There is about a one in 80 chance of a mother having twins. There are two types of twins. If two egg cells join with two separate sperm, non-identical* twins will be born. They each have different chromosomes so they can be of different sexes. Identical twins are born when a fertilized egg splits into two and each half develops into a baby. The twins look the same because they have identical chromosomes. Identical twins share a placenta. Non-identical* twins have one each.