Table 1.
Main characteristics of included studies in the meta-analysis
| Study | Country | Cohort design | Duration | Population | Definition of immediate/delayed transfer | Sample size (cycle) | Age (years) | Ovarian stimulation protocol | Trigger agent | Endometrial preparation | Embryo stage | Outcomes |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| [13] | France | Retrospective, single-center | 2012–2015 | Women undergoing first FET after a freeze-all cycle | Within the first menstrual cycle/after one or more menstrual cycles following oocyte retrieval (cycle 1/≥ 2) | 474 | Mean, immediate, 33.7; delayed, 33.9 | GnRH antagonist, long agonist or short agonist | GnRH agonist or hCG | Artificial cycle | Blastocyst | CPR, LBR, PLR |
| [14] | China | Retrospective, single-center | 2014–2017 | Women undergoing first FET after a freeze-all cycle | Cycle 1/≥ 2 | 4404 | Mean, immediate, 31.6; delayed, 31.9 | GnRH antagonist or long agonist | hCG | Artificial or natural cycle | Cleavage, morula or blastocyst | CPR, LBR, PLR |
| [15] | Australia | Retrospective, multi-center | 2000–2014 | Women undergoing first FET after a failed fresh ET or a freeze-all cycle | 25–35/50–70 days from embryo cryopreservation to FET | 4994 | Median, immediate, 36.0; delayed, 35.5 | GnRH antagonist, long agonist or short agonist | hCG | Artificial or natural cycle | - | CPR, LBR, PLR |
| [16] | Israel | Retrospective, single-center | 2009–2016 | Women undergoing first FET after a failed fresh ET | ≤ 22/> 22 days from oocyte retrieval to the start of FET cycle | 198 | Mean, immediate, 32.8; delayed, 34.1 | GnRH antagonist, long agonist or short agonist | hCG | Natural or modified natural cycle | Cleavage or blastocyst | CPR, LBR |
| [17] | China | Retrospective, single-center | 2013–2016 | Women undergoing first FET after a freeze-all cycle | Cycle 1/≥ 2 | 2998 | Mean, immediate, 30.6; delayed, 30.9 | Short agonist or progestin-primed ovarian stimulation | GnRH agonist, hCG or both | Artificial or modified natural cycle | Cleavage or blastocyst | CPR, LBR, PLR |
| [18] | USA | Retrospective, single-center | 2013–2016 | Women undergoing first FET after a failed fresh ET or a freeze-all cycle | Cycle 1/≥ 2 | 344 | Mean, immediate, 33.7; delayed, 33.5 | GnRH antagonist or long agonist | GnRH agonist, hCG or both | Artificial or natural cycle | Blastocyst | CPR, LBR, PLR |
| [19] | Spain | Retrospective, single-center | 2012–2014 | Women undergoing first FET after a freeze-all cycle | Cycle 1/≥ 2 | 512 | Mean, immediate; 34.7; delayed, 35.3 | GnRH antagonist or long agonist | GnRH agonist or hCG | Artificial cycle | Cleavage or morula | CPR, LBR, PLR |
| [20] | Turkey | Retrospective, single-center | 2015–2016 | Women undergoing first FET after a freeze-all cycle | 32–46/≥47 days from oocyte retrieval to FET | 1121 | ≤ 42 | GnRH antagonist | GnRH agonist, hCG or both | Artificial cycle | Blastocyst | LBR |
| [21] | Belgium | Retrospective, single-center | 2010–2014 | Women undergoing first FET after a failed fresh ET | ≤ 22/> 22 days from oocyte retrieval to the start of FET cycle | 1183 | Mean, immediate, 32.4; delayed, 32.5 | GnRH antagonist | hCG | Artificial, natural or modified natural cycle | Cleavage or blastocyst | CPR, LBR |
| [22] |
Belgium/ Vietnam |
Retrospective, two-center | 2010–2015 | Women undergoing first FET after a freeze-all cycle | Cycle 1/≥ 2 | 333 | Mean, immediate, 30.9; delayed, 31.8 | GnRH antagonist | GnRH agonist | Artificial cycle | Cleavage or blastocyst | CPR, PLR |
| [23] | China | Retrospective, single-center | 2016–2018 | Women undergoing first FET after a freeze-all cycle | Cycle 1/≥ 2 | 1540 | Mean, immediate, 31.4; delayed, 31.0 | GnRH antagonist, long agonist, ultra-long agonist, short agonist, ultra-short agonist or mild stimulation | GnRH agonist or hCG | Artificial, natural or stimulated cycle | Cleavage | CPR, LBR, PLR |
| [24] | Israel | Retrospective, single-center | 2010–2015 | Women undergoing first FET after a failed fresh ET | < 50/≥ 50 days from oocyte retrieval to FET | 129 | Mean, immediate, 29.9; delayed, 29.6 | Long agonist | hCG | Artificial cycle | Cleavage or blastocyst | CPR, LBR, PLR |
CPR, clinical pregnancy rate; ET, embryo transfer; FET, frozen embryo transfer; GnRH, gonadotropin-releasing hormone; hCG, human chorionic gonadotropin; LBR, live birth rate; PLR, pregnancy loss rate