【Objective】 To compare the impact of pregnancy outcomes when frozen blastocyst was transferred 2 to 3 hours after being thawed or 16 to 20 hours after being cultured overnight. 【Methods】 Retrospectively analyze the 523 cycles of thawed blastocyst transfer from September 2010 to October 2012. All the cycles are divided into two groups based different culture time before transfer: those blastocysts transferred 2 to 3 hours after being thawed are labeled as 0TET group(n = 247) and those cultured overnight before being transferred the next day are included in 1TET group (n = 276). The pregnancy outcomes are compared between the two groups.【Results】 Compared with 0TET group

1TET group with less embryo transferred (1.6 vs.1.8

P < 0.001) has higher clinical pregnancy rate (57.6% vs. 56.3%)

implantation rate (46.3% vs. 44.1%) and lower miscarriage rate (9.4% vs. 12.2%) as well as lower multiple-pregnancy rate (28.3% vs. 36.7%). However

all differences between the two groups have not yet reached the statistical significance except number of blastocyst transferred. Hormone replaced treatment as endometrium preparation method for blastocyst transfer has higher clinical pregnancy rate (62.8% vs. 53.0%

P = 0.027). Blastocysts frozen at the fifth day after fertilization have higher implantation potential than those frozen at the sixth day (implantation rate 57.6% vs. 36.1%

P = 0.004) based analysis of cycle data of single blastocyst transfer. 【Conclusion】 Overnight culture of thawed blastocyst before transfer does not affect the main pregnancy outcomes. The factors related with pregnancy outcomes are endometrium preparation methods and blastocyst developmental stages. To decrease the multiple-pregnancy rate

single embryo transfer of D5 should be encouraged.