| twins, dizygotic | Two offspring produced in the same pregnancy from separate ova fertilised at the same time. (12 Dec 1998) |
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| twins, monozygotic | Two offspring developed from one fertilised ovum. (12 Dec 1998) |
| uniovular twins | Identical twins, twins which are the result of a single zygote (fertilized egg) splitting into two cell masses and becoming two individuals. The twins are genetically identical and are always of the same sex (both males or both females). Compare: dizygotic twins. (09 Oct 1997) |
| fraternal twins | Fraternal twins are siblings who have shared a common uterine environment. They are due to fertilization of two different ova by different sperm. Fraternal twins are also called dizygotic twins. (12 Dec 1998) |