Impact of endometrial thickness alteration in response to progesterone administration on neonatal outcomes in frozen embryo transfer cycles: analysis of 6331 singleton newborns

doi:10.21203/rs.3.rs-5278874/v1

Background

To assess the impact of progesterone-induced changes in endometrial thickness (EMT) on singleton infant outcomes during frozen-thawed embryo transfer (FET) cycles.

Methods

This retrospective observational study included a total of 6331 singleton live births resulting from frozen-thawed Day 3 embryo transfer. Endometrial thickness (EMT) was assessed using transvaginal ultrasound one day prior to progesterone administration and on the day of frozen embryo transfer (FET) to examine any variations in EMT. The study population comprised 6331 women, who were categorized into three groups based on changes in EMT: the EMT increase group, the EMT decrease group, and the EMT stable group. The primary outcomes investigated in this study were mean birthweight, low birthweight (LBW), and small-for-gestational age (SGA). A multivariable linear regression analysis was performed to explore the association between changes in EMT following progesterone administration and newborns' birthweight.

Results

Despite any fluctuations in EMT levels on the day of FET compared to one day prior to progesterone administration, there were no statistically significant differences observed in the absolute mean birthweight of singletons among the three groups (3355.30 ± 502.69 vs. 3351.30 ± 474.79 vs. 3344.26 ± 514.54, P = 0.753). In comparison to the stable EMT group, the decreased EMT group had incidences of LBW and SGA in term infants at 1.1% (adjusted odds ratio [aOR]: 1.645, 95% confidence interval [CI]: 0.818–3.307) and 2.7% (aOR: 1.141, 95% CI:0.783–1.662), respectively; however, there was no significant association between the increased EMT group and risks of LBW (aOR: 1.310, 95% CI:0.723–2.375) or SGA (aOR:0.912, 95% CI:0.660–1.261). The multiple linear regression analysis revealed that both gestational age and infant gender exerted significant influences on singleton birthweight, while any alteration in endometrial thickness subsequent to progesterone administration did not yield a statistically significant impact on singleton birthweight.

Conclusions

The extent of EMT may exhibit variability, either increasing, decreasing, or remaining stable on the day of frozen embryo transfer (FET) compared to one day prior to progesterone administration. However, it is important to note that changes in EMT following progesterone administration do not demonstrate an independent association with adverse perinatal outcomes in term infants during FET cycles.

Birthweight

frozen-thawed embryo transfer

endometrial thickness change

Neonatal outcomes

Since the advent of the first child conceived through in vitro fertilization (IVF) in 1978, there has been a significant increase in the number of children born worldwide using assisted reproductive technology (ART), with delivery rates steadily rising each year. The health implications associated with these ART-conceived children have garnered widespread public concern. Generally, the majority of offspring resulting from IVF exhibit good overall health status. However, pregnancies resulting from IVF are associated with an elevated risk of adverse perinatal outcomes, including preterm birth (PTB), low birthweight (LBW), and small for gestational age (SGA)(1, 2). The etiology of these inferior outcomes in ART singletons is multifactorial. Pinborg A et al. discovered that subfertility constitutes a significant risk factor for adverse perinatal outcomes in ART singletons. Furthermore, they observed that even within the same mother, an ART singleton exhibits poorer outcomes compared to its non-ART sibling. Consequently, they postulated that factors associated with hormone stimulation and/or IVF techniques themselves may also contribute (e.g., embryo culture media and duration of culture, as well as embryo cryopreservation)(2). Emerging evidence further suggests that various factors associated with IVF procedures, such as patient characteristics, ovarian stimulation protocols, insemination techniques, cryopreservation methods, and the embryo culture medium employed, may also contribute to these suboptimal outcomes(3–8).

The assessment of birthweight in the ART population has been a frequent practice. Birthweight serves as a crucial parameter for evaluating neonatal health, as it is closely linked to mortality risk within the first year, developmental challenges during childhood, and susceptibility to various diseases in adulthood(9). LBW, in relation to the duration of pregnancy and gestational age, has been found to be associated with elevated rates of coronary heart disease as well as its related conditions including stroke, hypertension, and type 2 diabetes(10).

