odds ratio [OR], 1.45; 95% confidence interval [CI], 1.15-1.82) and 42% greater odds of live birth (OR, 1.42; 95% CI, 1.13-1.79) compared with those with oocyte retrieval dates in winter. A 41% greater odds of clinical pregnancy (OR, 1.41; 95% CI, 1.16-1.71) and 34% greater odds of live birth (OR, 1.34; 95% CI, 1.10-1.62) were observed among transfers with an average temperature at oocyte retrieval in the highest tertile (17.2-33.3 °C) compared with those in the lowest tertile (-17.2-6.7 °C). There were no consistent associations between clinical outcomes and day length at oocyte retrieval or between season, day length, or temperature at transfer of thawed embryos. CONCLUSION(S): Warmer temperatures at oocyte retrieval are associated with higher odds of clinical pregnancy and live birth among frozen ET cycles. The consistent associations seen with oocyte retrieval dates and the lack of associations observed with ET dates suggest that any seasonality effects on in vitro fertilization success are related to ovarian function and not uterine receptivity.

Characteristics of the cytoplasmic halo during fertilisation correlate with the live birth rate after fresh cleaved embryo transfer on day 2 in minimal ovarian stimulation cycles: a retrospective observational study.

BACKGROUND: Information regarding the influence of cytoplasmic events during fertilisation on the clinical outcome remains limited. The cytoplasmic halo is one of these events. A previous study that used time-lapse technology found an association of the presence and morphokinetics of the cytoplasmic halo with cleavage patterns, development to the blastocyst stage, and the ongoing pregnancy rate after blastocyst transfer. Therefore, the cytoplasmic halo may be a useful predictor of the pregnancy outcome after cleaved embryo transfer. This study evaluated the ability of the cytoplasmic halo to predict a live birth after fresh cleaved embryo transfer on day 2, and sought to identify factors potentially influencing the presence and morphokinetics of the halo. METHODS: A total of 902 embryos cultured in the EmbryoScope+® time-lapse system and subjected to single fresh cleaved embryo transfer were retrospectively analysed. The presence and duration of a cytoplasmic halo were annotated. The initial positions of the pronuclei were also observed. The correlation between the cytoplasmic halo and live birth was evaluated and the association of the cytoplasmic halo with patient, cycle, and embryonic characteristics was determined. RESULTS: Absence of a cytoplasmic halo was associated with a significant decrease in the likelihood of a live birth after fresh cleaved embryo transfer. Prolongation of the halo, especially the duration of central repositioning of cytoplasmic granules, had an adverse impact on the live birth rate. The characteristics of the cytoplasmic halo were not affected by the ovarian stimulation method used, female age, the serum steroid hormone level on the day of trigger, or semen quality. However, the cytoplasmic halo was significantly affected by male age, oocyte diameter, and the initial position of the male pronucleus. CONCLUSIONS: Absence or prolongation of the cytoplasmic halo was negatively correlated with the live birth rate after fresh cleaved embryo transfer. The characteristics of the cytoplasmic halo were strongly associated with oocyte diameter, male age, and the initial position of the male pronucleus. These findings indicate that the characteristics of the cytoplasmic halo can be used to select more competent embryos for transfer at the cleavage stage.

Defining recurrent implantation failure: a profusion of confusion or simply an illusion?

Recurrent implantation failure (RIF) is a poorly defined clinical scenario marked by failure to achieve pregnancy after multiple embryo transfers. The causes and definitions of implantation failure are heterogeneous, posing limitations on study design as well as the interpretation and application of findings. Recent studies suggest a novel, personalized approach to defining RIF. Here, we review the implantation physiology and definitions of the implantation rate, failure, and RIF.

Novel therapeutic options for treatment of recurrent implantation failure.

Despite the challenges in studying recurrent implantation failure, progress is currently being made in therapeutic options to help those who suffer from recurrent implantation failure. Three of the most promising therapeutic options for recurrent implantation failure include immune therapies such as peripheral blood mononuclear cells, platelet rich plasma and subcutaneous granulocyte-colony stimulating factor.

A review of the pathophysiology of recurrent implantation failure.

Implantation is a critical step in human reproduction. The success of this step is dependent on a competent blastocyst, receptive endometrium, and successful cross talk between the embryonic and maternal interfaces. Recurrent implantation failure is the lack of implantation after the transfer of several embryo transfers. As the success of in vitro fertilization has increased and failures have become more unacceptable for patients and providers, the literature on recurrent implantation failure has increased. While this clinical phenomenon is often encountered, there is not a universally agreed-on definition-something addressed in an earlier portion of this Views and Reviews. Implantation failure can result from several different factors. In this review, we discuss factors including the maternal immune system, genetics of the embryo and parents, anatomic factors, hematologic factors, reproductive tract microbiome, and endocrine milieu, which factors into embryo and endometrial synchrony. These potential causes are at various stages of research and not all have clear implications or immediately apparent treatment.

