If two or more babies grow in a woman’s uterus at the same time, they’re known as “multiples.” Sometimes they look exactly alike — identical — and sometimes they don’t look any more alike than typical siblings — fraternal. They happen in different ways.
A fertilized egg is made with one egg and one sperm. If it splits in two, which happens sometimes, you have identical twins. If one of those eggs separates again, you’ll have identical triplets, and so on. All the babies start with the same gene set: They’re either all boys or all girls, and they’ll look alike. Identical multiples happen in three or four of every 1,000 live births.
Sometimes, more than one egg comes out of a woman’s ovary in a single month. If each one is fertilized by different sperm, fraternal multiples happen. Unlike identicals, the genes of fraternals are as different as any other sibling with the same parents. These kind of multiples are born more often than identicals.
Same Mom, Different Dad
If a woman has two or more eggs during their fertility window, it’s possible that each one could be fertilized at different times — even by different men. So it’s possible for multiples to be born with different fathers.
Parents of twins often say their children have a special language they use only with each other. Apparently, the communication starts early. One study found that by the 14th week of pregnancy, twins make intentional movements toward each other. More research is needed to see if this holds true for other multiples — triplets, quadruplets, etc.
Culled from webmd.com by Renee A. Alli