Identical twins

Identical twins start out as genetically identical: they have the same alleles. They both share the same sack in the womb. They are always of the same sex, and are monozygotic or MZ twins (mono = one; zygote = fertilised egg). This contrasts with fraternal twins, who are formed by two separate eggs fertilised by two separate sperms, and who are not always the same sex (DZ = dizygotic). Both types of twin are carried in the same uterus at the same time, so their birth environment is the same.

Research shows that the frequency of monozygotic twinning is one in 240 births. Fraternal twins are twice as common.^[1] In vitro fertilization (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.^[2]

1. ↑ King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics, 7th ed. Oxford. p461
2. ↑ "Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause" (Press release). European Society of Human Reproduction and Embryology. 2007. Archived from the original on 2007-09-21. Retrieved 2008-09-30.