Do frozen embryo transfers modify the epigenetic control of imprinted genes and transposable elements in newborns compared with fresh embryo transfers and natural conceptions?

OBJECTIVE: To determine whether the epigenetic control of imprinted genes (IGs) and transposable elements (TEs) differs at birth between fresh or frozen embryo transfers and natural conceptions. DESIGN: Prospective study. SETTING: University hospital. PATIENT(S): A total of 202 singleton births were divided into three groups: 84 natural pregnancies (controls), 66 in vitro fertilization/intracytoplasmic sperm injection with fresh embryo transfers, and 52 vitro fertilization/intracytoplasmic sperm injection with frozen embryo transfers. INTERVENTION(S): None. MAIN OUTCOME MEASURE(S): Pyrosequencing was used to assess the DNA methylation profiles of three IGs (H19/IGF2:IG-DMR [two sequences], KCNQ1OT1:TSS-DMR, and SNURF:TSS-DMR) and two TEs (LINE-1 and HERV-FRD) in cord blood and placenta. The quantitative reverse transcriptase polymerase chain reaction was used to study the transcription of three IGs (H19, KCNQ1, and SNRPN) and two TEs (LINE-1 and ORF2). RESULT(S): After adjustment, the placental DNA methylation levels of H19/IGF2 were lower in the fresh embryo transfer group than in the control (H19/IGF2-seq1) and frozen embryo transfer (H19/IGF2-seq2) groups. The DNA methylation rate for LINE-1 was lower in placentas from the fresh embryo transfer group than in placentas from the control and frozen embryo transfer groups and for HERV-FRD compared with controls. In cord blood, DNA methylation levels were not significantly associated with the mode of conception. The relative expression of LINE-1 and ORF2 was decreased in both cord blood and placental tissues from fresh embryo transfer conceptions compared with natural conceptions and frozen embryo transfer conceptions. CONCLUSION(S): Compared with natural conceptions and frozen embryo transfers, fresh embryo transfers were associated with methylation and/or transcription changes in some TEs and IGs, mostly in placental samples, which could indicate altered placental epigenetic regulation resulting from ovarian stimulation protocols.

Endometrial thickness changes after progesterone administration do not affect the pregnancy outcomes of frozen-thawed euploid blastocyst transfer: a retrospective cohort study.

OBJECTIVE: To evaluate whether the change in endometrial thickness from progesterone administration day to transfer day is related to pregnancy outcomes in single frozen-thawed euploid blastocyst transfer cycles. DESIGN: Observational cohort study. SETTING: Single reproductive medical center. PATIENT(S): All patients were transferred with a single biopsied euploid blastocyst, and their endometrium was prepared with hormone replacement therapy (HRT). INTERVENTION(S): The endometrial thickness on the day of blastocyst transfer and progesterone administration was measured by transvaginal ultrasound, and the difference between them and the change ratio were calculated. MAIN OUTCOME MEASURE(S): Clinical pregnancy rates and live birth rates. RESULT(S): Endometrial ultrasound images of 508 euploid blastocyst transfer cycles using HRT were evaluated by transvaginal ultrasound. Overall, pregnancy outcomes were comparable in different groups of endometrial thickness changes. The results of multiple logistic regression showed that the clinical pregnancy rate and live birth rate did not significantly increase with the increase in endometrial thickness change ratios (per 10%) in the fully adjusted model as a continuous variable. In the adjustment model as a categorical variable, there was no statistical difference in pregnancy outcomes among the groups with changes in endometrial thickness. Interaction analysis showed that after adjusting for confounders, there was no statistically significant interaction between the endometrial thickness change ratio and pregnancy outcomes in all subgroups. CONCLUSION(S): In the euploid blastocyst transfer cycle of preparing the endometrium with HRT, the endometrial thickness change ratio after progesterone administration was not related to pregnancy outcomes.

Serum luteal phase progesterone in women undergoing frozen embryo transfer in assisted conception: a systematic review and meta-analysis.

