To maximize birth rates, physicians who perform in vitro fertilization
(IVF) often transfer multiple embryos, but this increases the multiple-birth
risk. Live-birth and multiple-birth rates may vary by patient age and embryo
quality. One marker for embryo quality is cryopreservation of extra embryos
(if embryos are set aside for cryopreservation, higher quality embryos may
have been available for transfer).
To examine associations between the number of embryos transferred during
IVF and live-birth and multiple-birth rates stratified by maternal age and
whether extra embryos were available (ie, extra embryos cryopreserved).
Design and Setting
Retrospective cohort of 300 US clinics reporting IVF transfer procedures
to the Centers for Disease Control and Prevention in 1996.
A total of 35,554 IVF transfer procedures.
Main Outcome Measures
Live-birth and multiple-birth rates (percentage of live births that
A total number of 9873 live births were reported (multiple births from
1 pregnancy were counted as 1 live birth). The number of embryos needed to
achieve maximum live-birth rates varied by age and whether extra embryos were
cryopreserved. Among women 20 to 29 years and 30 to 34 years of age, maximum
live-birth rates (43% and 36%, respectively) were achieved when 2 embryos
were transferred and extra embryos were cryopreserved. Among women 35 years
of age and older, live-birth rates were lower overall and regardless of whether
embryos were cryopreserved, live-birth rates increased if more than 2 embryos
were transferred. Multiple-birth rates varied by age and the number of embryos
transferred, but not by whether embryos were cryopreserved. With 2 embryos
transferred, multiple-birth rates were 22.7%, 19.7%, 11.6%, and 10.8% for
women aged 20 to 29, 30 to 34, 35 to 39, and 40 to 44 years, respectively.
Multiple-birth rates increased as high as 45.7% for women aged 20 to 29 years
and 39.8% for women aged 30 to 34 years if 3 embryos were transferred. Among
women aged 35 to 39 years, the multiple-birth rate was 29.4% if 3 embryos
were transferred. Among women 40 to 44 years of age, the multiple-birth rate
was less than 25% even if 5 embryos were transferred.
Based on these data, the risk of multiple births from IVF varies by
maternal age and number of embryos transferred. Embryo quality was not related
to multiple birth risk but was associated with increased live-birth rates
when fewer embryos were transferred.