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See my blog for more information about this graph!

This graph uses data from:  (p. 90)

Here is a related report, and some "key findings" from the report:

o In 2009, 1 in every 30 babies born in the United States was a twin, 
  compared with 1 in every 53 babies in 1980.

o The twin birth rate rose 76 percent from 1980 through 2009, 
  from 18.9 to 33.3 per 1,000 births.

o If the rate of twin births had not changed since 1980, 
  approximately 865,000 fewer twins would have been born in the 
  United States over the last three decades.

o Twinning rates rose by at least 50 percent in the vast majority 
  of states and the District of Columbia.

o Over the three decades, twin birth rates rose by nearly 100 percent 
  among women aged 35.39 and more than 200 percent among women aged 40 and over. 

o The older age of women at childbirth in 2009 compared with three 
  decades earlier accounts for only about one-third of the rise in 
  twinning over the 30 years.

The incidence of multiple births in the United States was quite stable at 
about 2 percent of all births from 1915 (the earliest year for which 
reliable data are available) through the 1970s. By the early 1980s, 
however, twinning rates, and those for triplet and higher-order multiples 
(triplet/+) began to rise, ultimately leading to what has been called 
"an epidemic of multiple pregnancies".

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