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When two fertilized eggs are implanted in the uterine wall at the same time, dizygotic (DZ) or fraternal twins (also known as “non-identical twins,” “dissimilar twins,” “biovular twins,” and, colloquially in the case of females, “sororal twins”) occur. Fraternal twins are created when two eggs are fertilized separately by two distinct sperm cells. Dizygotic and biovular refer to the formation of two zygotes from two eggs, or ova. Fraternal twins are essentially two regular siblings who happened to be born at the same time, as they are made up of two distinct eggs fertilized by two distinct sperm.

Due to chromosomal crossover during meiosis, dizygotic twins, like any other siblings, will almost always have distinct sequences on each chromosome. Dizygotic twins share just half of their DNA, similar to siblings who were conceived and born at different dates. Like any other siblings, Dizygotic twins may have a similar appearance, especially if they are the same age. On the other hand, Dizygotic twins might have drastically distinct appearances (for example, be of opposite sexes).

There is a genetic tendency for dizygotic twinning, according to studies. However, only the mother influences the likelihood of conceiving twins; there is no known mechanism for a male to trigger the release of the double ovum. Dizygotic twinning occurs at a rate ranging from six per thousand births in Japan (comparable to monozygotic twins) to fourteen or more per thousand in some African countries.

Older moms are also more likely to have dizygotic twins, with twinning chances more than doubling in moms over 35. The rate of fraternals has risen dramatically with the introduction of technology and practices to assist women in becoming pregnant.

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