1.   Because the rate of identical twinning is nearly identical worldwide, differences in twinning rates, overall, are determined almost entirely by varying rates of fraternal twin births.

2.   Calvert said horse breeders try to avoid twin births.

3.   However, he said these figures apply to single or twin births and additional risks are associated with septuplets.

4.   Twin births are seen as an evil omen in many cultures around the world.