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STUDY QUESTION Are the IVF parameters and the steroidogenic luteal characteristics of random-start IVF cycles different from conventional cycles in cancer patients? SUMMARY ANSWER No; controlled ovarian stimulation cycles randomly started at late follicular phase (LFP) and luteal phase (LP) are totally comparable to those conventional IVF cycles started at early follicular phase (EFP) in terms of the expression of the enzymes involved in cholesterol utilization and steroid hormone biosynthesis pathways, gonadotropin receptor expression and, estradiol (E2) and progesterone (P4) production in addition to the similarities in ovarian response to gonadotropin stimulation, oocyte yield, fertilization rate and embryo development competency in cancer patients. WHAT IS KNOWN ALREADY Random start ovarian stimulation protocols are commonly employed for oocyte and embryo freezing for fertility preservation in cancer patients with time constraints who do not have sufficient time to undergo ovarian stimulation initiated conventionally at EFP of the next cycle. No data is available regarding the molecular steroidogenic features of these cycles analyzed together with the clinical IVF characteristics in cancer patients. We aimed to address this question in this study to help understand how similar the random start cycles are to the conventional start ones. STUDY DESIGN, SIZE, DURATION A clinical translational research study conducted in 62 cancer patients undergoing IVF for fertility preservation between the years 2017 and 2022. PARTICIPANTS/MATERIALS, SETTING, METHODS Sixty-two patients who were diagnosed with different types of cancer and underwent ovarian stimulation for oocyte (n = 41) and embryo (n = 21) cryopreservation using GnRH antagonist protocol and human menopausal gonadotropins before receiving cancer treatment/surgery were enrolled in the study. For patients with breast cancer and endometrial cancer the aromatase inhibitor letrozole was used with gonadotropin stimulation. Ovarian stimulation was initiated conventionally at EFP in 22 patients and served as control while it was started at LFP in 20, and mid-LP in the other 20 patients. The luteinized granulosa cells (GCs) were recovered from follicular aspirates during oocyte retrieval procedure and used for the experiments separately for each individual patient. The expression of the enzymes involved in sex steroid biosynthesis (StAR, 3β-HSD, Aromatase) and cholesterol synthesis (3-hydroxy 3-methylglutaryl Co-A reductase (HMG-Co-A reductase)), utilization (hormone sensitive lipase (HSL)), and storage (Acetyl-Coenzyme A acetyltransferase 1 (ACAT-1)), and gonadotropin receptor expression status were analyzed using immunoblotting and RT-PCR methods. Laser confocal immunofluorescence imaging was applied to analyze and compare the expression patterns of the steroidogenic enzymes and their relation with mitochondria. In vitro E2 and P4 production by the cells were compared among the groups. MAIN RESULTS AND THE ROLE OF CHANCE Baseline demographic and IVF characteristics of the patients undergoing the conventional start and random start IVF cycles were similar. Duration of gonadotropin stimulation was significantly longer in LFP and LP start cycles in comparison to the conventional ones. Ovarian response to gonadotropin stimulation, mature and total oocyte yield, fertilization and Day 5 blastulation rates of the embryos were comparable between the conventional versus random start cycles. When the luteal GCs of these random start cycles were analyzed we could not find any gross differences between these cycles in terms of the viability index and gross light microscopic morphologic features. More detailed analysis of the molecular luteal characteristics of the cells using RT-PCR, immunoblotting methods revealed that the expression profiles of the gonadotropin receptors, and the enzymes involved in sex steroid biosynthesis and cholesterol synthesis/utilization, and the steroidogenic activity of the luteal GCs of the random start cycles are almost identical to those of the conventional start cycles. Confocal image analysis demonstrated similar patterns in the signal expression profiles of the steroidogenic enzymes and their co-localization within mitochondria. LARGE SCALE DATA N/A. LIMITATIONS, REASONS FOR CAUTION Caution should be exercised when interpreting our data and counseling cancer patients seeking fertility preservation because it is still unclear if previous exposure to cancer drugs, different ovarian pathologies or infertility etiologies, previous ovarian surgery and/or any other underlying diseases that are concomitantly present with cancer may cause a difference between conventional and random start stimulation protocols in terms of IVF parameters, luteal function and reproductive outcome. Relatively low number of patients in each stimulation protocol and pooling of luteal GCs for each patient rather than individual analysis of each follicle and oocyte are additional limitations of our study. WIDER IMPLICATIONS OF THE FINDINGS Our findings provide reassurance that random start protocol offers cancer patients an equally good prospect of fertility preservation as conventional IVF. STUDY FUNDING/COMPETING INTEREST(S) Funded by the School of Medicine, the Graduate School of Health Sciences of Koc University and Koç University Research Center for Translational Medicine (KUTTAM), equally funded by the Republic of Turkey Ministry of Development Research Infrastructure Support Program. All authors declare no conflict of interest. TRIAL REGISTRATION NUMBER N/A.

