Maternal and obstetric outcomes are influenced by developmental stage and cryopreservation of transferred embryos after clomiphene citrate-based minimal stimulation IVF

Abstract STUDY QUESTION Is the embryo transfer (ET) method associated with maternal and perinatal outcomes after minimal stimulation IVF using clomiphene citrate (CC)? SUMMARY ANSWER The incidence of pregnancy complications and adverse perinatal outcomes was influenced by the developmental stage (cleavage versus blastocyst stages) and cryopreservation (fresh versus vitrified) of the transferred embryos. WHAT IS KNOWN ALREADY Pregnancies resulting from IVF are associated with higher risks of adverse perinatal outcomes compared to natural conceptions; therefore, the next focus in reproductive medicine should be to assess whether these increased risks are attributable to IVF. Pregnancy complications and perinatal outcomes should be considered in addition to pregnancy outcomes when selecting the ET method, however, studies that describe the influence of transfer methods on perinatal and maternal outcomes are limited. STUDY DESIGN, SIZE, DURATION This study retrospectively analysed a large single-centre cohort. The clinical records of 36 827 women who underwent oocyte retrieval (during a CC-based minimal stimulation cycle) followed by their first ET at the fertility treatment centre between January 2008 and December 2017 were retrospectively analysed. The patients underwent a single fresh cleavage-stage ET (SFCT), single vitrified-warmed cleavage-stage ET (SVCT) or single vitrified-warmed blastocyst transfer (SVBT). This study only included one cycle per patient. PARTICIPANTS/MATERIALS, SETTING, METHODS Oocyte retrieval was performed following CC-based minimal ovarian stimulation. The embryos were transferred 2–3 days after retrieval or vitrified at the cleavage or blastocyst stage. The vitrified embryos were then warmed and transferred within the natural cycles. Pregnancy complications and perinatal outcomes were stratified according to the transfer methods used. Multivariate logistic regression analysis was performed to evaluate the effect of ET methods on the prevalence of pregnancy complications and congenital anomalies. MAIN RESULTS AND THE ROLE OF CHANCE The rates of clinical pregnancy and delivery were significantly different among the groups. We analysed pregnancy complications in 7502 singleton births (SFCT, 3395 cycles; SVCT, 586 cycles; and SVBT, 3521 cycles). Multivariate logistic regression analysis revealed that the adjusted odds ratio (AOR) for hypertensive disorders in pregnancy was significantly lower in the SVBT group than in the SFCT group [AOR, 0.72; 95% CI, 0.56–0.92]. The AOR for low-lying placenta was lower in the SVBT group than in the SFCT group (AOR, 0.34; 95% CI, 0.19–0.60). The AOR for placenta previa was lower in the SVCT and SVBT groups than in the SFCT group (AOR, 0.21; 95% CI, 0.07–0.58 versus AOR, 0.53; 95% CI, 0.38–0.75, respectively). A total of 7460 follow-up data on neonatal outcomes was analysed. The AOR for preterm delivery was lower in the SVBT group than in the SFCT group (AOR, 0.78; 95% CI, 0.64–0.94). The AOR for low birthweight was significantly lower after SVCT and SVBT than after SFCT (AOR, 0.68; 95% CI, 0.46–0.98 versus AOR, 0.57; 95% CI, 0.48–0.66, respectively). The AOR for small for gestational age was lower in the SVCT and SVBT groups than in the SFCT group (AOR, 0.68; 95% CI, 0.46–0.98 versus AOR, 0.44; 95% CI, 0.36–0.55, respectively). The AOR for large for gestational age babies was higher in the SVBT group than in the SFCT group (AOR, 1.88; 95% CI, 1.62–2.18). The incidence of each congenital anomaly was similar among the groups. LIMITATIONS, REASONS FOR CAUTION The study data were collected through self-reported parental questionnaires on maternal and neonatal outcomes. Our findings were not compared with the incidence of pregnancy complications and congenital anomalies in natural pregnancies. Furthermore, this study was retrospective in nature; therefore, further studies are required to ascertain the generalizability of these findings to other clinics with different protocols and/or different patient demographics. WIDER IMPLICATIONS OF THE FINDINGS This study demonstrated reassuring outcomes for SVBT (in terms of a lower incidence of pregnancy complications) compared to SFCT. Our findings provide valuable knowledge that will help improve perinatal and maternal outcomes in CC-based stimulation and inform couples of the possible benefits and risks of each type of ET method. STUDY FUNDING/COMPETING INTEREST(S) This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors. The authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER N/A.