The measurement of endometrial thickness (EMT) through transvaginal ultrasound (TVU) has gained widespread acceptance as a prognostic indicator for endometrial receptivity; however, the findings remain subject to controversy (11–18). In the majority of previous studies, monitoring of endometrial thickness occurred during ovarian stimulation in fresh embryo transfer cycles or during the proliferation phase prior to progesterone administration in frozen-thawed embryo transfer cycles. A plethora of evidence has consistently demonstrated that a reduction in peak endometrial thickness is significantly associated with diminished chances of pregnancy in both fresh and frozen-thawed embryo transfer (FET) cycles(11, 19–22). Barker et. al conducted an evaluation on the correlation between endometrial thickness during the luteal phase (specifically on the day of embryo transfer) and pregnancy outcomes in the oocyte donation model(18). The dynamic nature of endometrial physiology and the differentiation between the follicular and luteal phases in both natural menstrual cycles and IVF treatment are widely acknowledged(23–25). The endometrial pattern undergoes characteristic changes, transitioning from pattern A (a triple-line pattern comprising a central hyper-echoic line surrounded by two hypoechoic layers) to pattern B (an intermediate isoechogenic pattern with similar reflectivity as the surrounding myometrium and a poorly defined central echogenic line), and finally to pattern C (a homogeneous, hyperechogenic endometrium). Numerous published studies have examined the correlation between endometrial patterns and outcomes of assisted reproductive techniques, yielding conflicting conclusions(25–29).

To date, limited research has been conducted to investigate the potential correlation between changes in endometrial thickness in response to progesterone and neonatal outcomes. This study aims to analyze the variation in endometrial thickness (from the day of embryo transfer to the day prior to progesterone administration) and explore its impact on neonatal outcomes within FET cycles.

Study design and patients

This retrospective study was conducted at the Department of Assisted Reproduction, Ninth People's Hospital of Shanghai Jiao Tong University School of Medicine (Shanghai, China), involving a cohort of 6331 women who underwent frozen embryo transfer (FET) cycles between January 2010 and December 2019 in our center. The inclusion criteria encompassed women aged ≤ 40 years with a body mass index (BMI) < 30 kg/m² who received FET cycles in our center and had embryo transfer on day 3. Exclusion criteria comprised women with uterine malformation, endometriosis, adenomyosis, as well as endometrial polyps or submucosal myomas identified through hysteroscopy that could potentially impact embryo implantation. This observational study obtained approval from the Ethics Committee (Institutional Review Board) of Shanghai Ninth People's Hospital without any intervention. This study adhered to the principles outlined in the Helsinki Declaration.

Treatment protocol

Endometrial preparation for frozen embryo transfer (FET) involved both artificial cycles (estrogen-progesterone cycle, EP) and natural cycles (NCs). In the NC group, all patients underwent ultrasonic evaluation starting from day 10 of their menstrual cycle. The endometrial thickness (EMT) and mean diameter of the dominant follicle were assessed by the same physician. Once the diameter of the dominant follicle reached approximately 18mm and EMT reached 7mm, a blood sample was collected to measure progesterone, estradiol (E₂), and luteinizing hormone (LH) levels. If the E2 level exceeded 150 pg/mL while progesterone remained below 1 ng/mL, one of two procedures was conducted based on serum LH values: if LH was less than 20 mIU/mL, a bolus dose of 5,000 IU human chorionic gonadotropin (hCG) was administered on the same night; three days later, progesterone supplementation commenced. Finally, five days after hCG injection, cleavage-stage embryos were transferred. In cases where the serum LH concentration was ≥ 20 mIU/mL, hCG administration took place on the same afternoon followed by progesterone administration after a two-day interval. The transfer of cleavage-stage embryos occurred four days later. The evaluation of endometrial thickness (EMT) on the day of hCG administration was documented using transvaginal ultrasound imaging.

For EP cycles, patients initiated sequential oral administration of micronized estradiol (8mg/day) on cycle day 3. EMT ultrasound measurement was conducted 12–14 days after the initiation of micronized estradiol. Progesterone supplementation commenced when EMT reached 7mm. Patients initiated oral administration of yellow Fematon tablets, twice daily at a dosage of two tablets, each containing 2 mg micronized estradiol and 10 mg dydrogesterone (Abbott Healthcare Products B.V.). Additionally, soft vaginal P capsules were administered twice daily at a dosage of 200 mg each (Laboratoires Besins International). EMT on the day prior to progesterone administration was also documented through transvaginal ultrasound. Embryos were warmed and transferred on day 3 following P supplementation, with a maximum limit of two embryos for transfer.

Please refer to previous studies for comprehensive information regarding the endometrial preparation process for frozen embryo transfer (FET) and the measurement method of endometrial thickness (EMT)(21, 30, 31).

EMT assessment

Based on variations in EMT measurements between the day of embryo transfer and the day prior to progesterone administration, the patients were categorized into three distinct groups: individuals exhibiting a decline in EMT levels (Group 1), those with stable EMT values (Group 2), and subjects displaying an increase in EMT levels (Group 3).

