['To compare the respective pregnancy outcomes of cycles undergoing fresh elective single-blastocyst transfer (eSBT) and double-blastocyst transfer (DBT) after IVF.', 'Retrospective medical record review.', 'Academic medical center.', 'Couples undergoing fresh blastocyst transfer after IVF.', 'One thousand, four hundred, ninety-nine consecutive IVF cycles from January 1, 2002 through March 31, 2006 at the Massachusetts General Hospital Fertility Center were reviewed. Patients undergoing fresh blastocyst transfer (eSBT, n = 52; DBT, n = 187) were identified.', 'Respective pregnancy outcomes and the trend in twin gestations in all cycles undergoing blastocyst transfer, before and after the introduction of the eSBT program.', 'Statistically similar rates of biochemical pregnancy (76.9% vs. 77.6%), clinical pregnancy (61% vs. 63.4%), live birth (53.8% vs. 54.4%), and pregnancy loss (20% vs. 18.6%) per embryo transfer were observed for fresh eSBT and DBT cycles, respectively. Twin rates for eSBT were statistically significantly lower than for DBT cycles (3.1% vs. 51%). Fresh eSBT and DBT cryopreserved 2.8 vs. 1.7 blastocysts per cycle, respectively. Twenty-four months after the start of eSBT, the twin rate per transfer for all cycles undergoing blastocyst transfer (1, 2, or 3 blastocysts) was statistically significantly reduced from 47.2% to 22.9%, whereas the twin rate per transfer for all embryo transfers in women younger than 35 years of age was statistically significantly reduced from 28.8% to 15.6%.'] Does single-blastocyst transfer decrease twin gestation without affecting pregnancy outcome?