Criteria for considering studies for this review
Types of studies
Only truly randomized clinical trials will be eligible for inclusion. Quasi and pseudo-randomized clinical trials will be excluded. Cross-over trials will be included for completeness, but only the data from the first phase will be pooled in the meta-analysis because the design is not valid in the context of subfertility trials (Vail 2003).
Types of participants
All women undergoing IVF or ICSI will be eligible for inclusion.
Types of interventions
1) Trials comparing IVF cycles using only frozen-thawed embryo transfers until all frozen embryos are used versus IVF cycles using fresh and subsequent frozen-thawed embryo transfers until all frozen embryos are used.
2) Trials comparing the first frozen-thawed embryo transfer in IVF cycles using only frozen-thawed embryo transfers versus the first fresh embryo transfer in regular IVF cycles.
Types of outcome measures
Effectiveness: live birth rate per randomized woman, defined as the complete expulsion or extraction from its mother of a product of fertilization, irrespective of the duration of the pregnancy, which, after such separation, breathes or shows any other evidence of life, such as heart beat, umbilical cord pulsation, or deﬁnite movement of voluntary muscles, irrespective of whether the umbilical cord has been cut or the placenta is attached (Zegers-Hochschild 2009).
Adverse effect: ovarian hyperstimulation syndrome (OHSS) per woman.
Ongoing pregnancy rate, defined as the number of ongoing pregnancies per woman randomized (demonstrated by the presence of a gestational sac with fetal heart beat on ultrasound at ≥ 12 weeks of gestation).
Clinical pregnancy, defined as the number of clinical pregnancies per woman randomized (demonstrated by a pregnancy diagnosed by ultrasonographic visualization of one or more gestational sacs or deﬁnitive clinical signs of pregnancy). It includes ectopic pregnancy. Note: multiple gestational sacs are counted as one clinical pregnancy (Zegers-Hochschild 2009).
Time to pregnancy, defined as the time between start of treatment (first day of the last menstrual cycle) and clinical pregnancy.
Multiple pregnancy rate, defined as the number of multiple pregnancies per woman.
Miscarriage rate, defined as the number of miscarriages per woman.
Congenital disorders, defined as the number of congenital abnormalities at birth per all clinical pregnancies.
Pregnancy complications (including ectopic pregnancy, fetal growth disorders, preterm birth < 37 weeks, pregnancy induced hypertension (PIH), (pre-) eclampsia, HELLP (H: haemolysis; EL: elevated liver enzymes; and LP: low platelets in the blood)) per woman.
Birth weight of babies born per woman.
Search methods for identification of studies
We will search for all published and unpublished randomized controlled trials (RCTs) on cryopreservation of all embryos and transfer in a subsequent non-hyperstimulated cycle, without language restriction and in consultation with the Menstrual Disorders and Subfertility Group (MDSG) Trials Search Co-ordinator.
We will search the following electronic databases, trial registers, and websites:
Other electronic sources of trials will include:
The search strategies to be used in CENTRAL, MEDLINE, EMBASE, and PsycINFO are presented in the Appendices: Appendix 1; Appendix 2; Appendix 3; Appendix 4; Appendix 5
Searching other resources
In order to obtain additional relevant data, we will examine reference lists of eligible articles and contact the study authors where necessary. We will also handsearch relevant journals and conference abstracts that are not covered in the MDSG Register.
Data collection and analysis
Selection of studies
Two authors (KMW and SM) will screen the titles and abstracts retrieved by the search and retrieve the full texts of all potentially eligible studies. These two authors will independently examine these full text articles for compliance with the inclusion criteria and select studies eligible for inclusion in the review. We will correspond with study investigators, as required, to clarify study eligibility. Disagreements as to study eligibility will be resolved by discussion or by a third review author. We will document the selection process with a PRISMA flow chart.
Data extraction and management
Two review authors (KMW and MvW) will independently extract data from eligible studies using a data extraction form designed and pilot-tested by the authors. A third co-author (SR) will resolve any disagreements. Where studies have multiple publications the authors will collate multiple reports of the same study so that each study rather than each report is the unit of interest in the review; such studies will have a single study ID with multiple references.
We will correspond with study investigators for further data on methods or results, or both, as required
Assessment of risk of bias in included studies
Two review authors will independently assess the included studies for risk of bias in the following domains.
1. Sequence generation
A low risk of bias will be allocated if the investigators describe a random component in the sequence generation process such as:
2. Allocation concealment
A low risk of bias will be allocated if the participants and investigators enrolling participants cannot foresee assignment because one of the following, or an equivalent method, is used to conceal allocation:
A low risk of bias is allocated if blinding of participants, scientists, and clinicians or nurses has been ensured. However, in this study design it is ethically not possible to blind participants and clinicians. Lack of blinding may not increase the risk of bias if follow-up is complete and outcomes are unequivocal (live birth).
4. Completeness of outcome data
A low risk of bias will be allocated if there are no missing data, which means live birth rate and length of follow-up are stated, loss to follow-up is accounted for, and an intention-to-treat (ITT) analysis has been carried out.
5. Selective outcome reporting
A low risk of bias is allocated if all of the study's primary, secondary, and additional outcomes that are of interest in the review have been reported in a pre-specified way.
6. Other sources of bias
A low risk of bias is allocated if the study:
is free of commercial funding;
reports multiple pregnancy rate in the case of an embryo transfer policy of multiple embryos per treatment cycle;
has no other source of bias identified (e.g. imbalance in prognostic factors at baseline).
