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Live-birth rates and multiple-birth risk of assisted reproductive technology pregnancies conceived using thawed embryos, USA 1999-2000.

BACKGROUND
Increasing use of assisted reproductive technology treatments has been associated with the current rise in multiple births in the USA. Embryo cryopreservation and subsequent thawed embryo transfer may favourably impact the multiple-birth risk by relieving some pressure that patients and providers may feel to transfer several embryos in a single cycle. The study objective was to examine both live-birth rates and multiple-birth risk in thawed cycles.


METHODS
The authors used a population-based sample of 21 555 assisted reproductive technology procedures performed in US clinics in 1999 and 2000 that used thawed embryos derived from the patient's oocytes.


RESULTS
Both patient age and the number of embryos transferred were independent predictors of live birth. Even among women aged 20-29 years, the transfer of three embryos resulted in an increase in the live-birth rate compared with cycles in which one or two embryos were transferred. This increase in success was accompanied by an increased multiple-birth risk. In all age groups up to 40 years, the transfer of just two embryos resulted in a multiple-birth risk of 16-17%. The multiple-birth risk increased with the number of embryos transferred.


CONCLUSIONS
Patient age and the number of embryos transferred significantly affect live-birth and multiple-birth rates among women who use thawed embryos.