> "My understanding is that naturally twins are a rare phenomenon, but much more likely if doing something like in vitro or artificial insemination?"
There are various factors that increase the chances of having fraternal (non-identical) twins. Genetics (a women who is a twin herself is more likely to have twins), age (older women are more likely to have twins), and, of course, fertility drugs which stimulate ovulation can result in more eggs being released and thus more likely to produce twins.
Identical twins, on the other hand, are a natural phenomenon that occurs in about 1 in 250 pregnancies. There are no known factors that increase the chances of identical twins.
There are various factors that increase the chances of having fraternal (non-identical) twins. Genetics (a women who is a twin herself is more likely to have twins), age (older women are more likely to have twins), and, of course, fertility drugs which stimulate ovulation can result in more eggs being released and thus more likely to produce twins.
Identical twins, on the other hand, are a natural phenomenon that occurs in about 1 in 250 pregnancies. There are no known factors that increase the chances of identical twins.