All ultrasound measurements were conducted by a senior technician utilizing the GE Voluson E8 (GE Healthcare, Austria). EMT was determined as the maximum distance between the endometrial-myometrial interface in the mid-sagittal plane, following our previously established methodology(21, 30, 31). In NCs, the assessment of endometrial thickness before progesterone administration commenced from the day of human chorionic gonadotropin (hCG) administration. Conversely, in EP cycles, this evaluation was conducted based on the final ultrasound prior to initiating progesterone administration. Additionally, for all patients, transvaginal ultrasound was performed on the morning of embryo transfer to ensure exclusion of any endometrial cavity fluid or other unfavorable conditions that could impede successful embryo transfer.

Outcome measures and definitions

Live birth was defined as the delivery of a viable neonate after 24 or more weeks of gestation. In frozen cycles, gestational age (GA) was calculated from the day of embryo transfer, which was designated as Day 17 due to cryopreservation on Day 3(32). Preterm birth and very preterm birth were defined as delivery occurring prior to 37 and 32 completed gestational weeks, respectively. Small for gestational age (SGA) and very small for gestational age (VSGA) were defined as birthweight falling below the 10th and 3rd percentiles for the corresponding gestational age, respectively. Large for gestational age (LGA) was defined as birth weight exceeding the 90th percentile for the given gestational age, while very large for gestational age (VLGA) was defined as birthweight surpassing the 97th percentile. Low birthweight (LBW), very low birthweight (VLBW), high birthweight (HBW), and very high birth weight (VHBW) were classified as having a birthweight less than 2500g, less than 1500g, greater than 4000g, and greater than 4500g, respectively. In addition, the Z-score was utilized to compute birthweight adjusted for gestational age and neonatal gender using the following formula: Z-score= (x − µ)/σ, where x represents the weight of an infant, µ denotes the mean birthweight for infants of the same sex and gestational age in the reference group, and σ signifies the standard deviation of the reference group. Birthweight percentiles as well as Z-score calculation were based on Chinese singleton newborns from a reference population stratified by neonatal gender and gestational age(33). Data regarding pregnancy and neonatal outcomes were extracted from electronic medical records.

Statistical analysis

The baseline characteristics and neonatal outcomes were compared using one-way analysis of variance for continuous data, while χ² or Fisher's exact tests were employed for categorical data, as deemed appropriate. Multivariable logistic regression analysis was conducted to identify any associations between changes in EMT and neonatal outcomes, while multiple linear regression was employed to investigate the independent impact of EMT change on birthweight. The following confounding factors were included in all multivariable analyses: maternal age and body mass index (BMI), paternal age, parity, duration of infertility, previous small-for-gestational-age (SGA) infants, number of embryos transferred, type of endometrial preparation, rank of frozen embryo transfer cycle, pregnancy complications such as gestational diabetes, hypertension, pre-eclampsia and placenta previa), newborn gender and gestational age at birth. The reference group for comparison was the stable EMT group. Statistical significance was defined as a p-value less than 0.05. All data were analyzed using the Statistical Package for the Social Sciences for Windows (SPSS version 25.0).

In this observational study, the final dataset comprised 6331 women who met the inclusion criteria, with no loss to follow-up. The baseline demographic and cycle characteristics of all patients are presented in Table 1 based on EMT changes. Among all FET cycles, EMT decreased in 1746 patients and increased in 2835 patients on the day of embryo transfer. Comparison among the three groups did not reveal any significant differences for maternal age and BMI, paternal age, infertility cause, previous SGA babies, or FET cycle rank. However, parity, infertility duration, FET endometrial preparation, and number of embryos transferred differed significantly across EMT change categories. Neonatal outcomes stratified by changes in EMT status are also presented in Table I and Table II. There were no significant differences observed across the three groups in terms of newborn gender, distribution of gestational age, and occurrence of pregnancy-related complications. Similarly, the mean birthweight did not significantly differ among the three groups (3355.30 ± 502.69g vs. 3351.30 ± 474.79 vs. 3344.26 ± 514.54, P = 0.753). However, when considering the mean birthweight Z-scores categorized by EMT changes, a significant variation was noted between groups: Group 3 exhibited lower Z-scores (0.34 ± 1.07) compared to Group 1 (0.43 ± 1.04) and Group 2 (0.38 ± 1.04), with statistical significance at P < 0.05.