Objective To investigate the predictive value of endometrial thickness (EMT) for live birth when a lower threshold of EMT is not employed for embryo transfer (ET). Design Retrospective study Setting Academic assisted reproduction center Patient(s) All women who underwent fresh or frozen-thawed ET at the Koc University Hospital Assisted Reproduction Unit between October 2016 and August 2019 Intervention(s) After ruling out endometrial pathology, blastocyst transfer was planned regardless of the EMT in the absence of increased serum progesterone level on the trigger day in fresh embryo transfer cycles or before commencing progesterone treatment in artificially prepared frozen-thawed ET cycles. Main Outcome Measure(s) The primary outcome was live birth. Live birth and miscarriage rates per ET were stratified according to fresh and frozen-thawed ET cycles for each millimeter of endometrial thickness. Receiver operator characteristic curve analyses were performed to evaluate the predictive value of EMT for live birth. Result(s) A total of 560 ET cycles, 273 fresh and 287 frozen-thawed, were included in the study. Relevant patient characteristics as well as EMTs were similar between women who achieved a live birth and those who did not after fresh or frozen-thawed ET. There was no linear association between EMT and live birth or miscarriage rates. Area under the curve values for EMT to predict live birth after fresh, frozen-thawed, and all ETs were 0.56, 0.47, and 0.52, respectively. Conclusion(s) Our results showed that the EMT was not predictive for live birth in either fresh or frozen-thawed ET cycles. Once intracavitary pathology and inadvertent progesterone exposure were excluded, women with thinner EMTs should not be denied their potential for live birth because it is comparable to that of those with thicker EMT.

Research question Does subcutaneous progesterone provide similar live birth or ongoing pregnancy rates as vaginal progesterone in frozen embryo transfer (FET) cycles? Design Retrospective cohort study (n = 214 women), consisting of 107 women who received subcutaneous progesterone for FET in artificial cycles and 107 women receiving vaginal progesterone who were matched for age and treatment cycle rank acted as controls. All embryos were transferred in an artificial cycle with 6 mg per day oral oestradiol valerate starting on the second or third day of the menstrual cycle. Patients underwent transvaginal ultrasound on the 10th day of priming, and subcutaneous progesterone (50 mg/day) or vaginal progesterone (180 mg/day) was started if the endometrium had a trilinear pattern regardless of its thickness. Embryo transfer was carried out on the sixth day of progesterone administration. Oestradiol and progesterone were continued until a negative pregnancy test, 10 days after the transfer, or until the completion of 10th gestational week. Main outcome measures were live birth or ongoing pregnancy rates. Results Baseline characteristics were similar between the groups. Positive pregnancy test rates (64.5% versus 58.9%; P = 0.40; RR 1.1; 95% CI 0.89 to 1.35), live birth or ongoing pregnancy rates (39.3% versus 35.5%; P = 0.57; RR 1.11; 95% CI 0.78 to 1.56) and miscarriage rates (29% versus 25.5%; P = 0.68; RR 1.08; 95% CI 0.76 to 1.55) were similar in the subcutaneous progesterone and vaginal progesterone groups, respectively. Conclusions Subcutaneous progesterone seems to be an effective alternative to vaginal progesterone in patients undergoing FET. Randomized controlled trials comparing it with different progesterone preparations, routes and protocols are needed to better define its role.

