One Baby or More?

Humans usually have one baby at a time, but sometimes twins are born. Most often they are from two ova fertilized by two sperm (lower left), resulting in dizygotic twins. Sometimes, however, one zygote splits in two (lower right), resulting in monozygotic twins; if each of these zygotes splits again, the result is monozygotic quadruplets.

Usually, one ovulated ovum with 23 chromosomes and one sperm with 23 chromosomes combine to form a single zygote with 46 chromosomes. Hours later, this zygote duplicates into two cells. Months later, one baby with 46 chromosomes is formed. Sometimes, in the case of monozygotic (identical) twins, hours later one zygote with 46 chromosomes splits into two, and months later, identical twins are formed. Sometimes, in the case of dizygotic twins, two ova with 23 chromosomes each are ovulated. When two sperms with 23 chromosomes each combine with the two ova, two zygotes with 46 chromosomes each are formed. Hours later, each zygote splits into identical cells. Months later, fraternal twins are formed.