Identical twins (monozygotic twins) occur when a single fertilized egg splits and develops into two individuals. Eight to 12 days after conception, the embryonic layers that will split to form monozygotic twins begin to develop into specific organs and structures. It's believed that when the embryo splits later than this — usually between 13 and 15 days after conception — separation stops before the process is complete, and the resulting twins are conjoined.
An alternative theory suggests that two separate embryos may somehow fuse together in early development.
What might cause either scenario to occur is unknown.
April 10, 2013
- Gabbe SG, et al. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, Pa.: Saunders Elsevier; 2012. http://www.mdconsult.com/das/book/body/208746819-4/0/1528/0.html. Accessed Feb. 10, 2013.
- Wen XR, et al. Thoraco-omphalopagus conjoined twins: Impact of ultrasound assessment on successful surgical separation. Clinical Imaging. 2013;37:138.
- Mandy GT. Neonatal outcome, complications, and management of multiple births. http://www.uptodate.com/home. Accessed Feb. 10, 2013.
- Jackson OA, et al. Conjoined twin separation: Lessons learned. Plastic and Reconstructive Surgery. 2012;129:956.
- Chen CP, et al. Conjoined twins detected in the first trimester: A review. Taiwanese Journal of Obstetrics & Gynecology. 2011;50:424.
- Rhodes JL, et al. Preoperative planning for the separation of omphalopagus conjoined twins The role of a multicomponent medical model. Journal of Craniofacial Surgery. 2013;24:175.
- Mutchinik OM, et al. Conjoined twins: A worldwide collaborative epidemiological study of the International Clearinghouse for Birth Defects Surveillance and Research. American Journal of Medical Genetics Part C: Seminars in Medical Genetics. 2011;157:274.
- Cunningham FG, et al. Williams Obstetrics. 23rd ed. New York, N.Y.: The McGraw-Hill Companies; 2010. http://www.accessmedicine.com/resourceTOC.aspx?resourceID=46. Accessed Feb. 11, 2013.
- Chasen ST, et al. Twin pregnancy: Prenatal issues. http://www.uptodate.com/home. Accessed Feb. 10, 2013.
- Sharma D, et al. Cephalothoracopagus janiceps asymmetros twins: Antenatal sonographic diagnosis. Journal of Clinical Ultrasound. 2013;00:1.
- Rios LT, et al. Prenatal diagnosis and postnatal findings of cephalothoracopagus janiceps disymmetros: A case report. Case Reports in Medicine. 2012;00:1.
- Roque H, et al. Monoamniotic twin pregnancy. http://www.uptodate.com/home. Accessed Feb. 10, 2013.
- Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. Feb. 15, 2013.
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