Twinning Rates

Rates of twins vary by society but according to the latest CDC National Vital Statistics Report (2004), the approximate twin birth rate (number of twin births to total live births) in the U.S. is about 32.2 per 1,000 live births. Even though twins and other multiples are much more common today than they once were, you're still pretty unique if you are born as a twin as only about one of every 32 children born in the United States in 2004 has a twin. These odds used to be much higher--in 1964, for instance, there were 41,259 sets of twins born in the United States, with 10.4 per 1,000 births.

As the tables below show, the numbers of twin births has risen slightly in the United States each year from 1996 to 2004, reaching a record-number 132,219 twins born in the most recent year for which data is available. The numbers of triplet, quadruplet and quints-and-higher births has dropped slightly or stayed statistically balanced in five of the past six years. The total number of births in the U.S. rose slightly in 2004 to 4,112,052 births after a 2% gain in 2003 to 4,089,950 births. The birth numbers were virtually unchanged in 2002 from the previous year (4,021,726 births in 2002 vs. 4,025,933 births in 2001). The overall birth rate (14.1 births per 1,000 total population) also increased 1% in 2003 in the United States.

With the recent overall stability in the American birth rate coupled with the record number of twin births, it meant back in 2001 for the first time in U.S. recorded history that more than 3 percent of all births were twins. That number has risen slightly each of the past three years for which data exists and is now 3.2 percent.

Since 1990, the twin birth rate has now risen 42% (from 22.6 to 32.3 per 1,000 live births), while rising 70% since 1980 (from 18.9 to 32.2 per 1,000 live births). As mentioned, the rate for triplets and other higher-order multiples has slowed down slightly from its 1998 high of 193.5, but it has still risen close to 400 percent (from 37.0 to 176.9 per 100,000 live births) since 1980.

Other data released by the National Center for Health Statistics regarding the numbers of multiples born in the U.S. in 2004 indicates that births of multiples is still greatly impacted by the age of the mother and continues to rise each year. Since 1990, the twin birth rate has soared dramatically among women age 45-54 (from 23.8 per 1,000 in 1990 to 189.7 per 1,000 in 2002). In 2003, a remarkable 19.9 percent of all births to women age 45-54 were twins.

Other countries also show increases in twinning. A report by M2 Communications reported that twice as many twins were born in Norway in 1999 as were born in that country in 1986. Latest figures in that Scandanavian country show there were 1031 pairs of twins born in 1999, 21 sets of triplets and one set of quadruplets. According to the book The Two-Headed Boy & Other Medical Marvels by Jan Bonderon (Cornell University Press, 2000), there were 112 sets of quadruplets, 16 sets of quintuplets, six sets of sextuplets and 1 set of septuplets born in England and Wales between 1952-88. Contrast that with numbers from the Office of National Statistics (UK) in 1999 which shows 8,636 sets of twins, 267 sets of triplets and four sets of quads-or-higher born in either England or Wales out of a total of 622,000 live births. In the United Kingdom, there were eight sets of quadruplets or higher, 323 sets of triplets and 9,893 pairs of twins born in 1998.

The rates in Australia have also risen slightly. In 1978, about 1% of all pregnancies in Australia resulted in multiple births (according to the Australian Bureau of Statistics); in 1998 that number had increased to 1.5%. In 1998, there were 245,898 total births in Australia with 3700 multiple births (sets of multiples). In the region of New South Wales, there were 1218 multiple births (1191 sets of twins and 27 sets of triplets) among the 85,449 total births in 1998.

While it is difficult to see historical data on the chances of having twins and higher-order multiples for the early 1900's or other centuries, the book Multiple Human Births by Horatio Hackett Newman (Doubleday, 1940) does make reference to some numbers. One study detailed in that book is by an American biologist named Greulich, who looked at birth records of 21 (undisclosed) countries between 1915-25. That data showed that among 120,000,000 births during that period, there were 1,408,912 twin births, 15,738 triplet births and 179 quadruplet births. This equates out to a ratio of 1:85.2 for twins, 1 to (87.2)² for triplets and 1 to (87.5)³ for quadruplets.

In terms of highest twinning rates by state during the three-year period from 2002-04, the biggest percentage of twins were born in Massachusetts (a rate of 45.2 twins per 1,000 births and 10,805 twins total) followed by New Jersey (41.3) and Connecticut (40.5, 5,136 twins). Rhode Island (38.3), Maryland (37.3) and New Hampshire (36.0) ranked 4-6 as the trend of more twins being born in the NE United States certainly was validated. New Mexico had the lowest rate of twins born during the timeframe (23.8, 1,996 twins) with other lower rates being registered in Utah (26.1, 3,914 twins), and Alaska and Arizona (both 26.4, Alaska with 802 twins and Arizona with 7,184). The fewest twins during this period were born in Wyoming (just 542 twins, 27.0 rate).

Twinstuff 10:32, 12 June 2007 (CDT)