Given the significance of gestational age at delivery as a crucial factor in determining newborn birthweight, we conducted a subgroup analysis by stratifying term (≥ 37 weeks) or preterm births (< 37 weeks). As depicted in Table III and Table IV, there was no statistically significant disparity observed in the mean birthweight of singletons among the three groups within the term (≥ 37 weeks) category. Compared to the stable group of EMT, the incidence rates of low birth weight (LBW) and small for gestational age (SGA) in term infants were 1.1% (adjusted odds ratio [aOR]: 1.645, 95% confidence interval [CI]: 0.818–3.307) and 2.7% (aOR: 1.141, 95% CI:0.783–1.662), respectively, in the decreased EMT group; however, there was no significant association between the increased EMT group and risks of LBW (aOR: 1.310, 95% CI:0.723–2.375) or SGA (aOR:0.912, 95% CI:0.660–1.261). In singletons delivered before < 37 weeks of gestation, Group 2 exhibited a statistically significant increase in mean birthweight compared to the other two groups (P = 0.025) for preterm infants. The incidence of LBW in preterm infants was highest in Group 3, however, there were no statistically significant differences observed among the three groups. It is noteworthy that Group 3 exhibited the highest occurrence of SGA preterm infants (aOR: 0.893, 95% CI: 0.090–8.898).

Multiple linear regression analyses were conducted to examine the association between change in EMT and birthweight (Table V). Even after adjusting for various potential confounders, any alteration in EMT following progesterone administration did not have a statistically significant impact on newborns' birthweight. Furthermore, newborn gender (P < 0.001) and gestational age (P < 0.001) independently served as predictors for birthweight.

In this retrospective analysis of 6331 live-born singletons, we present the initial evidence indicating that any alteration in EMT subsequent to progesterone administration does not exert a significant impact on the birthweight of newborns in frozen embryo transfer (FET) cycles. Neither a decrease nor an increase in EMT following progesterone administration is associated with elevated risks of PTB, LBW, SGA, or pregnancy-related complications in term infants. Upon adjusting for several potential confounders, both newborn gender (P < 0.001) and gestational age (P < 0.001) emerge as independent predictors of birthweight.

In recent decades, considerable attention has been devoted to investigating the impact of EMT on the success rates of in vitro fertilization (IVF). Numerous studies have consistently demonstrated that a thin endometrial lining exerts an adverse influence on pregnancy outcomes(11, 22, 34, 35). Limited research has been dedicated to exploring the potential association between EMT and neonatal birthweight. In the initial investigation conducted by Chung et al., they examined various factors influencing adverse perinatal outcomes in pregnancies following IVF treatment, revealing a twofold increased risk of adverse perinatal outcome in individuals with EMT ≤ 10 mm compared to those with EMT > 12 mm(OR: 2.04; 95% CI: 1.09–3.83)(36). Additionally, a significant protective effect on infant outcome was observed with an increase in endometrial thickness (OR: 0.89; 95% CI: 0.80–0.99). Specifically, for every millimeter decrease in endometrial thickness, there was a relative increase of 12% (1/0.89 = 1.12) in the risk of adverse perinatal outcomes(36). However, no statistically significant difference in risk was observed when the analysis was limited to singletons (aOR: 1.66, 95% CI: 0.73–3.81)(36). Another study revealed a positive correlation between EMT administered on the day of HCG trigger and neonatal birthweight, particularly in pregnancies complicated by obstetric factors(37). The study also revealed that the increase in EMT no longer exhibited a significant association with birth weight in uncomplicated pregnancies(37). Oron et al. demonstrated that an endometrial thickness of less than 7.5 mm was associated with a significantly increased risk for adverse obstetric outcomes (adjusted OR: 1.53; 95% CI: 1.03–2.42; P = 0.04). However, no significant difference in singleton live birth weight was observed(16). Du et al. demonstrated that an EMT ≤ 7.5 mm was significantly associated with an elevated risk of LBW in 2,847 singletons resulting from fresh cycles (38). In a recent large-scale retrospective study involving 5,220 singleton newborns, it was observed that individuals with an endometrial thickness (EMT) of less than 8 mm had an adjusted odds ratio (aOR) of 1.57 (95% CI: 1.09–2.26) for low birth weight (LBW) in frozen embryo transfer (FET) cycles(39). This finding is consistent with previous research conducted at our center (21, 40).

In most of the aforementioned studies, EMT was assessed during the proliferative phase prior to progesterone administration. However, it is important to note that endometrial development exhibits distinct physiological characteristics between the follicular and luteal phases. During the follicular phase, estrogen exerts its influence on the endometrium, promoting EMT and facilitating the linear growth of both endometrial glands and blood vessels(23). Under the influence of progesterone, endometrial proliferation ceases within 2–3 days after ovulation(23). Limited research has been conducted on the association between changes in EMT following progesterone administration and pregnancy as well as neonatal outcomes.

