The estimated twin birth rate in the United States is 31.1 twins per 1,000 live births. So, if you take the number of twins born in the U.S. for the most recent year statistics are available, 2006, and divide by three, you end up with just under 46,000.
The incidence of twins per 1,000 births has risen substantially in recent years. In 1995, there were 24.8 twins per 1,000 live births; in 2006, that proportional figure had increased to 32.1. The states with the highest incidences of twin birth rates are Connecticut, Massachusetts, New Jersey and Nebraska. The state with the lowest twin incidence rate of birth is New Mexico. Go figure.
The age of the mother also has a lot to do with the chance of twins being born. Essentially, the older she is, the higher the favorable odds are. Remarkably, in the year 2002 for example, 18.9 percent of all twin births in the United States were delivered by women ages 45 to 49. As fertility treatments and other scientific advancements have allowed women to have children far past the standard biological clock time frame, it appears to have also correlated, for whatever reason, with more twin births. The rate has remained high, edging up a little further in 2006 to 20% of all twin births coming from women 45 or older.
Another interesting wrinkly in the twin statistics is that the ratio of double births per 1,000 is substantially lower among Hispanic women than it is for Caucasian moms. In 2006, for example, the rate for Hispanic women was 21.8 per 1,000, as opposed to 36.0 for Caucasian women and 36.8 for African-American moms. There is a dark underside to all these statistics. Twins are five times more likely to die within a month of being born than single specimen children. Assisted birth methods such as in-vitro fertilization meanwhile were responsible in 2004 for 17 percent of all twin births in the U.S.