Study question Do livebirth rate (LBR), obstetric and perinatal outcomes vary between frozen embryo transfers (FET) in the first or subsequent menstrual cycles following a freeze-all approach? Summary answer Immediate FET has a higher LBR and similar perinatal outcomes than delayed FET. Quantitative synthesis of available literature shows an increased LBR with immediate transfer. What is known already Whether FET should be done in the first menstrual cycle following oocyte collection (OC) is controversial and the duration of a possible detrimental effect of supraphysiological sex steroid levels on pregnancy outcome is unknown. A multinational survey centers showed that, 61% of clinicians prefer to wait for a washout period before proceeding to FET, even after a failed fresh embryo transfer. Limited number of studies compared FET in the first menstrual cycle with delayed FET in a subsequent cycle with varying results. There is limited data on obstetric outcomes of pregnancies resulting from FET in the first menstrual cycle. Study design, size, duration 198 women who underwent a freeze-all cycle followed by FET between July 2017 and June 2020 were included. 119 FET in the first menstrual cycle (30 days from oocyte collection) were retrospectively compared. MEDLINE was searched on 01 January 2021 using relevant keywords. Cohort studies comparing immediate versus delayed transfer following freeze all cycles were included and quantitative summary for LBR was obtained. Participants/materials, setting, methods Freeze-all was undertaken when (i) the woman is deemed to be at high risk for OHSS, (ii)serum progesterone level is > 1.5 ng/ml on the day of trigger, (iii)preimplantation genetic testing is planned, (iv)the woman will undergo surgery prior to ET, (v)couple preference. Main results and the role of chance Baseline characteristics were similar between the groups except for antral follicle count (22 vs 18, MD = 5, 95% CI = 0 to 8), and number of metaphase-two oocytes (13 vs 10, MD = 3, 95% CI = 1 to 6) all of which were significantly higher in the immediate transfer group. Clinical pregnancy rate (CPR) per ET was similar in two groups (50.4% vs 44.3%, RR = 1.14, 95% CI = 0.84 to 1.54). Miscarriage rate per pregnancy was significantly lower (12.3 vs 31.1, RR = 0.40, 95% CI = 0.19 to 0.84) and LBR per ET was significantly higher (42.9 vs 26.6, RR = 1.61, 95% CI = 1.06 to 2.46) in the immediate transfer group. Median gestational age at delivery was similar (267.5 (262.5–273) vs 268 (260–271.5) days, MD = 1.00, 95% CI= –4.00 to 5.00). Median birthweight was significantly higher in the delayed transfer group (3520 vs 3195 grams, MD= –300, 95% CI= –660 to –20 grams). Birthweight percentile, height at birth and head circumference were similar between groups. Literature search revealed 1712 studies from which nine were eligible for quantitative summary. Cumulative risk ratio showed a 10% increase in LBR with immediate transfer compared to delayed transfer (RR = 1.10, 95% CI = 1.01 – 1.20, I2=67%, 17369 embryo transfers). Limitations, reasons for caution Our study is limited by its retrospective design and relatively limited sample size for multivariate analyses. Yet, it is reassuring that the majority of our findings are consistent with previous publications. Wider implications of the findings: The hypotheses generated by our retrospective findings, i.e., FET in the immediate menstrual cycle resembling fresh ETs with strong trends towards lower birthweight and lower incidence of preeclampsia is noteworthy for the design of future studies, and these outcomes should be followed and reported. Trial registration number None

Study question Does the presence of endometrioma during ovarian stimulation affect blastulation and clinical pregnancy rates (CPR)? Summary answer Blastulation rates were similar in women with endometrioma compared to women without. Likewise, CPR were comparable. What is known already Although relationship of endometriosis and subfertility is well-established, its mechanism is still under investigation. Decreased oocyte quality, resulting from anatomical and/or inflammatory factors is one of the prominent culprits. Most studies regarding endometriosis and oocyte quality are highly heterogeneous and effect of endometriosis on oocyte quality is yet to be determined. Blastulation is thought as a surrogate marker for oocyte quality. Thus, it may be possible that detrimental effect of the presence of endometrioma during ovarian stimulation can be indirectly assessed by blastulation. Study design, size, duration Records of all women who underwent assisted reproductive technology treatment at Koc University Hospital Assisted Reproduction Unit between 2016 and October 2020 were screened for this retrospective study. All women who had endometrioma(s) during ovarian stimulation were included in the study group (EG) (n = 