Although a limited number of studies have assessed the changes in EMT during IVF stimulation(24, 25, 27, 41–43), their findings exhibit some conflicting conclusions. The prognostic utility of the EMT change, occurring between the third day of gonadotrophin stimulation and the day of hCG administration, in predicting pregnancy occurrence was reported to be insignificant by Zhao et al. Furthermore, their findings indicated that a combined assessment of EMT change and type failed to accurately predict clinical outcomes(27). However, unfortunately, they neglected to investigate the correlation between changes in EMT following progesterone administration and the subsequent pregnancy and neonatal outcomes. Haas, J. et al. investigated the disparity in EMT between the late estrogen phase and the day of embryo transfer in 274 FET cycles, revealing a robust inverse correlation of high significance between pregnancy outcomes and changes in EMT(41). However, there were several limitations in their study. Firstly, the EMT was assessed using different measurement methods: transabdominal ultrasound on the day of embryo transfer and vaginal ultrasound at the end of the estrogen phase. Secondly, only women undergoing the hormonal preparation protocol for the FET cycle were included. Lastly, they solely focused on evaluating ongoing pregnancy rates in the FET cycle and did not investigate any potential association between changes in EMT after progesterone administration and neonatal outcomes. Another study reported that an increased endometrial thickness following progesterone administration was associated with improved pregnancy outcomes(42). However, the relationship between changes in EMT after progesterone administration and neonatal outcomes was not investigated. Our previous study demonstrated that irrespective of any changes in epithelial-mesenchymal transition (EMT) following progesterone administration, there was no significant impact on the clinical pregnancy rate (CPR) and live birth rate during the frozen embryo transfer (FET) cycle(31). However, our previous investigation did not explore the association between EMT alterations after progesterone administration and neonatal outcomes.

The present study, aimed at addressing the limitations of previous investigations, examined the precise impact of progesterone administration on EMT change and its association with neonatal outcomes in FET cycles. Our findings, based on a cohort of over 6000 singleton newborns conceived through FET cycles, unequivocally demonstrate that any alteration in EMT following progesterone administration does not exert a significant impact on the outcomes of term newborns in FET cycles. In this study, results from multiple linear regression analysis revealed that both newborn gender and gestational age independently predicted birthweight, which is consistent with existing literature(21, 44, 45). Male neonates have been predicted to weigh 130g higher than female neonates. Each weekly increase in gestational age has been associated with a 172g rise in birthweight. To account for potential bias related to infant gender and gestational age, Z-scores were calculated and compared across the three groups, revealing consistently lower scores in Group 3. In our cohort, significant differences were observed among the three groups regarding baseline and cycle characteristics such as parity, infertility duration, FET endometrial preparation, and number of embryos transferred. However, based on the linear regression model analysis, these aforementioned confounders did not exert any influence on neonatal birthweight. Moreover, considering that pregnancy outcomes may impact birthweight, we conducted additional analyses both including and excluding this factor; nevertheless, consistent results were obtained.

Several limitations were encountered in this study. Despite our meticulous review of the database using strict inclusion and exclusion criteria, the retrospective design constrained our findings. Additionally, to mitigate potential confounding effects of culture duration on FET outcomes, we focused exclusively on neonatal outcomes following day 3 embryo transfers(46). A major strength of this study was the use of data from a single center, ensuring consistency in practice. Endometrial preparation protocols were highly consistent throughout the study period, and endometrial thickness measurements were conducted by the same doctor at our center. While there may have been some bias due to inter-observer variability or changes over time in measurement methods, we accounted for potential confounders that could have impacted our findings.

The present large-scale single-center study provides valuable insights into the impact of progesterone administration-induced EMT changes on pregnancy outcomes in frozen embryo transfer (FET) cycles. Our findings indicate that any alterations in EMT following progesterone administration do not significantly affect the outcomes of term newborns in FET cycles. This novel discovery offers reassuring information for patients undergoing FET with EMT changes due to progesterone administration, regarding the health of their neonates. However, further prospective cohort studies with extended follow-up periods are warranted to validate these conclusions.

