Editing Twin (section)

===Mirror image twins===
Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9–12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs ([[situs inversus]]).<ref>{{cite journal |vauthors=Gedda L, Sciacca A, Brenci G, etal |title=Situs viscerum specularis in monozygotic twins |journal=Acta Genet Med Gemellol (Roma) |volume=33 |issue=1 |pages=81–5 |year=1984 |pmid=6540028|doi=10.1017/S0001566000007546 |doi-access=free}}</ref> If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image.<ref>[https://www.proactivegenetics.com/faq-twins-genetics.html#55 Twins genetics, details of environmental impacts] {{Webarchive|url=https://web.archive.org/web/20210831070813/http://www.proactivegenetics.com/faq-twins-genetics.html#55 |date=2021-08-31}}. Proactivegenetics.com. Retrieved on 2013-08-10.</ref> The term "mirror image" is used because the twins, when facing each other, appear as matching reflections.<ref>{{Cite news|url=https://wstwinregistry.org/2015/10/01/what-are-mirror-image-twins/|title=What are mirror image twins? {{!}} Washington State Twin Registry {{!}} Washington State University|date=2015-10-01|work=Washington State Twin Registry|access-date=2018-09-05|language=en-US|archive-date=2018-09-05|archive-url=https://web.archive.org/web/20180905141034/https://wstwinregistry.org/2015/10/01/what-are-mirror-image-twins/|url-status=live}}</ref>