Introduction
Fresh cleavage-stage embryo transfer (ET) may be advantageous to embryonic development since the uterus can maintain the homeostasis of the embryo's environment more effectively than in vitro conditions (Fernandez-Shaw et al., 2015;Glujovsky et al., 2016). However, the pregnancy rate is higher after blastocyst transfer than after cleavagestage ET. Furthermore, cryopreservation techniques enable embryos to avoid the detrimental effects that ovarian stimulation exerts on endometrial function (Ubaldi et al., 1997;Kolibianakis et al., 2002;Shapiro et al., 2011). Therefore, an optimal and patient-friendly strategy for ET (i.e. cleavage stage versus blastocyst stage or fresh versus cryopreserved) should be selected for individual patients to obtain better pregnancy outcomes.
Pregnancies resulting from IVF (multiple or single) are associated with a higher risk of adverse perinatal outcomes, such as low birthweight (LBW), small for gestational age (SGA) (Qin et al., 2017), preeclampsia and placental anomalies (Romundstad et al., 2006;Vermey et al., 2019), compared to pregnancies that were conceived naturally. Therefore, the next focus in reproductive medicine should be to assess whether the increased risks are attributable to ART. Hence, in selecting ET methods, perinatal and maternal outcomes should be considered in addition to pregnancy outcomes. However, studies describing the influence of transfer methods on perinatal and maternal outcomes are limited. An earlier study reported that neonatal health parameters, including the prevalence of congenital malformations, after blastocyst transfers are similar to or slightly better than those after cleavage-stage ETs (Ishihara et al., 2014). Furthermore, a cumulative meta-analysis study reported that pregnancies resulting from frozen ETs (FET) have relatively lower risks of preterm delivery (PTD), LBW and SGA babies compared to those resulting from fresh ETs, which have an increased risk of hypertensive disorders of pregnancy (HDP and large for gestational age (LGA) babies) (Maheshwari et al., 2018). However, these studies included several methods of ovarian stimulation. Ovarian stimulation protocols may adversely affect perinatal outcomes, such as LBW (Santos et al., 2010); therefore, the influence of transfer methods on perinatal and maternal outcomes should be evaluated in patients who received treatment under the same protocol. That is, the ovarian stimulation method in the oocyte retrieval cycles should be uniform to unify the effect of ovarian stimulation on embryos. Furthermore, studies on perinatal and maternal outcomes in clomiphene citrate (CC)-based minimal stimulation cycles are limited. Therefore, we analysed retrospectively a large single-centre cohort stratified by the ET method they received and assessed the perinatal and maternal outcomes after CCbased minimal ovarian stimulation for IVF.

WHAT DOES THIS MEAN FOR PATIENTS?
Pregnancies resulting from IVF treatment have higher risks (related to health of the mother and child) compared to natural conceptions. Therefore, effectiveness (pregnancy outcomes) and safety (maternal and child health) should be considered by clinicians when choosing treatment strategies, such as embryo transfer (ET) methods. The ET methods can be roughly divided into two types: fresh ET and frozen ET (FET). The outcomes of fresh ETs can be adversely affected by the medicine for egg growth promotion. By using FET, these adverse effects can be avoided; however, the embryos might be damaged by the cryopreservation (freezing) procedure. In this study, we investigated whether there is a link between ET method and the safety of mothers and babies during pregnancy and delivery. The study reviewed 10 years' worth of data from a single infertility centre and found that FET in the natural cycle had a higher chance of ending in a live birth and a lower risk of pregnancy complications compared to fresh ETs in the egg retrieval cycle. These findings provide valuable information that will help improve clinical outcomes and maternal and child health and will also help couples when considering the possible benefits and risks of each type of ET method.