EMT

Endometrial Thickness

FET

Frozen-thawed Embryo Transfer

IVF

In vitro Fertilization

ART

Assisted Reproductive Technology

SGA

Small Gestational Age

VSGA

Very small for Gestational Age

LGA

Large for Gestational Age

VLGA

Very Large for Gestational Age

LBW

Low Birthweight

VLBW

Very Low Birthweight

HBW

High Birthweight

VHBW

Very High Birthweight

PTB

Preterm Birth

Ethics approval and consent to participate

This observational study obtained approval from the Ethics Committee (Institutional Review Board) of Shanghai Ninth People's Hospital without any intervention. This study adhered to the principles outlined in the Helsinki Declaration. Written informed consent was waived due to the retrospective study.

Consent for publication

Not applicable.

Availability of data and materials

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Competing interests

The authors declare no competing interests.

Funding

This work was supported by the National Natural Science Foundation of China (grant numbers：82271732, 82001502) and Shanghai Science and Technology Innovation Fund (grant numbers: 22Y21900800).

Author Contributions Statement

KYP and CQJ provided overall supervision for the study, including overseeing procedures, conception, design, and completion. YS, ZYW, GHY and LYL were responsible for data collection. YJ, ZJ, and DT contributed to data analysis and manuscript drafting. YJ, ZJ, and DT made equal contributions to this article. All authors participated in the final interpretation of study data and revisions of the manuscript.

Acknowledgments

The authors express their gratitude to the nurses and laboratory staff of the Department of Assisted Reproduction for their invaluable contributions to this research. Additionally, the authors extend their appreciation to the infertile couples who willingly participated in this study.

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Nelissen EC, Van Montfoort AP, Coonen E, Derhaag JG, Geraedts JP, Smits LJ, et al. Further evidence that culture media affect perinatal outcome: findings after transfer of fresh and cryopreserved embryos. Hum Reprod. 2012;27(7):1966–76.
Q JZYWHLXM. C, Y F, Effect of in vitro culture period on birth weight after vitrified-warmed transfer cycles: analysis of 4,201 singleton newborns. 2019;111(1):97–104.

Table I Baseline demographics, cycle characteristics and perinatal outcomes according to endometrial thickness change.

	Group1 (n=1746)	Group2 (n=1750)	Group3 (n=2835)	P value
Maternal age (y)	31.82±3.87	31.66±3.76	31.66±3.74	0.329^a
Maternal BMI (kg/m²)	21.58±2.94	21.53±2.85	21.54±3.01	0.854^a
Paternal age (years)	33.92±5.03	33.79±4.85	33.71±4.93	0.361^a
Infertility cause				0.142^b
Female	977(56.0)	1006(57.5)	1540(54.3)
Male	252(14.4)	263(15.0)	428(15.1)
Mixed	367(21.0)	316(18.1)	608(21.4)
Unexplained	150(8.6)	165(9.4)	259(9.1)
Parity, n (%)				0.000^b
First	874(50.1)	941(53.8)	1593(56.2)
High order	872(49.9)	809(46.2)	1242(43.8)
Infertility duration (y)	3.64±2.67	4.03±2.78	3.65±2.65	0.000^a
FET cycle rank, n (%)				0.185^b
First	1097(62.8)	1130(64.6)	1857(65.5)
High order	649(37.2)	620(35.4)	978(34.5)
FET endometrial preparation, n (%)				0.000^b
Ovulatory cycle	1110(63.6)	1156(66.1)	1962(69.2)
Artificial cycle	636(36.4)	594(33.9)	873(30.8)
Number of embryos transferred, n (%)				0.029^b
1	150(8.6)	164(9.4)	205(7.2)
2	1596(91.4)	1586(90.6)	2630(92.8)
Previous SGA newborns, n (%)	8(0.5)	3(0.2)	11(0.4)	0.314^c
Newborn gender, n (%)				0.599^b
Female	852(48.8)	866(49.5)	1360(48.0)
Male	894(51.2)	884(50.5)	1475(52.0)
Gestational age (weeks) , n (%)				0.367^b
<37	129(7.4)	110(6.3)	184(6.5)
≥37	1617(92.6)	1640(93.7)	2651(93.5)
Pregnancy-related complications	156(8.9)	173(9.9)	238(8.4)	0.229^b
Birth weight (g)	3355.30±502.69	3351.30±474.79	3344.26±514.54	0.753^a
Z-score	0.43±1.04	0.38±1.04	0.34±1.07	0.023^a

SGA - small for gestational age. ^aOne-way ANOVA. Values are mean±SD. ^bPearson chi-square test. Values are number (percentage). ^cFisher’s exact test. Values are number (percentage). Z-score: EMT increase group vs EMT decrease group (P=0.007).

