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Monozygotic Twins

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Updated April 28, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Definition:

Monozygotic twins are formed when one zygote, created with one egg and one sperm, splits into two, leading to two embryos. Each of those embryos then develops as a separate fetus. Monozygotic twins are also known as identical twins, because they share practically identical genes. This means that monozygotic twins are the same sex, share physical characteristics and even may have similar personalities.

It was once thought that monozygotic twins shared the exact same DNA, but in fact, environmental factors can influence DNA over time. Some studies have found that slight DNA differences are more common in older identical twins than in very young identical twins. This is likely due to spending more time apart and therefore being exposed to different environments.

Contrary to popular belief, your chances of having monozygotic twins are not related to your family history. If there is more than one set of identical twins in a family, it is more likely due to luck or external environmental factors - but not family genetic history. (It's unclear which environmental factors may increase the risk of monozygotic twins, but the research is ongoing.)

Twins who are conceived with the help of fertility treatments are generally dizygotic, or non-identical twins. However, IVF treatment may slightly boost your chances of having identical twins. Why this occurs is unclear.

Semi-Identical Twins and Conjoined Twins

A rare form of monozygotic twins is semi-identical or half-twins. This occurs when one egg is fertilized by two separate sperm.

Conjoined twins are another rare form of monozygotic twinning, where the twins do not fully separate when the zygote splits. They may share multiple organs. Most conjoined twins die in utero or are stillborn.

In some cases, conjoined twins who survive may be surgically separated, but this surgery is risky and cannot always be attempted or completed successfully.

Monozygotic Twins, Their Amniotic Sacs and Placentas

Most of the time, monozygotic twins have separate amniotic sacs but share one placenta. The technical term for this is monochorionic-diamniotic (or Mo-Di), and it occurs between 60% and 70% of the time with monozygotic twins. Sharing one placenta increases risks to the pregnancy, due to the possibility of twin-to-twin transfusion syndrome. The pregnancy must be watched very carefully if Mo-Di twins are diagnosed.

Another possibility is for the twins to each have his or her own placenta and amniotic sac. This is known as dichorionic-diamniotic (or Di-Di) twins. The risks in a Di-Di pregnancy are lower than with a Mo-Di pregnancy. There is a misconception that Di-Di twins are always fraternal (non-identical), but this is not true. About 30% of monozygotic twins are Di-Di.

The riskiest combination is when twins share one amniotic sac and one placenta. This only occurs in monozygotic twins and never with non-identical twins. This is known as monochorionic-monoamniotic (Mo-Mo) twins, and it is the relatively rare, occurring in only 5% of twin pregnancies.

With Mo-Mo twins, the biggest risk is that the babies can become entangled in the umbilical cords. There is also a risk of twin-to-twin transfusion syndrome and a higher risk of prematurity. Early studies found that only 50% of Mo-Mo twins survived, but later studies have found more encouraging results, with the perinatal (the period immediately before and after birth) mortality rate closer to 20%.

More about twins:

Sources:

Conjoined twins. Mayo Clinic. Accessed online November 10, 2011. http://www.mayoclinic.com/health/conjoined-twins/DS00869

Cordero L, Franco A, Joy SD. "Monochorionic monoamniotic twins: neonatal outcome." J Perinatol. 2006 Mar;26(3):170-5.

Mario F. Fraga, Esteban Ballestar, Maria F. Paz, Santiago Ropero, Fernando Setien, Maria L. Ballestar, Damia Heine-Suñer, Juan C. Cigudosa, Miguel Urioste, Javier Benitez, Manuel Boix-Chornet, Abel Sanchez-Aguilera, Charlotte Ling, Emma Carlsson, Pernille Poulsen, Allan Vaag, Zarko Stephan, Tim D. Spector, Yue-Zhong Wu, Christoph Plass, and Manel Esteller. "Epigenetic differences arise during the lifetime of monozygotic twins." Proc Natl Acad Sci U S A. 2005 July 26; 102(30): 10604-10609.

Racowsky ,Catherine; Schlegel, Peter N.; Fauser, Bart C.; Carrell, Douglas T. Biennial Review of Infertility: Volume 2, 2011. Publisher: Springer; 1st Edition. edition (June 9, 2011)

Roqué H, Gillen-Goldstein J, Funai E, Young BK, Lockwood CJ. "Perinatal outcomes in monoamniotic gestations." J Matern Fetal Neonatal Med. 2003 Jun;13(6):414-21.

Also Known As: identical twins
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  4. Fertility Treatments: Clomid, Fertility Drugs, IUI, and IVF
  5. Twins, Triplets, and Higher Order Multiples
  6. Monozygotic Twins - Identical Twins Definition

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