Trends in multiple births conceived using assisted reproductive technology, United States, 1997-2000.

OBJECTIVE To examine trends in multiple births conceived using assisted reproductive technology (ART) in the United States between 1997 and 2000 and to estimate the proportion of all US multiple births attributable to ART use. METHODS We analyzed population-based data of 109 519 live-born infants who were conceived in the United States using ART and born between 1997 and 2000 and population-based data of 15 856 809 live-born infants who were delivered in the United States between 1997 and 2000. Multiple birth rates (the number of live-born infants delivered in multiple gestation pregnancies per 1000 live births) and the proportion of all US multiple births attributable to ART were evaluated. RESULTS The twin rate for ART patients increased between 1997 and 2000, reaching 444.7 per 1000 live births in 2000, whereas the triplet/+ rate declined substantially from 134.3 to 98.7 per 1000 live births from 1997-2000. From 1997-2000, the proportion of multiple births in the United States attributable to ART increased from 11.2% to 13.6%, whereas the proportion attributable to natural conception decreased from 69.9% to 64.5%. In 2000, the proportion of triplet/+ births attributable to ART and to natural conception was 42.5% and 17.7%, respectively. The contribution of ART to multiple births increased substantially with maternal age, from 11.6% for triplet/+ infants born to women aged 20 to 24 to 92.8% for women aged 45 to 49 years. CONCLUSIONS The contribution of ART to twin birth rates continues to increase, but the contribution of ART to triplet/+ birth rates has declined.