Table II Birth weights and birth weights categories of live born singletons

	Group1 (n=1746)	Group2 (n=1750)	Group3 (n=2835)	P value
Newborn gender, n (%)				0.599^b
Female	852(48.8)	866(49.5)	1360(48.0)
Male	894(51.2)	884(50.5)	1475(52.0)
Mean birthweight (g)	3355.30±502.69	3351.30±474.79	3344.26±514.54	0.753^a
Normal weight, 2500–4000 g, n (%)	1513(86.7)	1550(88.6)	2476(87.3)	0.070^b
LBW (< 2500 g), n (%)	64(3.7)	63(3.6)	105(3.7)
HBW (> 4000 g), n (%)	145(8.3)	110(6.3)	201(7.1)
VHBW (> 4500 g), n (%)	15(0.9)	23(1.3)	29(1.0)
VLBW (< 1500 g), n (%)	9(0.5)	4(0.2)	24(0.8)
Very small for gestational age (<3rd percentile), n (%)	26(1.5)	33(1.9)	49(1.7)	0.133^b
Small for gestational age (<10th percentile), n (%)	47(2.7)	55(3.1)	111(3.9)
Normal gestational age (≥10th percentile, ≤90th percentile), n (%)	1327(76.0)	1364(77.9)	2177(76.8)
Large for gestational age (>90th percentile)	242(13.9)	212(12.1)	334(11.8)
Very large for gestational age (>97th percentile), n (%)	104(6.0)	86(4.9)	164(5.8)

SGA - small for gestational age. LBW, low birth weight; VLBW, very low birth weight; HBW, high birth weight; VHBW, very high birth weight

^aOne-way ANOVA. Values are mean±SD. ^bPearson chi-square test. Values are number (percentage).

Values are number (percentage). Z-score: EMT increase group vs EMT decrease group (P=0.007).

Table III Birth weights and birth weights categories of live born singletons by gestational age.

	Group1	Group2	Group3	P value
Gestational age≥37 weeks
N(%)	1617	1640	2651
Newborn gender, n (%)				0.505^b
Female	788(48.7)	817(49.8)	1272(48.0)
Male	829(51.3)	823(50.2)	1379(52.0)
Mean birthweight (g)	3423.65±424.872	3400.65±418.856	3410.05±423.527	0.294^a
Normal weight, 2500–4000 g, n (%)	1439(89.0)	1485(90.5)	2390(90.2)	0.222^b
LBW (< 2500 g), n (%)	18(1.1)	24(1.5)	32(1.2)
HBW (> 4000 g), n (%)	145(9.0)	109(6.6)	200(7.5)
VHBW (> 4500 g), n (%)	15(0.9)	22(1.3)	29(1.1)
Very small for gestational age (<3rd percentile), n (%)	23(1.4)	28(1.7)	42(1.6)	0.807^b
Small for gestational age (<10th percentile), n (%)	44(2.7)	49(3.0)	91(3.4)	0.402^b
Large for gestational age (>90th percentile), n (%)	230(14.2)	204(12.4)	323(12.2)	0.096^b
Very large for gestational age (>97th percentile), n (%)	103(6.4)	80(4.9)	147(5.5)	0.178^b
Gestational age < 37 weeks
N	129	110	184
Newborn gender, n (%)				0.732^b
Female	64(49.6)	49(44.5)	88(47.8)
Male	65(50.4)	61(55.5)	96(52.2)
Mean birthweight (g)	2498.57±605.965	2615.59±629.418	2396.39±732.408	0.025^a
Normal weight, 2500–4000 g, n (%)	74(57.4)	65(59.1)	86(46.7)	0.067^b
VLBW (< 1500 g), n (%)	9(7.0)	4(3.6)	24(13.0)
LBW (< 2500 g), n (%)	46(35.7)	39(35.5)	73(39.7)
HBW (> 4000 g), n (%)	0(0.0)	1(0.9)	1(0.5)
VHBW (> 4500 g), n (%)	0(0.0)	1(0.9)	0(0.0)
Very small for gestational age (<3rd percentile), n (%)	3(2.3)	5(4.5)	7(3.8)	0.632^c
Small for gestational age (<10th percentile), n (%)	6(4.7)	11(10)	27(14.7)	0.017^c

LBW, low birth weight; VLBW, very low birth weight; HBW, high birth weight; VHBW, very high birth weight

^aOne-way ANOVA. Values are mean±SD.

^bPearson chi-square test. Values are number (percentage). ^cFisher’s exact test. Values are number (percentage).

Table IV Crude and adjusted ORs for birth weights and birthweights categories in singleton births by gestational age.