71). They were matched with women diagnosed with tubal factor or unexplained infertility who underwent oocyte pickup within the same period to form the control group (CG) (n = 104). Participants/materials, setting, methods All women underwent antagonist or long protocol. All embryos were cultured until blastocyst stage regardless of the number of oocytes or embryos available. Size/location of endometriomas, number of oocytes retrieved, number of available blastocysts, positive pregnancy test per cycle and clinical pregnancy rate per cycle were recorded. Blastulation rate was calculated as number of available blasts divided by the number of metaphase-II oocytes. Embryos were transferred in a fresh or artificially prepared frozen-thawed cycle. Main results and the role of chance There were 71 women in EG and 104 women in CG, which included 30 women with tubal and 74 with unexplained infertility. Median endometrioma size was 26 mm(22–33). Twenty-three patients in EG had history of endometrioma excision (31.3%). Median age [35.0 years (31.0–39.0) vs 34 (32.0–36.0), p = 0.26] and serum AMH levels [1.8 (1.1 - 4.2) vs 2.3 (1.3 - 3.7) ng/dL, p = 0.91] were similar in EG and CG, respectively. Body mass index in kg/m2 [21.8 (20.2–24.6) vs 24 (21.5–27.9), p Positive pregnancy test per cycle was similar at 53.5% vs 61.5% in EG and CG, respectively (p = 0.3). CPR were similar between the EG and CG (45% vs 58%, respectively, p = 0.10). Limitations, reasons for caution Retrospective design, lack of live birth information are the main limitations of our study. Wider implications of the findings: Presence of endometrioma during ovarian stimulation does not seem to adversely affect blastulation rates. While this is reassuring regarding oocyte quality, further research is required to assess its effect on live birth. Trial registration number Not applicable

Study question Does endometrial thickness (EMT) predict live birth (LB) after fresh and frozen-thawed embryo transfer (ET) and is there a lower EMT cut-off for ET? Summary answer Once intracavitary pathology and inadvertent progesterone exposure is excluded, EMT is not predictive for LB. EMT is not linearly associated with probability of LB. What is known already EMT is commonly used as a marker of endometrial receptivity and in turn, assisted reproductive technology treatment success. ET is often cancelled or postponed if EMT is below an arbitrary cut-off. However, the available evidence on the relationship between EMT and LB rates is conflicting and too dubious to hold such strong stance. An overwhelming majority of the studies on the subject are retrospective, they use different arbitrary cut off values ranging between 6 to 9 mm with heterogeneous stimulation and transfer protocols. Study design, size, duration Records of all women who underwent fresh or frozen-thawed ET in Koc University Hospital Assisted Reproduction Unit between October 2016 - August 2019 were retrospectively screened. All women who underwent fresh or frozen-thawed blastocyst transfer during the study period were included. Every woman contributed to the study with only one transfer cycle for each category, i.e., fresh ET and frozen-thawed ET. Participants/materials, setting, methods After ruling out endometrial pathology, EMT was measured on the day of ovulation trigger for fresh ET cycles, and on the day of progesterone commencement for frozen-thawed ET. ET was carried out, regardless of EMT, if there was no suspicion of inadvertent progesterone exposure, i.e., due to follicular phase progesterone elevation in fresh or premature ovulation in frozen ET cycles. Main results and the role of chance 560 ET cycles, 273 fresh and 287 frozen-thawed, were analyzed. EMT varied from 4mm to 18mm. EMT were similar between women who achieved a LB and who did not after fresh ET [10.5 (9.2 – 12.2) mm and 9 (8 – 11) mm, respectively, p = 0.11]. Ovarian stimulation characteristics and proportion of women who received a single embryo were similar (69% vs 68.3%, respectively, p = 0.91). Women who achieved a LB was significantly younger than those who did not [35 (32–38) and 37 (33–41), respectively, p Area under curve values of EMT for predicting LB in fresh, frozen-thawed and all ET were 0.56, 0.47 and 0.52, respectively. EMT and LB rate were not linearly correlated in fresh or frozen-thawed ET cycles. Limitations, reasons for caution Although our study is retrospective, no women was denied ET due to EMT in our center. Only patients undergoing ET were included in the analysis, which may introduce bias due to the selection of couples who were competent enough to produce at least one blastocyst fit for transfer. Wider implications of the findings: Since women with thin endometrium had reasonable chance for LB even in the absence of a cut-off for EMT in this unique dataset, delaying or denying ET for any given EMT value alone does not seem justified. Further studies in which ET is carried out regardless of EMT are needed. Trial registration number Not applicable

