Fresh versus frozen embryo transfers for assisted reproduction

  • Protocol
  • Intervention

Authors


Abstract

This is the protocol for a review and there is no abstract. The objectives are as follows:

To determine whether IVF with frozen-thawed embryo transfers only (freeze-all strategy) has a positive effect on fertility outcomes, including live birth rate, compared to standard IVF with fresh and frozen-thawed embryo transfers.

Background

Description of the condition

Subfertility is defined as the failure to conceive after one year of unprotected intercourse (Van Voorhis 2007). One in six couples experience subfertility at least once during their reproductive lifetime and approximately 10% of couples worldwide are subfertile (ESHRE 2010; CDC 2011). Common causes of subfertility include low semen quality, fallopian tubal occlusion, ovulatory disorders, and endometriosis (Hull 1985). Low semen quality can manifest itself as low sperm concentration, low motility, or low numbers of sperm with normal morphology. Fallopian tubes can be blocked or damaged by infection, or there can be adhesions of the tubes or ovaries caused by surgery or endometriosis. One of the most common causes of ovulatory disorders (such as variable cycle length, oligomenorrhoea or amenorrhoea) is polycystic ovarian syndrome (PCOS) (Knochenhauer 1998). Couples who fail to conceive spontaneously are diagnosed as having unexplained infertility if no cause can be found (that is, there is evidence of ovulation with normal serum levels of luteal progesterone, tubal patency, and normal semen parameters).

Description of the intervention

Assisted reproductive technology (ART) has rapidly evolved as a therapy for subfertility for all the above-mentioned indications. ART involves handling of gametes (eggs and sperm cells) or embryos outside the human body with the ultimate goal being to achieve pregnancy leading to the birth of a healthy child. The most widely used treatment for subfertility is in vitro fertilization (IVF) with or without intracytoplasmic sperm injection (ICSI) followed by the transfer of one or more embryos into the uterus. When there are additional embryos they are frozen, allowing them to be used in a future cycle should the couple not conceive from the fresh transfer or to be used later if the couple wishes to have another child.

IVF has revolutionized reproductive medicine as we know it (Steptoe 1978). It was originally applied in an unstimulated natural cycle, without any form of ovarian (hyper) stimulation. Although pregnancies occurred in the unstimulated cycle, two drawbacks of this initial approach were the risk of premature ovulation and the relative inefficiency, since cycles often failed because no oocytes could be collected even if ovulation had occurred (Rongieres-Bertrand 1999). These drawbacks fuelled extensive research into ovarian stimulation protocols to prevent premature ovulation and to obtain more oocytes and embryos. This led to the use of exogenous gonadotrophins and gonadotrophin-releasing hormone (GnRH) agonists. However, two important complications of ovarian stimulation arose, (1) ovarian hyperstimulation syndrome (Elchalal 1997; Beerendonk 1998) and (2) multiple pregnancies (Nygren 2001).

Modern-day IVF and ICSI cycles are now characterized by controlled ovarian stimulation (COS) regimens to obtain multiple oocytes for IVF or ICSI (Hillier 2013). Subsequently fresh transfer of the morphologically best embryo(s) is performed. Even so, many women fail to achieve a pregnancy after transfer of one or more fresh embryos. Recent technical improvements in cryopreservation have led to increased chances of embryo survival after thawing and subsequently increased pregnancy rates per frozen-thawed embryo transfer (CDC 2011a; Wong 2014 in press). In fact, pregnancy rates after frozen-thawed embryo transfer are now almost equal to pregnancy rates after fresh transfer. This has fueled the call for a new IVF strategy where no fresh transfer is conducted and all available embryos are cryopreserved and thawed and transferred in a subsequent cycle without ovarian hyperstimulation (Mastenbroek 2011; Maheshwari 2013).

How the intervention might work

The use of ovarian hyperstimulation with exogenous gonadotropins in IVF and ICSI cycles does not only lead to the maturation of multiple oocytes but is also associated with altered endometrial development that may impair receptivity of the endometrium for the transferred embryo (Kolb 1997; Check 1999; Nikas 1999; Kolibianakis 2002; Papanikolaou 2005; Horcajadas 2007; Haouzi 2009). This effect may be caused by supraphysiologic levels of estradiol (E2) and progesterone (P) during the follicular phase and could lead to morphologic and biochemical endometrial alterations and a more advanced endometrium than in natural cycles (Kolibianakis 2002; Bourgain 2003; Venetis 2013). As a consequence this may result in embryo-endometrium asynchrony. One possible way to overcome the negative effect of ovarian hyperstimulation on the endometrium is the cryopreservation of all embryos in the hyperstimulated cycle with subsequent frozen-thawed embryo transfer to the uterus in a non-hyperstimulated cycle, a so-called 'freeze-all' strategy. In contrast to a standard IVF or ICSI treatment, transfer of one or more fresh (non-frozen) embryo(s) within the hyperstimulated IVF cycle is thus not performed in the freeze-all strategy. In this review we will refer to the freeze-all strategy as IVF with frozen-thawed embryo transfers only, while the standard IVF or ICSI treatment will be referred to as IVF with fresh and, in case of the availability of supernumerary embryos, frozen-thawed embryo transfers. In both strategies frozen-thawed embryo transfers are conducted in a non-hyperstimulated cycle.