Study patients
In this study, women were included when their first ET was determined. All clinical records of women who underwent oocyte retrieval (during a CC-based minimal stimulation cycle) followed by their first ET at the Kato Ladies Clinic between January 2008 and December 2017 were analysed retrospectively ( Supplementary Fig. S1). The patients underwent a single fresh cleavage-stage ET (SFCT), single vitrified-warmed cleavage-stage ET (SVCT) or single vitrified-warmed blastocyst transfer (SVBT). This study included only one cycle per patient. Only the data of patients with singleton pregnancies were included. The follow-up data from all patients who delivered were used for the analysis of pregnancy complications. Data on the completed follow-up on neonatal outcomes were compared among the three ET methods. Data of patients who had cervical incompetence were excluded from the analysis of neonatal outcomes. We classified infertility as ovulation (irregular menstruation caused by polycystic ovary syndrome or diminished ovarian reserve), tubal factor (diagnosed by hysterosalpingography), endometrial factor (diagnosed by hysteroscope), male factor (diagnosed by semen test), combined and unexplained (patients not diagnosed with any cause).

Ethical approval
This retrospective cohort study was approved by the Institutional Review Board of . Written informed consent for the analysis of embryonic, pregnancy, maternal and perinatal outcomes was obtained from all patients at the time of first consultation.

Minimal ovarian stimulation cycle IVF
The detailed protocol for minimal stimulation with CC has been previously reported (Kato et al., 2018;Karakida et al., 2020;Nishihara et al., 2020). In brief, CC (50-100 mg/day; Fuji Pharma Co., Ltd., Tokyo, Japan) was administered orally (with an extended regimen) from the third day of the retrieval cycle to the day before induction of final oocyte maturation. Ovulation was triggered using a nasal spray containing the GnRH agonist, buserelin (Suprecur; Mochida Pharmaceutical Co., Ltd., Tokyo, Japan or Buserecur; Fuji Pharma Co., Ltd.).
Oocyte retrieval was performed 30-36 h after triggering, using a 21-G needle (Kitazato Corporation, Shizuoka, Japan) without anaesthesia or follicular flushing. Cumulus-oocyte complexes were collected, washed and transferred to human tubal fluid medium (Kitazato Corporation) with paraffin oil at 5% CO 2 in air at 37 C for culture, until either conventional IVF was performed 3 h later  or, in cases of ICSI, denudation was performed 4 h after oocyte retrieval Ezoe et al., 2020). All embryos were cultured at 37 C (gas phase: 5% O 2 , 5% CO 2 and 90% N 2 ) with 100% humidity in a water jacket or with nonhumidified incubators (Astec Co. Ltd, Fukuoka, Japan). Embryo vitrification and warming were performed using Cryotop (Kitazato Corporation), as previously described (Mori et al., 2015).

