Sometimes during ovulation a woman releases two or even more mature eggs. If all the eggs are fertilized, the woman may have a multiple birth, usually twins. Often, when two eggs are fertilized, one disappears very early in the pregnancy and only a single baby is born. When both eggs are fertilized and survive, the woman has twins. The twins will be fraternal, meaning each egg was fertilized separately by a different sperm. They won't have an identical appearance and may even be different sexes.