	Group1	Group2	Group3
Gestational age≥37 weeks
Very small for gestational age (<3rd percentile)
Crude OR (95% CI)	1.204(0.690-2.099)	Reference	1.079(0.666-1.748)
AOR (95% CI)	1.368(0.732-2.554)	Reference	1.064(0.619-1.827)
Small for gestational age (<10th percentile)
Crude OR (95% CI)	1.140(0.815-1.593)	Reference	0.933(0.700-1.244)
AOR (95% CI)	1.141(0.783-1.662)	Reference	0.912(0.660-1.261)
Large for gestational age (>90th percentile)
Crude OR (95% CI)	0.808(0.677-0.963)	Reference	0.972(0.826-1.143)
AOR (95% CI)	0.822(0.676-1.000)	Reference	0.950(0.793-1.138)
Very large for gestational age (>97th percentile)
Crude OR (95% CI)	0.754(0.558-1.018)	Reference	0.766(0.550-1.067)
AOR (95% CI)	0.874(0.661-1.155)	Reference	0.848(0.624-1.154)
Low birth weight(<2500g)
Crude OR (95% CI)	1.319(0.713-2.440)	Reference	1.216(0.713-2.071)
AOR (95% CI)	1.645(0.818-3.307)	Reference	1.310(0.723-2.375)
High birth weight (>4000g)
Crude OR (95% CI)	0.791(0.621-1.007)	Reference	0.918(0.734-1.149)
AOR (95% CI)	0.784(0.600-1.025)	Reference	0.872(0.680-1.117)
Very high birth weight (>4500g)
Crude OR (95% CI)	1.452(0.751-2.809)	Reference	1.229(0.704-2.147)
AOR (95% CI)	1.679(0.805-3.502)	Reference	1.200(0.653-2.204)
Gestational age < 37 weeks
Small for gestational age (<10th percentile)
Crude OR (95% CI)	2.278(0.814-6.376)	Reference	0.646(0.307-1.361)
AOR (95% CI)	9.992(0.587-17.227)	Reference	0.893(0.090-8.898)
Low birth weight(<2500g)
Crude OR (95% CI)	0.864(0.514-1.450)	Reference	0.576(0.356-0.930)
AOR (95% CI)	0.895(0.195-4.104)	Reference	0.087(0.019-0.388)

CI: confidence interval, OR: odds ratio, AOR: adjusted odds ratio

Analyses were adjusted for maternal age and BMI, paternal age, infertility cause, parity, infertility duration, FET cycle rank, the type of endometrial preparation, number of embryos transferred, newborn sex, previous SGA newborns.

Table V Results of multiple linear regression analysis on the birthweight of live-born singletons

Model	Non-standardized coefficients		Standardized coefficients	t	P value
Model	B	Std. error	Beta	t	P value
Constant	-3335.343	146.076		-22.833	0.000
FET cycle rank, High order (vs First)	6.485	10.922	0.006	0.594	0.553
Maternal age (y)	0.115	1.986	0.001	0.058	0.954
Paternal age (y)	0.179	1.484	0.002	0.121	0.904
Duration of infertility(y)	-3.046	2.054	-0.016	-1.483	0.138
Parity, High order（vs First )	-8.629	10.794	-0.009	-0.799	0.424
Maternal BMI (kg/m²)	-0.655	1.767	-0.004	-0.370	0.711
FET endometrial preparation, Artificial cycle (vs Ovulatory cycle)	-12.873	11.218	-0.012	-1.148	0.251
EMT: before progesterone administration (mm)	9.462	4.925	0.041	1.921	0.055
EMT: on the day of FET (mm)	-10.407	4.545	-0.051	-2.290	0.022
EMT change
Remain stable(reference)
Increased	-22.158	15.251	-0.022	-1.453	0.146
Decreased	1.561	15.245	0.001	0.102	0.918
No. of embryo transferred	5.084	18.880	0.003	0.269	0.788
Pregnancy-related complications, yes (vs no)	-23.406	19.387	-0.013	-1.207	0.227
Infant gender boys (vs girls)	130.122	10.362	0.130	12.558	0.000
Gestational age (weeks)	172.641	3.275	0.560	52.707	0.000

No competing interests reported.

Impact of endometrial thickness alteration in response to progesterone administration on neonatal outcomes in frozen embryo transfer cycles: analysis of 6331 singleton newborns

Status:

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Abstract

Background

Methods

Results

Conclusions

Background

Methods

Study design and patients

Treatment protocol

EMT assessment

Outcome measures and definitions

Statistical analysis

Results

Discussion

Conclusions and Future Prospects

Abbreviations

Declarations

References

Tables

Additional Declarations

Status:

Version 1