response: I don't actually know, though the twin rate rose 76% between 1980 and 2009, ending up at 33.3 for every 1,000 births.
original_response: Well, the twin birth rate in the US rose from 18.9 to 33.3 for every 1,000 births between 1980 and 2009
history: I have a twin in real life and when people see us together, I get real sick of them asking if we are twins! How interesting! A developing fetus is called a singleton if they are alone but a multiple if there is more than one developing! Those embryos sure like multiplying! I wonder if twins are more rarer than triplets?
knowledge: The human twin birth rate in the United States, rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births.