Why it is important to do this review

The relative effectiveness of IVF treatment with frozen-thawed embryo transfers only (freeze-all strategy) compared to standard IVF treatment with fresh and frozen-thawed embryo transfers is unclear. A previous systematic review reported that a freeze-all strategy was associated with better pregnancy outcomes than a strategy that also included fresh embryo transfers (Roque 2013). However, only three randomized trials were included in the review and no outcomes were reported for live births. Therefore this systematic review aims to investigate whether there is a difference in live birth rate between these two treatment strategies.

Objectives

To determine whether IVF with frozen-thawed embryo transfers only (freeze-all strategy) has a positive effect on fertility outcomes, including live birth rate, compared to standard IVF with fresh and frozen-thawed embryo transfers.

Methods

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

Primary outcomes
  1. 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).

  2. Adverse effect: ovarian hyperstimulation syndrome (OHSS) per woman.

Secondary outcomes
  1. 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).

  2. 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).

  3. Time to pregnancy, defined as the time between start of treatment (first day of the last menstrual cycle) and clinical pregnancy.

  4. Multiple pregnancy rate, defined as the number of multiple pregnancies per woman.

  5. Miscarriage rate, defined as the number of miscarriages per woman.

  6. Congenital disorders, defined as the number of congenital abnormalities at birth per all clinical pregnancies.

  7. 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.

  8. 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.

Electronic searches

We will search the following electronic databases, trial registers, and websites:

  • Menstrual Disorders and Subfertility Group (MDSG) Trials Register;

  • Cochrane Central Register of Controlled Trials (CENTRAL);

  • MEDLINE;

  • EMBASE;

  • PsycINFO;

  • CINAHL.

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:

  • using a computerized random number generator;

  • using a random numbers table.

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:

  • central computer randomization;

  • serially numbered, sealed opaque envelopes.

3. Blinding

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'.

Data synthesis

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.

Sensitivity analysis

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.

Acknowledgements

We would like to acknowledge the team at the Cochrane Menstrual Disorders and Subfertility Group for their assistance, and especially Trials Search Co-ordinator Marian Showell for the literature search.

Appendices

Appendix 1. MDSG keywords

Menstrual disorders and Subfertility database search for KMW1790

Keywords CONTAINS "cryopreservation"or "frozen embryo transfer" or "frozen embryos" or "frozen-thawed cycle" or "frozen-thawed embryo transfer" or "frozen-thawed embryos" or "FET" or "cryopreserved embryos" or "cryopreserved-thawed embryos" or "vitrified"or "vitrification"or"fresh v cryopreserved"or Title CONTAINS "cryopreservation"or "frozen embryo transfer" or "frozen embryos" or "frozen-thawed cycle" or "frozen-thawed embryo transfer" or "frozen-thawed embryos" or "FET" or "cryopreserved embryos" or "cryopreserved-thawed embryos" or "vitrified"or "vitrification"or "fresh v cryopreserved"

Appendix 2. MEDLINE search strategy

1 exp Cryopreservation/
2 exp Freezing/
3 (cryopreservat$ or cryofixation or cryonic suspension).tw.
4 freez$.tw.
5 thaw$.tw.
6 exp Vitrification/
7 Vitrif$.tw.
8 froze$.tw.
9 or/1-8
10 exp embryo transfer/ or exp fertilization in vitro/ or exp sperm injections, intracytoplasmic/ or exp ovulation induction/
11 embryo$.tw.
12 blastocyst$.tw.
13 vitro fertili?ation.tw.
14 ivf.tw.
15 icsi.tw.
16 intracytoplasmic sperm injection$.tw.
17 ovulation induc$.tw.
18 (ovar$ adj3 hyperstim$).tw.
19 (ovar$ adj3 stimulat$).tw.
20 exp Superovulation/ or Superovulat$.tw.
21 or/10-20
22 9 and 21
23 randomized controlled trial.pt.
24 controlled clinical trial.pt.
25 randomized.ab.
26 placebo.tw.
27 clinical trials as topic.sh.
28 randomly.ab.
29 trial.ti.
30 (crossover or cross-over or cross over).tw.
31 or/23-30
32 exp animals/ not humans.sh.
33 31 not 32
34 22 and 33