Embryo transfer
The ET method to be used was determined after consultations with patients at the initiation of oocyte retrieval cycles. In our clinic, SFCT was basically proposed for the first ET to simplify the first treatment cycle. However, the freeze-all strategy was chosen if a CC-induced thin endometrium was observed. In cases where an endometrial polyp was observed during the oocyte retrieval cycle or where the day chosen for the transfer was inconvenient for the patient, the freeze-all strategy was also chosen. Furthermore, in some cases, SVBT was chosen for the first ET owing to issues with the fallopian tubes and previous ectopic pregnancies. The patient's preferences were also considered. SFCTs and SVBTs were performed as previously described (Kato et al., 2012;Nishihara et al., 2020;Onogi et al., 2020). ET was performed under vaginal ultrasound guidance using a specially designed soft silicone inner catheter (Kitazato Corporation); a single embryo was placed in a minimal volume in the upper part of the uterine cavity. In SFCT cycles, the cleavage-stage embryo was transferred on Day 2 or 3 after oocyte retrieval in the CC-based minimal stimulation cycle. In SVCT and SVBT cycles, the cleavage-stage embryo or blastocyst was transferred on Day 2 or 5, respectively, after ovulation in a natural cycle. Oral dydrogesterone (30 mg/day; Mylan EPD G.K., Tokyo, Japan) was administered routinely during the early luteal phase after the transfers. Maternal and neonatal outcomes were obtained from the questionnaire filled by patients after the infant's 1-month examination. All pregnant women were invited to respond to the questionnaire at 9 weeks of gestation, in the second trimester, and after delivery. If they did not respond, we contacted them and asked about their outcomes.

Study outcomes
The primary outcomes were maternal and obstetric outcomes and major congenital anomalies. Maternal and obstetric outcomes included HDP, gestational diabetes mellitus, haemolysis-elevated liver enzymeslow platelet count syndrome, preterm premature rupture of membrane, low-lying placenta, placenta previa, placenta accreta, placental abruption and caesarean section, while neonatal outcomes included gestational age [27 weeks, 28-31 weeks, 32-36 weeks, 37-41 weeks and 42 weeks], birthweight [<1000 g, 1000-1499 g, 1500-2499 g and 2500 g], SGA and LGA. When the edge of the placenta was <20 mm from the cervix but not overlying it, it was classified as a lowlying placenta. When the placenta completely covered the cervix, it was classified as placenta praevia (Jansen et al., 2020).
The questionnaire requested information on the following: date and mode of delivery, sex, birthweight, length of the newborn(s), presence of any birth defect or anomaly and pregnancy complications. A live birth was defined as any delivery at 22 weeks of gestation. PTD was defined as delivery occurring at <37 weeks of gestation. LBW and very LBW were defined as birthweights of <2500 g and <1500 g, respectively. Perinatal mortality was defined as the sum of stillbirths (22 pregnancy weeks) and early (within 7 days) neonatal deaths. SGA and LGA were defined as birthweights below the 10th percentile and above the 90th percentile, respectively, according to the Japanese national reference for neonates (Itabashi et al., 2010). Neonatal outcomes were obtained from questionnaires completed by mothers after the 1-month infant examination. Congenital anomalies were classified using the Q-codes of the International Statistical Classification of Diseases and Related Health Problems, 10th Revision, by reformatting the answers provided by the parents in the questionnaires (World Health Organization, 2016). Major congenital anomalies were classified according to the European Surveillance of Congenital Anomalies (EUROCAT) guidelines into the following 13 classes: nervous system; eyes; ears, face and neck; congenital heart defects; respiratory; orofacial clefts; digestive system; abdominal wall defects; urinary; genital; limb; other anomalies/syndromes; and chromosomal. According to the EUROCAT guidelines revised in November 2021, the following cases were not registered: cases of cerebral palsy; and cases with only minor defects, excluding those associated with major anomalies.

Statistical analyses
All statistical analyses were performed using JMP software (SAS, Cary, NC, USA). Proportion data were analysed using the Chi-squared test, while continuous parameters were compared using the one-way ANOVA, with significance determined using Tukey's test for post hoc analysis. Univariate logistic regression analysis was used to identify confounders that were potentially associated with maternal and perinatal outcomes. Furthermore, the association of the confounders with the ET method groups was analysed. Multivariate logistic regression analysis for the maternal and perinatal outcomes was used to adjust for bias (using the confounders) and verify the statistical significance (using Wald statistic). The parameters associated with maternal and perinatal outcomes or with the ET method groups were used as confounders. For the analysis of pregnancy complications, maternal age, BMI, smoking, previous delivery and cause of infertility were used as confounders. For the analysis of perinatal outcomes, maternal age, BMI, smoking, previous delivery, cause of infertility and infant sex were used as confounders. Odds ratios (ORs) and adjusted ORs (AORs) are reported with 95% CIs for each group.
The SFCT group was used as the reference for logistic regression analysis. A P-value of <0.05 was considered statistically significant.

