Identical twins grow after your fertilized egg splits in half and creates two genetically identical or monozygotic embryos. If the split happens in the first weeks after conception, each baby develops their own placenta and amniotic sac. If the split occurs later, the two embryos will most likely share the same amniotic sac. It will be difficult to differentiate between these twins just by looking at them and they will be the same sex.
Fraternal twins, or non-identical twins develop when two separate eggs are fertilised, creating two genetically different or dizygotic embryos. The babies each have their own placenta and amniotic sac. The only thing they share is your uterus. These twins may not only look different from each other they may even be opposite sexes.