OBJECTIVE: To investigate the association between luteal serum progesterone levels and frozen embryo transfer (FET) outcomes. DESIGN: Systematic review and meta-analysis. SETTING: Not applicable. PATIENT(S): Women undergoing FET. INTERVENTION(S): We conducted electronic searches of MEDLINE, PubMed, CINAHL, EMBASE, the Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, Web of Science, ClinicalTrials.gov, and grey literature (not widely available) from inception to March 2021 to identify cohort studies in which the serum luteal progesterone level was measured around the time of FET. MAIN OUTCOME MEASURE(S): Ongoing pregnancy or live birth rate, clinical pregnancy rate, and miscarriage rate. RESULT(S): Among the studies analyzing serum progesterone level thresholds <10 ng/mL, a higher serum progesterone level was associated with increased rates of ongoing pregnancy or live birth (relative risk [RR] 1.47, 95% confidence interval [CI] 1.28 to 1.70), higher chance of clinical pregnancy (RR 1.31, 95% CI 1.16 to 1.49), and lower risk of miscarriage (RR 0.62, 95% CI 0.50 to 0.77) in cycles using exclusively vaginal progesterone and blastocyst embryos. There was uncertainty about whether progesterone thresholds ≥10 ng/mL were associated with FET outcomes in sensitivity analyses including all studies, owing to high interstudy heterogeneity and wide CIs. CONCLUSION(S): Our findings indicate that there may be a minimum clinically important luteal serum concentration of progesterone required to ensure an optimal endocrine milieu during embryo implantation and early pregnancy after FET treatment. Future clinical trials are required to assess whether administering higher-dose luteal phase support improves outcomes in women with a low serum progesterone level at the time of FET. PROSPERO NUMBER: CRD42019157071.

The impact of medical therapies and factors related to treatment procedures in women with rheumatoid arthritis and inflammatory bowel disease receiving assisted reproduction: a nationwide cohort study.

OBJECTIVE: To examine whether medications used to treat rheumatoid arthritis (RA)/chronic inflammatory bowel disease (IBD), or factors related to the assisted reproductive technology (ART) procedures, impact the success of ART. In women with RA/IBD, initial studies have shown a reduced chance of a live-born child after ART. DESIGN: Cohort study. SETTING: Nationwide Danish health registries. PATIENTS: All Danish women with a fresh embryo transfer from January 1, 2006, through 2018. The cohorts comprised 1,824 embryo transfers in women with RA/IBD and 97,191 embryo transfers in women without RA/IBD. INTERVENTIONS: Observational, noninterventional study. MAIN OUTCOME MEASURE: Live birth per fresh embryo transfer. RESULTS: The chance of a live birth in women with RA/IBD receiving ART, compared with other women receiving ART, had an adjusted odds ratio (OR) of 0.79 (95% confidence interval [CI], 0.68-0.91). Prescribed corticosteroids before embryo transfer were positively associated with a live-born child (adjusted OR, 1.21; 95% CI, 1.12-1.31), while the use of antiinflammatory/immunosuppressive agents did not have significant importance. Intracytoplasmic sperm injection was associated with a reduced chance (adjusted OR, 0.94; 95% CI, 0.90-0.97). Type of hormone treatment protocol did not have significant importance, and transfer at the blastocyst stage was positively associated with a live-born child (adjusted OR, 1.54; 95% CI, 1.46-1.62). CONCLUSIONS: In women with RA and/or IBD, prescribed corticosteroid before embryo transfer and embryo transfer at the blastocyst stage were associated with successful ART. Intracytoplasmic sperm injection was associated with a slightly reduced chance. Antiinflammatory/immunosuppressive agents and type of hormone protocols did not have significant importance.

Comparing prediction of ongoing pregnancy and live birth outcomes in patients with advanced and younger maternal age patients using KIDScore™ day 5: a large-cohort retrospective study with single vitrified-warmed blastocyst transfer.

BACKGROUND: The KIDScore™ Day 5 (KS-D5) model, version 3, is a general morphokinetic prediction model (Vitrolife, Sweden) for fetal heartbeat prediction after embryo transfer that was developed based on a large data set that included implantation results from a range of clinics with different patient populations, culture conditions and clinical practices. However, there was no study to comparing their pregnancy and live birth prediction ability among different maternal age. The aim of this study is to analyze the performance of KS-D5 in predicting pregnancy and live birth in various maternal age groups after single vitrified-warmed blastocyst transfer (SVBT). METHODS: A total of 2486 single vitrified-warmed blastocyst transfer (SVBT) cycles were analyzed retrospectively. Confirmed fetal heartbeat positive (FHB+) and live birth (LB+) rates were stratified by Society for Assisted Reproductive Technology (SART) maternal age criteria (< 35, 35-37, 38-40, 41-42 and ≥ 43 years of age). Within each age group, the performance of the prediction model was calculated using the AUC, and the results were compared across the age groups. RESULTS: In all age groups, the FHB+ rates decreased as the KIDScore decreased (P <  0.05). Conversely, the AUCs increased as the maternal age increased. The AUC of the < 35 age group (0.589) was significantly lower than the AUCs of the 41-42 age group (0.673) and the ≥43 age group (0.737), respectively (P <  0.05). In all age groups, the LB+ rates decreased as the KIDScore decreased (P <  0.05). Conversely, the AUCs increased as the maternal age increased. The AUC of the ≥43 age group (0.768) was significantly higher than the AUCs of other age groups (P <  0.05; < 35 age group = 0.596, 35-37 age group = 0.640, 38-40 age group = 0.646, 41-42 age group = 0.679). CONCLUSIONS: In the present study, we determined that the KIDScore model worked well for prediction of pregnancy and live birth outcomes in advanced age patients.

