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.