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How are twins formed?

Twins form in one of two ways:

Identical twins occur when a single fertilized egg splits into two. Identical twins look almost exactly alike and share the exact same genes. Most identical twins happen by chance.

Fraternal twins occur when two, separate eggs are fertilized by two, separate sperm. Fraternal twins do not share the exact same genes — they are no more alike than they are to their siblings from different pregnancies. Fraternal twins tend to run in some families.

Multiple births can be fraternal, identical, or a combination. Multiples associated with fertility treatments are mainly fraternal.

This answer is based on the source infromation from the National Women's Health Information Center.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.