Appendix 3. EMBASE search strategy

1 exp Cryopreservation/
2 exp Freezing/
3 (cryopreservat$ or cryofixation or cryonic suspension).tw.
4 freez$.tw.
5 thaw$.tw.
6 exp Vitrification/
7 Vitrif$.tw.
8 froze$.tw.
9 or/1-8
10 exp embryo transfer/ or exp fertilization in vitro/ or exp intracytoplasmic sperm injection/
11 in vitro fertili?ation.tw.
12 icsi.tw.
13 intracytoplasmic sperm injection$.tw.
14 (blastocyst adj2 transfer$).tw.
15 ivf.tw.
16 exp superovulation/
17 superovulat$.tw.
18 exp ovulation induction/
19 blastocyst$.tw.
20 embryo$.tw.
21 vitro fertili?ation.tw.
22 ovulation induc$.tw.
23 (ovar$ adj3 stimulat$).tw.
24 (ovar$ adj3 hyperstim$).tw.
25 or/10-24
26 9 and 25
27 Clinical Trial/
28 Randomized Controlled Trial/
29 exp randomization/
30 Single Blind Procedure/
31 Double Blind Procedure/
32 Crossover Procedure/
33 Placebo/
34 Randomi?ed controlled trial$.tw.
35 Rct.tw.
36 random allocation.tw.
37 randomly allocated.tw.
38 allocated randomly.tw.
39 (allocated adj2 random).tw.
40 Single blind$.tw.
41 Double blind$.tw.
42 ((treble or triple) adj blind$).tw.
43 placebo$.tw.
44 prospective study/
45 or/27-44
46 case study/
47 case report.tw.
48 abstract report/ or letter/
49 or/46-48
50 45 not 49
51 26 and 50

Appendix 4. CENTRAL search strategy

1 exp Cryopreservation/
2 exp Freezing/
3 (cryopreservat$ or cryofixation or cryonic suspension).tw.
4 freez$.tw.
5 thaw$.tw.
6 exp Vitrification/
7 Vitrif$.tw.
8 froze$.tw.
9 or/1-8
10 exp embryo transfer/ or exp fertilization in vitro/ or exp sperm injections, intracytoplasmic/ or exp ovulation induction/
11 embryo$.tw.
12 blastocyst$.tw.
13 vitro fertili?ation.tw.
14 ivf.tw.
15 icsi.tw.
16 intracytoplasmic sperm injection$.tw.
17 ovulation induc$.tw.
18 (ovar$ adj3 hyperstim$).tw.
19 (ovar$ adj3 stimulat$).tw.
20 exp Superovulation/ or Superovulat$.tw.
21 or/10-20
22 9 and 21

Appendix 5. PsycINFO search strategy

1 (cryopreservat$ or cryofixation or cryonic suspension).tw.
2 freez$.tw.
3 thaw$.tw.
4 Vitrif$.tw.
5 froze$.tw.
6 or/1-5
7 exp reproductive technology/
8 icsi.tw.
9 intracytoplasmic sperm injection$.tw.
10 (blastocyst adj2 transfer$).tw.
11 assisted reproduct$.tw.
12 ovulation induc$.tw.
13 (ovari$ adj2 stimulat$).tw.
14 COH.tw.
15 superovulat$.tw.
16 infertil$.tw.
17 subfertil$.tw.
18 (ovari$ adj2 induction).tw.
19 ivf.tw.
20 vitro fertili?ation.tw.
21 (ovar$ adj3 hyperstimulat$).tw.
22 or/7-21
23 6 and 22
24 random.tw.
25 control.tw.
26 double-blind.tw.
27 clinical trials/
28 placebo/
29 exp Treatment/
30 or/24-29
31 23 and 30

Contributions of authors

Kai Mee Wong wrote the protocol. Sjoerd Repping and Sebastiaan Mastenbroek developed the concept of the study. Madelon van Wely, Fulco van der Veen, Sjoerd Repping, and Sebastiaan Mastenbroek provided feedback on the protocol.

Declarations of interest

The authors of this systematic review have no conflicts of interest to declare.

Sources of support

Internal sources

  • None, Not specified.

External sources

  • None, Not specified.

Ancillary