In vitro fertilization with single blastocyst-stage versus single cleavage-stage embryos

Single-embryo transfer has been recommended to reduce the incidence of multiple gestations when in vitro fertilization is performed in women under 36 years of age. We designed a prospective, randomized, controlled trial to determine whether there were any differences in the rates of pregnancy and delivery between women undergoing transfer of a single cleavage-stage (day 3) embryo and those undergoing transfer of a single blastocyst-stage (day 5) embryo.

We studied 351 infertile women under 36 years of age who were randomly assigned to undergo transfer of either a single cleavage-stage embryo (176 patients) or a single blastocyst-stage embryo (175 patients). Multifollicular ovarian stimulation was performed with a gonadotropin-releasing hormone antagonist and recombinant follicle-stimulating hormone.

The study was terminated early after a prespecified interim analysis (which included 50 percent of the planned number of patients) found a higher rate of pregnancy among women undergoing transfer of a single blastocyst-stage embryo (P=0.02). The rate of delivery was also significantly higher in this group than in the group undergoing transfer of a single cleavage-stage embryo (32.0 percent vs. 21.6 percent; relative risk, 1.48; 95 percent confidence interval, 1.04 to 2.11). Two multiple births occurred, both of monozygotic twins, both of which were in the group undergoing transfer of a single cleavage-stage embryo.

These findings support the transfer of a single blastocyst-stage (day 5) embryo in infertile women under 36 years of age.