Single center randomized study including 388 women aged less than 39 years, performing in vitro fertilization, to determine if blastocyst stage (Day 5 to 6) embryo transfer (ETs) improves implantation and pregnancy rate compared with cleavage stage (Day 2 to 3) ETs.

Embryos from in vitro fertilization are routinely transferred into the woman's uterus at cleavage stage (Day 2 to 3) or at blastocyst stage (Day 5 to 6).

The rational of blastocyst stage transfer is that blastocyst stage is the most biologically correct stage for embryos to be in the uterus, and longer culture in the laboratory may give the possibility to reduce the number of genetically abnormal embryos.

The investigators design a single center randomized trial, including 388 women aged less than 39 years, performing in vitro fertilization, to confront blastocyst stage embryo transfers (ETs) and cleavage stage ETs. Each patient is randomized with a non blind randomization, based on a computer model, on the first day after fertilization.

The primary outcomes are implantation and pregnancy rate. The secondary outcome is to verify if blastocyst embryo transfer leads to a higher multiple-pregnancy rate.

An interim analysis takes place at one third of the population. Follow up is of 4 weeks for implantation rate, and covers 8 weeks regarding pregnancy rate.

Inclusion Criteria:

• female patients undergoing IVF procedures
• age less than 39 years

Exclusion Criteria:

• less than 4 fertilized eggs in day 1 after IVF