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Causes
By Mayo Clinic staffIdentical twins occur when a single fertilized egg splits and develops into two fetuses. The split normally happens during the first 12 days after conception. When the fertilized egg splits later than this — usually between 13 to 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.
What causes the egg to separate or why this sometimes occurs later than normal isn't clear, but experts suspect that it may be due to a number of genetic and environmental factors.
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- Carnevale FC, et al. Importance of angiographic study in preoperative planning of conjoined twins: Case report. Clinics. 2006;61(2): 167-170.
- Pajkrt E, et al. First-trimester diagnosis of conjoined twins. Prenatal Diagnosis. 2005;25(9):820-826.
- Kaufman M. The embryology of conjoined twins. Child's Nervous System. International Society for Pediatric Neurosurgery. 2004;20(8-9):508-525.
- Gilbert S. Types of conjoined twins. In: Gilbert S. Developmental Biology. 8th ed.
- Sunderland, Mass.: Sinauer Associates, Inc; 2006. http://8e.devbio.com/article.php?id=112&search=conjoined%20twins. Accessed Sept. 2, 2008.
