From Romulus and Remus—the myth of the twin brothers who founded the city of Rome—to the synchronized speech of Australia's "Twinnies" Paula and Brigette Powers, identical siblings have long inspired ...

Twins occur in about one out of 90 human pregnancies in the US. As a rule, at conception, one sperm fertilizes a single egg to produce a zygote. Two thirds of twins are fraternal, with fertilization ...