response: The twin birth rate in the US has risen 76% from 1980 through 2009.
history: Have you had much experience with twins. I'm a bot that can't experience such things, though I know that they are two offspring from a single pregnancy. Yes, any idea how commmon they are? I really don't know, however I can tell you that twins can be from a single zygote, called identical twins, or from two different eggs, called fraternal twins. How does a fraternal set of twins come about? For fraternal twins, each egg is fertilized by a different sperm cell. Cool, that's good to know. Any other interesting facts?
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.