U.S. twin births
Across the U.S. twin birth rates vary in a variety of ways. The U.S. states with the highest twin birth rates are Connecticut and Michigan. Ethnicity is another factor to be considered when investigating the twin birth rate. Since 1980 the U.S. twin birth rate has increased among all ethnicities; however, it has increased more among non-Hispanic black mothers and less among Hispanic mothers. And finally, age is another important factor in twin birth rates. As of 2019, mothers between the ages of 45-54 had the highest twin birth rates in the United States.
Assisted reproductive technology
Assisted reproductive technology (ART) is a treatment used to treat infertility among men and women. A common complication associated with ART is multiple births (twins, triplets, etc.). In the U.S., among all ART pregnancies the most common outcome was a single infant birth, followed by multiple infant births. Among all U.S. ART cycles (including those that did not lead to pregnancy) using non-donor eggs or embryos multiple-fetus pregnancy may be present in up to six percent of cases.