Pregnancy complications after SFCT, SVCT and SVBT
A total of 36 827 ETs (SFCT, 23 738 cycles; SVCT, 3395 cycles; and SVBT, 9694 cycles) were performed during the study period (Table I). The rates of clinical pregnancy and delivery were significantly different among the groups. The follow-up data of 7502 patients who delivered were stratified according to the transfer methods used (Table II). The incidence of pregnancy complications was higher in the SFCT group than in the SVCT and SVBT groups. In particular, the incidence of HDP was higher in the SFCT group compared with the SVBT group. Furthermore, low-lying placenta and placenta previa were more frequently observed in the SFCT group than in the SVCT and SVBT groups. The association between transfer methods and pregnancy complications adjusted for maternal age and BMI was assessed using multivariate logistic regression analysis (Table III). The AOR for HDP was significantly lower in the SVBT group than in the SFCT group. The AORs for low-lying placenta and placenta previa were lower in the SVCT and SVBT groups than in the SFCT group.

Neonatal outcomes after SFCT, SVCT and SVBT
We obtained the completed follow-up data on 7477 (99.7%) cases. Of these, patients with cervical incompetence were excluded from the  Supplementary Fig. S1). There was no statistical difference in the stillbirth rate among the three groups. The rate of PTD was higher after SFCT than after SVCT and SVBT. The incidence of SGA was significantly higher in the SFCT group than in the SVCT and SVBT groups. Furthermore, the incidence of SGA was significantly higher in the SVCT group than in the SVBT group. The incidence of LGA was significantly higher in the SVBT group than in the SFCT and SVCT groups. The rates of infant death and birth defects were comparable among the groups. In the stillbirth cycles, the incidence of birth defect was also comparable among the three groups. Furthermore, multivariate logistic regression analysis demonstrated that the AOR for PTD was lower in the SVBT group that in the SFCT group (Table V). The AOR for LBW was significantly lower after SVCT and SVBT. A higher AOR for SGA and lower AOR for LGA were observed in the SFCT group compared to the SVCT and SVBT groups.

Detailed analysis of congenital anomalies
Congenital anomalies were categorized into 13 classes (Table VI and  Supplementary Table SI). The incidence of each congenital anomaly was similar among the groups in the live-birth cycles. The most frequent congenital anomaly was congenital heart defects in the live-birth cycles in all groups. In the stillbirth cycles, the incidence of each congenital anomaly was similar among the groups, and chromosomal anomalies were observed in all groups.

