When only one embryo is available, is it better to transfer on Day 3 or to grow on?

Research question: In patients with only one embryo on Day 3 post-IVF treatment, does transferring the embryo into the uterine environment achieve a higher pregnancy rate than growing the embryo on with a plan to transfer at Day 4-6? Design: This was a retrospective cohort study conducted in patients with only one viable embryo on Day 3 post-IVF treatment. Data were extracted from a standardized IVF database and included 1384 women who fulfilled this study's selection criteria. Outcomes of these embryos were followed up and stratified into two groups: embryos transferred on Day 3 and those grown on to Day 4-6. Pregnancy rate (biochemical and clinical) and live birth rates were analysed with logistic regression and adjusted using a parsimonious model for baseline patient characteristics. Results: Biochemical pregnancy (16.7% versus 9.5%, odds ratio [OR] 1.9, P = 0.001), clinical pregnancy (14.7% versus 6.8%, OR 2.35, P < 0.001) and live birth rates (9.7% versus 4.4%, OR 2.37, P = 0.002) were significantly higher in the Day 3 group than those in the group grown on to Day 4-6. These differences were still significant after adjusting for potential confounders (adjusted OR 2.60, 3.71, 4.08, respectively, P < 0.001). Conclusions: These findings support Day 3 cleavage-stage embryo transfer instead of growing on to Day 4-6 for blastocyst-stage transfer when only a single embryo is available.