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Causes
By Mayo Clinic staffIdentical twins (monozygotic twins) occur when a single fertilized egg splits and develops into two fetuses. The split normally happens during the first 12 days after conception. It's believed that when the fertilized egg splits later than this — usually between 13 and 14 days after conception — separation stops before the process is complete, and the resulting twins are conjoined. The degree to which the egg splits and when the split happens determines how and where the twins will be joined.
But, another theory suggests that two separate embryos may somehow fuse together in early development.
What might cause either scenario to occur is unknown.
- Cunningham FG, et al. Multifetal gestation. In: Cunningham FG, et al. Williams Obstetrics. 23rd ed. New York, N.Y.: McGraw Hill; 2010. http://www.accessmedicine.com/content.aspx?aID=6037146. Accessed Sept. 24, 2010.
- Winkler N, et al. The imaging spectrum of conjoined twins. Ultrasound Quarterly. 2008;24:249.
- Spitz L. First trimester diagnosis of conjoined twins. Prenatal Diagnosis. 2005;25:820.
- Amin S, et al. Live conjoined twins: A rare first trimester diagnosis during emergency department sonography. Emergency Medicine. 2010;39:e105.
- Roque H, et al. Monoamniotic twin pregnancy. http://www.uptodate.com/home/index.html. Accessed Sept. 24, 2010.
- Martinez-Frias ML, et al. Epidemiological and clinical analysis of a consecutive series of conjoined twins in Spain. Journal of Pediatric Surgery. 2009;44:811.
- Kokcu A, et al. Conjoined twins: Historical perspective and report of a case. Journal of Maternal Fetal Neonatal Medicine. 2007;20:349.
- Pajkrt E, et al. First-trimester diagnosis of conjoined twins. Prenatal Diagnosis. 2005;25:820.
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