Objective:  To evaluate trends in number of embryos transferred and resultant high-order multiple (HOM) pregnancy rates by Society for Assisted Reproductive Technology (SART)-member clinics between 1996 and 2003 and  to relate these practice patterns and outcomes to clinic compliance with SART-American Society for Reproductive Medicine (ASRM) embryo transfer guidelines.
Setting: Society for Assisted Reproductive Technology-member fertility centers in the United States.
Patient(s): Five hundred thirty-six thousand, five hundred twenty-four fresh, nondonor IVF cycles.
Main outcome measure(s): Number of embryos transferred; pregnancy rates; implantation rates; and proportion of singleton, twin, and HOM pregnancies.
Result(s): The number of embryos transferred declined each year. High-order multiple pregnancy rates also declined, whereas the twin rates remained stable. The most pronounced declines in number transferred occurred immediately after publication of SART-ASRM embryo transfer guidelines. After stratifying clinics according to mean and modal number of embryos transferred, clinics transferring the fewest embryos in women <35 years of age had the highest mean implantation and pregnancy rates. Furthermore, the percentage of clinics transferring two embryos to a majority of women <35 years of age increased from 3.3% in 1996 to 49.9% in 2003.
Conclusion(s): The implementation of SART-ASRM embryo transfer guidelines is associated with significant reductions in the number of embryos being transferred, along with reductions of HOM pregnancies. Initiatives to further reduce twin pregnancies and encourage singleton gestation outcomes are outlined.