How are twins formed?
- Fraternal (Non-identical) twins are formed when two egg cells are fertilized; each egg by a different sperm so that two embryos are formed. Fraternal twins can be of the same or opposite sex and they don't have to look at all alike.
- Identical twins are formed when one egg after being fertilized by one sperm, divides into two halves. The two halves are genetically identical. Identical twins are usually of the same sex.
- Conjoined (Siamese) twins are formed exactly like identical twins, but during the split, the embryo does not separate completely to form two separate babies and the twins develop attached to one another.
"George Files" by Parenting the Next Generation