These domains will be assessed by two authors (KMW, SM) with any disagreements resolved by consensus or by contacting the third author (SR). All judgments will be fully described. The conclusions will be presented in the risk of bias figures and incorporated into the interpretation of review findings.
Measures of treatment effect
For dichotomous data (for example live birth rates), we will use the numbers of events in the control and intervention groups of each study to calculate Mantel-Haenszel odds ratios (ORs). For continuous data (for example birth weight), if all studies report exactly the same outcomes we will calculate mean difference (MDs) between treatment groups. If similar outcomes are reported on different scales (for example change in weight) we will calculate the standardised mean difference (SMD). We will reverse the direction of effect of individual studies, if required, to ensure consistency across trials. We will treat ordinal data as continuous data. We will present 95% confidence intervals (CI) for all outcomes. Where data to calculate ORs or MDs are not available, we will utilise the most detailed numerical data available that may facilitate similar analyses of included studies (for example test statistics, P values). We will compare the magnitude and direction of effect reported by studies with how they are presented in the review, taking account of legitimate differences.
Unit of analysis issues
The primary analysis of the review will be per woman or couple randomized. Reported data that does not allow valid analysis (for example per embryo transfer or per oocyte) will not be pooled with the data of the primary analysis. However, if possible, contact will be made with the authors of these studies for additional relevant data and these data will be separately extracted from the included trials for completeness.
Reported multiple live births will be counted as one live birth event.
Only first-phase data from cross-over trials will be included.
If studies report only 'per cycle' data, we will contact authors and request 'per woman' data.
Dealing with missing data
The data will be analysed on an ITT basis as far as possible and attempts will be made to obtain missing data from the original triallists. Where these are unobtainable, imputation of individual values will be undertaken for the primary outcomes only. Any imputation undertaken will be subjected to sensitivity analysis. Live births will be assumed not to have occurred in participants without a reported outcome. For other outcomes, only the available data will be analysed. If studies report sufficient detail to calculate MDs but provide no information on associated standard deviations (SD), the outcome will be assumed to have a SD equal to the highest SD from other studies within the same analysis.
Assessment of heterogeneity
Heterogeneity will be considered when the clinical and methodological characteristics of the included studies are sufficiently similar for a meta-analysis to provide a clinically meaningful summary. Statistical analyses will be performed in accordance with the guidelines for statistical analysis developed by The Cochrane Collaboration (Higgins 2003; Higgins 2011). Heterogeneity between the results of different studies will be assessed by the I2 statistic, with an I2 value greater than 50% judged to indicate substantial heterogeneity (Higgins 2003; Higgins 2011).
Assessment of reporting biases
The authors aim to minimize the potential impact of publication and reporting biases by performing a comprehensive search for eligible studies and looking for duplication of data. If 10 or more studies are included in an analysis, a funnel plot will be used to investigate the possibility of small study effects (a tendency for the intervention to have a bigger impact in smaller studies).
If included studies do not report the primary outcome measure of live birth, and do not report interim outcomes such as clinical pregnancy either, informal assessment will be undertaken as to whether studies reporting the primary outcome measures reflect typical findings for the interim outcomes. Within-study reporting bias will be considered by looking at the protocols.
The assessment of reporting biases will be addressed in the 'risk of bias in included studies' section of the 'Results'.
Review Manager software will be used to perform the meta-analyses using a fixed-effect model to calculate pooled ORs and 95% CIs. To aid interpretation, findings for primary outcomes will be translated to absolute risks, expressed as percentages based on the 95% CIs. Results for continuous outcomes will be combined using MDs.
Prospectively, it is planned to present the analyses as:
cumulative rates for cryopreservation of all embryos and subsequent frozen embryo transfer in a cycle without COS until exhaustion of supply versus fresh embryo transfer and subsequent frozen embryo transfer in a cycle without COS until exhaustion of supply;
one fresh versus frozen embryo transfer attempt.
An increase in the risks of a particular outcome, either beneficial or detrimental, will be displayed graphically in the meta-analyses to the right of the centre line and a decrease in the odds of an outcome will be displayed to the left of the centre line.
Subgroup analysis and investigation of heterogeneity
Where data are available, we will conduct subgroup analysis for the primary outcome to determine the separate evidence within the following subgroups.
1) Timing of cryopreservation: trials in which the embryo cryopreservation was performed at an early developmental stage (up to and including Day 4) compared to trials in which the embryo cryopreservation was performed at later stages (Day 5 up to Day 6).
2) Method of cryopreservation: trials in which embryo cryopreservation was performed with vitrification compared to trials in which embryo cryopreservation was performed with slow freezing.
We will conduct sensitivity analyses for the primary outcomes to determine whether the conclusions are robust to arbitrary decisions made regarding the eligibility of studies and analysis. These analyses will include consideration of whether the review conclusions would have differed if:
1. eligibility was restricted to studies without high risk of bias;
2. a random-effects model had been adopted;
3. alternative imputation strategies had been implemented;
4. the summary effect measure was relative risk rather than OR.
Overall quality of the body of evidence: summary of findings table
We will prepare a summary of findings table using GRADEpro or Guideline Development Tool software. This table will evaluate the overall quality of the body of evidence for the primary review outcomes (live birth and OHSS) and clinical pregnancy using GRADE criteria (study limitations (that is risk of bias), consistency of effect, imprecision, indirectness, and publication bias). Judgments about evidence quality (high, moderate, or low) will be justified, documented, and incorporated into reporting of results for each outcome.