PubMed, Cochrane, Embase, and Web of Science electronic databases were searched for studies published in English up to March 2023, using Medical Subject Headings [MeSH] and keywords related to the study, no restriction on the year of publication. The database’s specific strategy was (‘Embryo Transfer’ OR ‘Embryo Transfers’ OR ‘Blastocyst Transfer’ OR ‘Blastocyst Transfers’ OR ‘Fertilizations in Vitro’ OR ‘IVF’ OR ‘In Vitro Fertilization’ OR ‘In Vitro Fertilizations’ OR ‘ICSI’ OR ‘Intracytoplasmic Sperm Injection’ OR ‘Intracytoplasmic Sperm Injections’) AND (‘Endometrial compaction’ OR ‘Endometrial thickness change’ OR ‘change, endometrial thickness’ OR ‘Endometrial thickness decreased’ OR ‘Endometrial thickness compacted’) AND (‘Pregnancy Outcome’ OR ‘Pregnancy Outcomes’ OR ‘Clinical outcomes’ OR ‘Live birth’ OR ‘Clinical pregnancy’ OR ‘Ongoing pregnancy’). Additionally, references of all included articles were screened, and literature retrieval was finalized. The entire retrieved literature was screened by two independent reviewers, and the included literature was collated using EndNote X9.3.3. Any articles with uncertainties were resolved through discussion and, if necessary, group discussion with a third investigator to reach a final consensus.

Studies that met the following criteria were considered eligible for systematic review: (1) The study was an observational cohort study; (2) The study population was all women who underwent IVF; (3) Report on the relationship between EMT changes and pregnancy outcomes; (4) Studies had at least one of the following outcomes: clinical pregnancy, ongoing pregnancy, live birth, and spontaneous abortion; (5) Published in English.

Studies with the following conditions will be excluded: (1) No clinical outcome or no available data; (2) Overview, conference abstracts, case reports, and case series; (3) Articles without complete research strategy.

Clinical pregnancy is defined as one or more gestational sacs seen in the uterine cavity detected by ultrasound. Ongoing pregnancy is defined as transvaginal ultrasound showing fetal heart activity at 12 weeks of gestation or later. Live birth is defined as surviving infants delivered at ≥ 24 weeks of gestation. Spontaneous abortion is considered to be the pregnancy loss of one or more gestational sacs previously observed before 24-week gestation.

According to the PRISMA guidelines, the following data were extracted independently by two authors from each eligible study: first author’s surname, publication year, country, study duration, study design, number of cycles, endometrial preparation protocol, endometrial measurement method, number of embryos transferred, embryo development stage, and pregnancy outcomes. Data from different subgroups within the same study were also extracted for possible synthesis. Disagreements between the two reviewers were resolved in the same manner as described above.

The quality of the included cohort studies was assessed independently by two reviewers using the Newcastle-Ottawa Scale (NOS), which assigns a maximum of nine points to each study based on three broad dimensions: selection (4 points), comparability (2 points), and outcome (3 points). Scores ranged from 0 to 9 points, with studies ≥ 7 points being considered high quality, 4-6 points indicating moderate quality, and < 4 points indicating low quality. Disagreements between the two reviewers were resolved in the same manner as described above.

Pregnancy outcomes were counted as dichotomous variables and expressed as RR with a 95% CI. The degree of heterogeneity was quantified by I² statistics, when I^2 = 0%, considered no heterogeneity between studies, when I² < 25%, considered mild heterogeneity between studies, when 25% ≤ I² < 50%, considered moderate heterogeneity between studies, when I² ≥ 50%, considered high heterogeneity between studies (16). According to heterogeneity, the results were calculated using a random effects model (Der Simonian-Laird) or fixed-effects model (Mantel-Haenszel). All statistical analyses were performed using Stata 17.0.

In total, 298 studies were retrieved and reviewed. 217 studies were retained after removing duplicates, with each title and abstract being evaluated by two reviewers. Subsequently, 28 full-text articles were screened for a full review, and 10 articles were excluded for the reasons outlined in the flowchart, leaving 18 studies (12–14, 17–31) that met the inclusion criteria. The PRISMA scheme for searching and selecting literature is shown in Figure 1 .

Table 1 shows the basic characteristics of the 18 studies, including first author’s surname, publication year, country, study duration, study design, number of cycles, endometrial preparation protocol, endometrial measurement method, number of embryos transferred, embryo development stage, outcome indicators, and quality score. Seven prospective studies and 11 retrospective studies were eligible for meta-analysis. Among the 18 included studies, 3 examined fresh ET, 15 examined FET. A total of 16,164 cycles were included, including 3022 fresh ET cycles, and 13,142 FET cycles (including 6481 hormone replacement therapy [HRT] cycles and 6661 natural cycles [NC] or modified natural cycles [mNC]).