Ovarian Hyperstimulation Syndrome Is Associated with a High Secondary Sex Ratio in Fresh IVF Cycles with Cleavage-Stage Embryo Transfer: Results for a Cohort Study.

The sex ratio at birth is defined as the secondary sex ratio (SSR). Ovarian hyperstimulation syndrome (OHSS) is a serious and iatrogenic complication associated with controlled ovarian stimulation (COS) during assisted reproductive technology (ART) treatments. It has been hypothesized that the human SSR is partially controlled by parental hormone levels around the time of conception. Given the aberrant hormonal profiles observed in patients with OHSS, this retrospective study was designed to evaluate the impact of OHSS on the SSR. In this study, all included patients were divided into 3 groups: non-OHSS (n=2777), mild OHSS (n=644), and moderate OHSS (n=334). Our results showed that the overall SSR for the study population was 1.033. The SSR was significantly increased in patients with moderate OHSS (1.336) compared to non-OHSS patients (1.002) (p=0.048). Subgroup analyses showed that increases in the SSR in patients with moderate OHSS were observed in the IVF group (1.323 vs 1.052; p=0.043), but not in the ICSI groups (1.021 vs 0.866; p=0.732). In addition, the elevated serum estradiol (E2) and progesterone (P4) levels in OHSS patients were not associated with SSR. In this study, for the first time, we report that a high SSR is associated with OHSS in patients who received fresh IVF treatments. The increases in SSR in OHSS patients are not attributed to the high serum E2 and P4 levels. Our findings may make both ART clinicians and patients more aware of the influences of ART treatments on the SSR and allow clinicians to counsel patients more appropriately.

Utilization of preimplantation genetic testing in the USA.

PURPOSE: To evaluate the use of preimplantation genetic testing (PGT) and live birth rates (LBR) in the USA from 2014 to 2017 and to understand how PGT is being used at a clinic and state level. METHODS: This study accessed SART data for 2014 to 2017 to determine LBR and the CDC for years 2016 and 2017 to identify PGT usage. Primary cycles included only the first embryo transfer within 1 year of an oocyte retrieval; subsequent cycles included transfers occurring after the first transfer or beyond 1 year of oocyte retrieval. RESULTS: In the SART data, the number of primary PGT cycles showed a significant monotonic annual increase from 18,805 in 2014 to 54,442 in 2017 (P = 0.042) and subsequent PGT cycles in these years increased from 2946 to 14,361 (P = 0.01). There was a significant difference in primary PGT cycle use by age, where younger women had a greater percentage of PGT treatment cycles than older women. In both PGT and non-PGT cycles, the LBR per oocyte retrieval decreased significantly from 2014 to 2017 (P<0001) and younger women had a significantly higher LBR per oocyte retrieval compared to older women (P < 0.001). The CDC data revealed that in 2016, just 53 (11.4%) clinics used PGT for more than 50% of their cycles, which increased to 99 (21.4%) clinics in 2017 (P< 0.001). CONCLUSIONS: A growing number of US clinics are offering PGT to their patients. These findings support re-evaluation of the application for PGT.

Adding new ingredients to the recipe for successful embryo transfers.

The success of a pregnancy and the birth of a heathy baby following embryo transfer are conditioned by many factors, including embryo quality and the uterine environment. While we keep looking for more indicators of embryo quality, it also is critical to understand what constitutes a favorable uterine environment leading to a successful pregnancy and birth. This issue of JARG offers new insights on both components-so called by some "the seed and the soil"-and also highlights the critical interactions between the two. Collectively, these publications are contributing to a better understanding of basic embryology and reproductive biology. They could lead to multiple applications to mitigate infertility issues; however, our knowledge base remains rudimentary when it comes to sorting out the 'soil or seed' dilemma. The call from all authors for more research in their respective areas resonates within the ART community. Recognizing the practical and ethical limitations of studies in human patients also reemphasizes the need for solid research in multiple animal models to better understand what constitutes the best recipe for successful embryo transfer outcomes.