The authors' concluded that double-embryo transfer produced significantly greater pregnancy and live birth rates compared to single-embryo transfer rates. The authors conclusions reflected the results of the review. Reliance on relatively few studies and small participant numbers combined with potential for publication bias suggests these conclusions should be treated with caution.

To compare single-embryo versus double-embryo transfer in fresh in vitro fertilisation (IVF) and intracytoplasmic sperm injection (ICSI) cycles with respect to implantation, ongoing pregnancy and live birth rates.

MEDLINE, EMBASE, Science Citation Index and Cochrane Central Register of Controlled Trials were searched from 1995 to 2008, without language restrictions. Search terms were reported.

Published and ongoing randomised clinical trials (RCTs) that compared single-embryo to double-embryo transfer in fresh IVF/ICSI cycles were eligible for inclusion. The review outcomes were implantation, ongoing clinical pregnancy and live births.

Most included participants were women who were receiving their first IVF cycle with good quality embryos and were less than 36 years old (where stated). The description of good quality embryos was defined as symmetrical blastomeres, four or five blastomeres on day two, at least seven blastomeres on day three, four to six cells at day two to day 10, less than 20% fragmentation, grade three or four or were not described.

The authors did not state how many reviewers assessed studies for inclusion.

The authors stated that two reviewers independently performed validity assessment. Randomisation was noted, but otherwise little detail was provided.

Ongoing pregnancy rate, live birth and implantation rate were extracted to calculate odds ratios (ORs) and 95% confidence intervals (CI).

Two reviewers independently extracted data.

A fixed-effect meta-analysis (Mantel-Haenszel) was used to synthesis odds ratios and 95% CIs. Heterogeneity was assessed using Cochrane's Q test, Breslow-Day and I² tests.

Seven RCTs (n=1,402) were included in the review.

Implantation rates were not significantly different between single-embryo and double-embryo transfer (five studies, n=987). Double-embryo transfer produced significantly higher clinical pregnancy rates (OR 2.06, 95% CI 1.64 to 2.60; six studies, n=1,321) and significantly higher live birth rates (OR 1.87, 95% CI 1.44 to 2.42; five studies, n=1,046) compared with single-embryo transfer. Heterogeneity was not found between the pooled studies. Quality assessment of studies was not reported.

Fresh double-embryo transfer had a significantly greater ongoing pregnancy rate and live birth rate than single-embryo transfer in populations suitable for assisted reproductive technology treatment.

This review addressed a clear research question supported by clear inclusion criteria. The authors searched several relevant databases without language restrictions, which reduced the chances of relevant studies being omitted and language bias being present. Unpublished studies were not sought systematically and publication bias was not assessed; therefore, publication bias could not be ruled out. Validity was assessed, but a lack of reported methodology hindered further evaluation of its adequacy. Some steps were taken throughout the review process to minimise errors and bias. Data were analysed using appropriate statistical techniques. The authors conclusions reflected the evidence presented. The review contained relatively few studies and small numbers of participants, which combined with potential for publication bias and lack of details on study validity mean the authors' conclusions should be treated with caution.

Practice: The authors stated that single embryo transfer was an appropriate transfer option for only a small minority of IVF patients and double embryo transfer had a large number of arguments in favour of its use.

Research: The authors stated that further research to evaluate short-term and long-term results, all relevant costs and their funding sources was necessary to to better compare single versus double embryo transfer strategies.

Not stated.

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.