STUDY QUESTION Are there any differences in the molecular characteristics of the luteal granulosa cells (GC) obtained from stimulated versus non-stimulated (natural) IVF cycles that may help explain the defective luteal phase in the former? SUMMARY ANSWER Luteal GC of stimulated IVF cycles, particularly those of agonist-triggered antagonist cycles, are less viable ex vivo, express LH receptor and anti-apoptotic genes at lower levels, undergo apoptosis earlier and fail to maintain their estradiol (E2) and progesterone (P4) production in comparison to natural cycle GC. WHAT IS KNOWN ALREADY Luteal function is defective in stimulated IVF cycles, which necessitates P4 and/or hCG administration (known as luteal phase support) in order to improve clinical pregnancy rates and prevent miscarriage. The luteal phase becomes shorter and menstruation begins earlier than a natural cycle if a pregnancy cannot be achieved, indicative of early demise of corpus luteum (premature luteolysis). Supra-physiological levels of steroids produced by multiple corpora luteae in the stimulated IVF cycles are believed to inhibit LH release directly via negative feedback actions on the hypothalamic-pituitary-ovarian axis resulting in low circulating levels of LH and a defective luteal phase. We hypothesized that some defects in the viability and steroidogenic activity of the luteal GC of the stimulated IVF cycles might contribute to this defective luteal phase in comparison to natural cycle GC. This issue has not been studied in human before. STUDY DESIGN, SIZE, DURATION A comparative translational research study of ex vivo and in vitro models of luteal GC recovered from IVF patients undergoing natural versus stimulated IVF cycles was carried out. Luteinized GC were obtained from 154 IVF patients undergoing either natural (n = 22) or stimulated IVF cycles with recombinant FSH and GnRH agonist (long) (n = 44), or antagonist protocol triggered conventionally either with recombinant hCG (n = 46) or with a GnRH agonist (n = 42). GC were maintained in vitro for up to 6 days. PARTICIPANTS/MATERIALS, SETTING, METHODS Cellular viability (YO-PRO-1 staining), the expression of the steroidogenic enzymes, pro-apoptotic genes [Bcl-2-associated death promoter (BAD), Bcl-2-associated X protein (BAX) and Caspase-3 (CASP3)], anti-apoptotic genes [RAC-alpha serine/threonine-protein kinase (AKT-1) and Bcl-2-like protein 2 (BCL2-L2)], LH receptor, vascular endothelial growth factor (VEGF) (using real-time quantitative PCR at mRNA level and western blot immunoprecipitation assay at protein level) and in vitro E2 and P4 production (electrochemiluminescence immunoassay) were compared in GC among the groups. MAIN RESULTS AND THE ROLE OF CHANCE Natural cycle GC were significantly more viable ex vivo (88%) compared to their counterparts of the stimulated IVF cycles (66, 64 and 37% for agonist and antagonist cycles triggered with hCG and GnRH agonist respectively, P < 0.01). They were also more capable of maintaining their vitality in culture compared to their counterparts from the stimulated IVF cycles: at the end of the 6-day culture period, 74% of the cells were still viable whereas only 48, 43 and 22% of the cells from the agonist and antagonist cycles triggered with hCG and agonist respectively, were viable (P < 0.01). The mRNA expression of anti-apoptotic genes (AKT-1 and BCL2-L2) was significantly lower, while that of pro-apoptotic genes (BAD, BAX and CASP3) was significantly higher in the stimulated cycles, particularly in the agonist-triggered antagonist cycles, compared to natural cycle GC (P < 0.01 for long protocol and antagonist hCG trigger, P < 0.001 for agonist trigger). The expression of steroidogenic enzymes (stAR, SCC, 3β-HSD and aromatase) and VEGF was significantly higher in the agonist and hCG-triggered antagonist cycles compared to natural cycle GC. Therefore, in vitro E2 and P4 production in cells from the stimulated IVF cycles was significantly higher than their counterparts obtained from the natural cycles in the first 2 days of culture. However, after Day 2, their viability and hormone production began to decline very rapidly with the most drastic decrease being observed in the agonist-triggered cycles. By contrast, natural cycle GC maintained their viability and produced E2 and P4 in increasing amounts in culture up to 6 days. In vitro P production and the mRNA and protein expression of LH receptor, VEGF and 3β-HSD were most defective in the agonist-triggered antagonist cycles compared to natural and agonist and hCG-triggered antagonist cycles. In vitro hCG treatment of a subset of the cells from the agonist-triggered cycles improved their viability, increased E2 and P4 production in vitro and up-regulated the mRNA expression