![From 'Monsters' to Modern Medical Miracles: selected moments in the history of conjoined twins from medieval to modern times. Banner containing six icons that link to the individual pages of the website. From left to right: 1. Image of three embryos, that links to the Embryology page. 2. Image of conjoined twins from a 15th century book, that links to the Age of Superstition page. 3. Image of Millie-Christine McCoy, that links to the Marvels on Exhibit page. 4. Image of Clara and Alta Rodriguez, that links to the Separation Surgeries page. 5. Image of a fine engraving of two sets of conjoined twins, that links to the Gallery of Images page. 6. Image of a title page, that links to the bibliography page. Below the banner, to the left of the links to each page is a large eighteenth century woodcut illustration of female twins joined at the back.](images/conjoined.png)
Embryology of Conjoined Twins
Identical twins develop when a single fertilized egg, also known as a monozygote, splits during the first two weeks of conception. Conjoined twins form when this split occurs after the first two weeks of conception. The monozygote does not fully separate and eventually develops into a conjoined fetus that shares one placenta, one amniotic sac, and one chorionic sac. Because the twins develop from a single egg, they will also be the same sex. The extent of separation and the stage at which it occurs determine the type of conjoined twin, i.e., where and how the twins will be joined.
The diagram at left illustrates the germ layers in embryos of conjoined twins. The proximity of the segments determines how much shared tissue there will be. The further apart the segments, the greater the likelihood that the organs will develop fully in each fetus. If the segments are at their farthest point, there will only be a minimum of tissue and cartilage joining the twins, i.e. omphalopagus twins will develop.
Classification of Conjoined Twins
Isidore Geoffroy Saint-Hillaire was the first teratologist to classify conjoined twins, using Greek etymology to describe the twins in terms of their shared anatomy. Many of his terms are still in use today. Listed below are some commonly occurring types of conjoined twins, examples of which may be viewed throughout this exhibit.
Last Reviewed: March 11, 2024