In five studies of FET-HRT (13, 14, 22, 23, 31) and one study with fresh oocyte donation (26), ECR was defined as the rate of change in EMT between the day of progesterone administration and the day of ET. In 7 other FET-HRT studies (12, 18–20, 24, 25, 28), ECR was defined as the rate of change at which EMT changed from the end of estrogen-only phase to the ET or the day before the ET. In 2 fresh ETs (21, 27) and 2 FET-mNC (17, 29) studies, ECR refers to the rate at which EMT changes from hCG triggered to ET. Youngster et al. (30) described ECR as the rate of change from the day of ovulation to the day of ET.

As shown in Figure 2 , a total of 15 studies documented CPR, and the pooled results indicated that endometrial compaction was not associated with CPR (RR [95% CI] = 0.98 [0.90,1.08]; I^2 = 69.76%), but there was high heterogeneity across the studies. Subgroup analysis revealed no statistically significant difference in CPR between endometrial compaction and non-compaction groups in the Fresh ET subgroup (RR [95% CI] = 0.96 [0.86,1.07]; I^2 = 0.00%) or the FET subgroup (RR [95% CI] = 1.00 [0.89,1.11]; I^2 = 73.85%). Moreover, we also performed subgroup analysis according to different preparation protocols, study design, measurement methods, and ECR. As shown in Table 2 , there was no correlation between endometrial compaction and CPR.

We analyzed 9 studies that met the requirements and the combined results are shown in Figure 3 , where endometrial compaction was not associated with the OPR (RR [95% CI] = 1.18 [0.95,1.47]; I^2 = 78.77%). There was high heterogeneity among studies, so we also performed a subgroup analysis with only one study in the Fresh ET group (RR [95% CI] = 0.96[0.84,1.11]; I^2 = 0.00%) and 8 studies in the FET group (RR [95% CI] = 1.25 [0.93,1.68]; I^2 = 80.64%), all of which showed no correlation between endometrial compaction and OPR.

Considering the high heterogeneity between studies, we conducted more subgroup analyses. In a subgroup analysis of different ultrasound measurement methods, the OPR of the endometrial compaction group in the ET cycle was significantly higher than that of the non-compaction group (RR [95% CI]=1.69 [1.26, 2.26]; I^2 = 29.27%), based on the combination of the first TVUS measurement and the second AUS measurement of EMT (see Figure 4 ). According to a subgroup analysis of different ECRs, the OPR was higher in the endometrial compaction group than in the non-compaction group (RR [95% CI] = 1.99 [1.61,2.47]; I^2 = 0.00%). In the other subgroups, similar results were not observed, as shown in Figure 5 . No statistical differences were found between the endometrial compaction and non-compaction groups in the subgroup analysis of other influencing factors, as shown in Table 3 .

As shown in Figure 6 , a total of 10 studies reported LBR using a fixed effects model (Mantel-Haenszel), and the combined results showed no statistically significant association between endometrial compaction and LBRs (RR [95% CI] = 0.97 [0.92,1.02]; I^2 = 0.00%). In subgroup analysis, the Fresh ET group (RR [95% CI] = 0.95 [0.85,1.07]; I^2 = 0.00%) and FET group (RR [95% CI] = 0.98 [0.92,1.04]; I^2 = 23.68%) showed no significant difference between the endometrial compaction and non-compaction.

In the subgroup analysis of TVUS + AUS, the endometrial compaction group showed a slight improvement in LBR (RR [95% CI]=1.27 [1.00,1.61]; I^2 = 62.28%). However, as shown in Figure 7 , no significant statistical difference in LBR was detected when TVUS was used for both measurements prior to ET (RR [95% CI] = 0.96 [0.91,1.02]; I^2 = 0.00%). As shown in Table 4 , there were no significant differences in pooled RRs of LBR after conducting other subgroups analyses.

As shown in Figure 8 , we analyzed all studies that met the criteria. Because of the low heterogeneity between studies, a fixed-effect model with the Mantel-Haenszel method was used. Combined results showed that endometrial compaction was not associated with spontaneous abortion (RR [95% CI] = 1.05 [0.89,1.23]; I^2 = 0.00%).

The funnel plot for each outcome was visually symmetric (see Supplementary Appendix 3-1 , 3-2 , 3-3 , 3-4 ). Furthermore, the regression-based Egger test did not show statistical significance (P = 0.0512, 0.120, 0.4295 and 0.0727 for clinical pregnancy, ongoing pregnancy, live birth and spontaneous abortion, respectively), suggesting that there was no significant publication bias in the studies that were included.