Discussion
In this large retrospective cohort study of 36 827 ET cycles, we confirmed that the clinical pregnancy and delivery rates were highest after SVBT, followed by SVCT, and then SFCT. Our results also showed that pregnancies resulting from SFCT had a significantly higher incidence of HDP and PTD compared to those resulting from SVBT. Furthermore, the incidences of placenta previa, low-lying placenta, LBW and SGA were significantly higher after SFCT than after SVCT and SVBT, while the LGA rate was significantly lower in the SFCT group. The rates of stillbirth, infant death and birth defect were comparable among all three groups. This study demonstrated the favourable outcomes of SVBT-improved pregnancy, maternal and perinatal outcomes. In contrast, compared to SVCT and SVBT, SFCT was associated with a higher risk of pregnancy complications, which might be linked to the type of ET (cleavage versus blastocyst or fresh versus frozen) or the endometrial preparation (ovarian stimulation cycle or natural cycle). FET is associated with improved neonatal outcomes with respect to the rates of PTD, LBW and SGA but has a higher risk of LGA compared to fresh ETs although their congenital anomaly rates are comparable (Kato et al., 2012;Maheshwari et al., 2018); this is consistent with our results. A number of studies reported that FET was associated with a higher risk of HDP (Imudia et al., 2013;Liu et al., 2013;Ishihara et al., 2014;Opdahl et al., 2015;Maheshwari et al., 2016); however, our results showed that fresh ET (SFCT) had a higher risk of HDP than   frozen blastocyst transfer (SVBT). The abovementioned studies were limited because the cohorts and interventions (embryonic stage, freezing method, endometrial preparation protocol and criteria in replacement cycles) varied. Furthermore, most studies that described an increased risk of HDP after FET included hormone replacement (HR) as part of endometrial preparation (Imudia et al., 2013;Liu et al., 2013;Ishihara et al., 2014). FET carried out in HR cycles is associated with a higher risk of HDP compared to natural cycles Moreno-Sepulveda et al., 2021); therefore, we considered that the risk of HDP was associated more with the method of endometrial preparation than with the type of transfer. Our results demonstrated that the incidences of placenta previa and low-lying placenta were significantly higher after fresh ET than after FET. A previous study reported that the risk of placental abnormalities, such as placenta previa, was lower after FET compared to fresh ET (Sazonova et al., 2011); our study confirmed this result. However, some studies reported that the incidence of placental abnormalities was comparable between fresh ET and FET (Liu et al., 2013;Ishihara et al., 2014). One study showed that fresh ETs in the stimulated cycles and FET in the HR cycles were both associated with a higher risk of placental abnormalities compared to FET in natural cycles (Rombauts et al., 2014). Thus, exogenous hormone administration for endometrial preparation might have a greater influence on the risk of placental abnormalities than the type of transfer.
Ovarian stimulation is associated with an increased risk of PTD, LBW and SGA compared to fresh ET in natural cycles (Jwa et al., 2019). In the process of implantation and placentation, hormones stimulate trophoblast differentiation and invasion, which are essential during implantation (Malassine and Cronier, 2002;Pereira et al., 2015).
Recent studies have suggested that the supraphysiologic oestradiol milieu generated during fresh IVF could alter the optimal periimplantation uterine environment, leading to abnormal placentation and ultimately, adverse perinatal and maternal outcomes, such as LBW, SGA, HDP and placenta previa (Bielefeldt et al., 1990;Farhi et al., 2010;Pereira et al., 2015Pereira et al., , 2017Saito et al., 2019;Huang et al., 2020). When the serum oestradiol level on the day of the trigger is increased by ovarian stimulation, the increase in adverse obstetric outcomes continues to rise in a linear fashion (Royster et al., 2016). In the present study, the serum oestradiol level on the day of maturation trigger was significantly higher in the SFCT group than in the SVCT and SVBT groups. Therefore, we hypothesized that a supraphysiological oestradiol level may alter normal angiogenesis and placentation, leading to adverse outcomes, such as LBW, SGA, HDP and placental abnormalities. Further investigations of potential alterations in placental and foetal development in pregnancies following fresh ET in a CC-based minimal stimulation cycle are needed.

