A total of 16 studies (2,010 elective single embryo transfer; 77,074 controls) were included in the review: eight RCTs (1,012 elective single embryo transfer; 1,064 double embryo transfer) and eight observational studies (998 elective single embryo transfer; 76,010 controls). Across RCTs sample sizes ranged from 53 to 661, across observational studies from 86 to 59,786. The quality of the RCTs was variable: five reported adequate methods of sequence generation and allocation concealment; three reported adequate blinding of patient-reported outcomes; all eight adequately addressed incomplete outcome data and were free of selective reporting; six were free of other biases. Most of the observational studies were of high quality. The greatest risk of bias was in the area of comparability where half of the studies were assessed to have a possible risk of bias.
Evidence from the RCTs suggested that elective single embryo transfer reduced the risk of a preterm birth compared with double embryo transfer (RR 0.37, 95% CI 0.25 to 0.55, NNT nine, four trials). The observational data did not show an effect of elective single embryo transfer on preterm birth (three studies). Heterogeneity (Ι²=71%) was observed in the analysis of the observational studies. Both randomised and observational evidence suggested that elective single embryo transfer reduced the risk of newborns with low birth weight compared with double embryo transfer (RCTs: RR 0.25, 95% CI 0.15 to 0.45, NNT six, three trials; observational: RR 0.51, 95% CI 0.29 to 0.91, NNT 24, three studies). No other significant results were found for comparisons of elective single embryo transfer and double embryo transfer.
When comparing elective single embryo transfer to spontaneous singleton pregnancies, evidence from observational studies showed that elective single embryo transfer increased the risk of preterm birth (RR 2.13, 95% CI 1.26 to 3.61, two studies). There was evidence of a high level of heterogeneity in this analysis (Ι²=78%). Findings for risk of low birth weight were non-significant. One observational study reported significantly lower risk of gestational diabetes mellitus, placenta previa and ectopic pregnancy for spontaneous conception compared with elective single embryo transfer.
The planned sensitivity analysis to investigate the effect of using blastocysts (on or after day 5) for elective single embryo transfer on study outcomes could not be performed because none of the included studies reported on this. Funnel plots could not be used to assess publication bias due to the small number of included studies.