of anti-apoptotic gene BCL-L2 together with steroidogenic enzymes stAR, SCC, 3B-HSD, LH receptor and VEGF. LARGE SCALE DATA Not applicable. LIMITATIONS, REASONS FOR CAUTION The limitations include analysis of luteinized GC only might not reflect the in vivo mechanisms involved in survival and function of the whole corpus luteum; GC recovered during oocyte retrieval belong to a very early stage of the luteal phase and might not be representative; effects of ovulation triggered with hCG may not equate to the endogenous LH trigger; the clinical characteristics of the patients may vary among the different groups and it was not possible to correlate stimulation-related molecular alterations in luteal GC with the clinical outcome, as no oocytes have been utilized yet. Therefore, our findings do not conclusively rule out the possibility that some other mechanisms in vivo may also account for defective luteal function observed in stimulated IVF cycles. WIDER IMPLICATIONS OF THE FINDINGS Ovarian stimulation is associated with significant alterations in the viability and steroidogenic activity of luteal GC depending on the stimulation protocol and mode of ovulation trigger. Reduced survival and down-regulated expression of 3B-HSD, LH receptor and VEGF leading to compromised steroid production in stimulated cycles, and particularly in the agonist-triggered cycles, may at least in part help explain why the luteal phase is defective and requires exogenous support in these cycles. STUDY FUNDING/COMPETING INTEREST(S) This study was funded by the School of Medicine, the Graduate School of Health Sciences of Koc University and Koc University Research Center for Translational Medicine (KUTTAM), equally funded by the Republic of Turkey Ministry of Development Research Infrastructure Support Program. All authors declare no conflict of interest.

The aim of this retrospective cohort study was to compare the effectiveness of the new flexible progestin primed ovarian stimulation (fPPOS) protocol with the flexible gonadotropin-releasing-hormone antagonist (GnRH-ant) protocol in women with decreased ovarian reserve (DOR). Twenty-seven women who underwent fPPOS and 54 age-matched women who received GnRH-ant for pituitary suppression were included in the study. All women had DOR and underwent oocyte cryopreservation. Three-hundred IU/day FSH was started on cycle day 2-3 and 0.25 mg/day GnRH-ant or 10 mg/day medroxyprogesterone acetate was started when the leading follicle reached 14 mm or serum oestradiol level was ≥200 ng/mL. The median duration of stimulation, day of commencing pituitary suppression and duration of suppression were similar in both groups, with 8, 5, and 5 days, respectively. The median number of cumulus-oophorous complexes (4.0 vs 5.5), metaphase-two oocytes (3 vs 4), the total number of oocytes cryopreserved (3.0 vs 4.5), and oocyte maturation rates (0.67 vs 0.70) were similar between the fPPOS and GnRH-ant groups, respectively. There was one case of premature ovulation in the fPPOS group and none in the GnRH-ant group (

Research question Do live birth rates (LBR), obstetric and perinatal outcomes vary between women who underwent frozen embryo transfer (ET) in the immediately subsequent menstrual cycle, and with those who underwent delayed frozen ET. Design Retrospective cohort study (n = 198) consisting of 119 women who underwent immediate transfer within 30 days of oocyte retrieval (OR) and 79 women who underwent delayed transfer which was performed after >30 days following OR. Either flexible antagonist or flexible progestin-primed ovarian stimulation protocols were started after a baseline ultrasonography on the second or third day of menstrual cycle. Only freeze all cycles were included in the study and all transfers were with hormonal endometrial preparation. Main outcome measures were LBR, birth weight, gestational day at birth and pregnancy complications. Results Peak estradiol level on trigger day (2746 vs 2081 pg/ml) and number of metaphase-two oocytes (13 vs 10) were significantly higher in the immediate transfer group. Clinical pregnancy rate per ET was similar between the groups (50.4% vs 44.3%). However, miscarriage rate per positive pregnancy was significantly higher (12.3% vs 31.1%) while LBR per ET was significantly lower (42.9% vs 26.6%) in the delayed transfer group. Median gestational age at delivery were 267.5 and 268 days in the immediate and delayed transfer groups. Median birthweight was significantly higher in the delayed transfer group (3520 vs 3195 g). Adjusted analyses also suggest similar LBR with immediate and delayed transfer. Conclusion(s) Frozen ET in the immediate menstrual cycle and delayed ET, after a freeze all strategy did not show significant difference in terms of LBR after adjustment. Obstetric and perinatal outcomes of frozen ET in the immediate menstrual cycle appear reassuring.