Strengths and limitations
The strength of our study is the large, single-centre cohort analysis since a large, uniform cohort is essential for the assessment of infrequent events such as minor obstetrical complications and congenital malformations. In the present study, all transferred embryos were derived from the oocytes retrieved in the CC-based minimal stimulation cycle, i.e. all embryos were exposed to a single treatment. This consistency helped us to exclude the effects of ovarian stimulation on the embryos. Furthermore, the laboratory and ET protocols and luteal support were uniform among the groups. In particular, FET was only performed in natural cycles, which simplified the comparison of the  fresh and frozen transfer methods. Moreover, to overcome some of the inherent design limitations, we adjusted for important pregnancyrelated confounders, such as maternal age and BMI, cause of infertility, smoking history and previous delivery, in our analyses to minimize possible flaws in our data. This improved the validity of our analysis, although unknown confounders and residual limitations may exist. First, the single-centre cohort analysis (which we mentioned as a strength above) could also be a limitation. The minimal stimulation protocolextended administration of CC, intra-nasal GnRH agonist for ovulation trigger and oral progesterone for luteal support-is not widely used. Therefore, external validity of the study findings could be limited. Data were collected using self-reported parental questionnaires on maternal and neonatal outcomes. Self-reported maternal and neonatal complications could be potentially erroneous, particularly where uncommon/ complex medical terms were involved. Furthermore, the collection rate of completed follow-up data on neonatal outcomes was 99.7%; some of the more severe outcomes could be associated with a lower chance of reply. Thus, a crosschecking process could have been more credible. Our findings were not compared with the incidence of pregnancy complications and congenital anomalies in natural pregnancy. Furthermore, this was a retrospective observational study, and there is a possibility that the difference in the patients' background characteristics, which may have been caused by the indication of the ET methods, affected maternal and perinatal outcomes. Further studies comparing similar groups of patients are needed to ascertain the generalizability of these findings. Furthermore, we conducted power analysis on each outcome (deliveries, pregnancy complications and birth defects) among the ET methods and detected a difference of 99.9% for delivery, 99.9% for pregnancy complication and 92.2% for birth defects. However, this study showed powers ranging from 5.0% to 99.9% in detecting a difference in each complication or congenital anomaly among the groups; therefore, the accuracy of some analysis results was low owing to the small sample size. Therefore, further studies with larger sample sizes are required to validate our findings.
In conclusion, we found reassuring outcomes with SVBT in terms of lower incidence of pregnancy complications (including PTD, LBW, SGA, HDP) and placental abnormalities, compared to SFCT. Even with the growing trend to 'freeze-all' and an 'elective FET' policy, considering the higher risk of LGA it is premature to apply this policy to all ART cycles. Additionally, some couples opt for the fresh ET approach to simplify their first treatment cycle. It is important for practitioners to facilitate individualized treatment according to the clinical situation. Lastly, we mentioned the potential effect of endometrial preparation methods on the endometrium in either fresh ET or FET. Regarding the adverse effect of ovarian stimulation on normal placentation, improved protocols (e.g. regimens that utilize minimal stimulation) could help alleviate negative effects. Even though our SFCT group used the minimal stimulation with CC alone, we found some adverse perinatal outcomes. Further studies comparing pregnancy and neonatal complications after fresh transfers in natural and CC-based minimal stimulation cycles are needed to confirm and clarify the association between ovarian stimulation and perinatal and maternal outcomes. A few clinics have adopted the use of CC for minimal stimulation. Therefore, our findings provide valuable knowledge that will improve the clinical outcomes of CC-based stimulation. It is crucial for practitioners to evaluate, and inform couples of, the possible benefits and risks involved with each ART treatment process.

Supplementary data
Supplementary data are available at Human Reproduction Open online.

Data availability
The datasets used and/or analysed during the present study are available from the corresponding author on reasonable request.
Authors' roles S.O. contributed to the interpretation and writing. K.E. contributed to the study design, data collection, analysis, interpretation and writing. N.K., H.H. and T.K. contributed to the data collection and interpretation of the data. K.Tak., K.Tan. and S.N. contributed to revising the manuscript. K.K. contributed to the study design, interpretation and writing. All authors read and approved the final manuscript.

Funding
This research did not receive any specific grants from funding agencies in the public, commercial, or not-for-profit sectors.  Nerve system, n (%) 11 ( Values are presented as n (%). SFCT, single fresh cleaved embryo transfer; SVBT, single vitrified-warmed blastocyst transfer; SVCT, single vitrified-warmed cleaved embryo transfer.
A P-value of <0.05 was considered statistically significant (Chi-squared test).