Etiology of Conjoined Twins – Madison Kramp

Since conjoined twins are such a mystery to science, I didn’t know exactly how to create a project to demonstrate how complex it really is. There are so many types of conjoined twins, which is why I chose to do a poster. I was able to write down uncommon facts, if surgery is an option, how common this disorder is, and what we do know about the development of these twins. 

With my project I made sure to cover three different course objectives. I covered the major steps of fetal development, the disorders during pregnancy and their cause, and touched base on the objective cleave, blastocyst formation, implantation, placental formation, gastrulation, organogenesis, and birth. I mostly described the cleavage of an embryo. In my project I described the steps of cell division for an embryo. I added an image to better demonstrate my description. Type A, B, and C relate to whether an embryo becomes a monozygotic (identical) or dizygotic (fraternal) egg. Conjoined, or siamese, twins are a type of fetal abnormality. It occurs when the two fetuses do not properly separate, as well as when they share a placenta and amniotic sacs. In typical identical twins (monozygotes), A placenta is shared but they each have their own amniotic sacs. This is hypothesized by the fission and fusion theory. The fission theory proposes the fertilized egg splits but doesn’t fully separate, resulting in two attached embryos. The fusion theory proposes the embryos properly split, however fuse together later in development. Even with these two theories, scientists still do not fully know the cause of conjoined twins, therefore there is no way to prevent conjoined twins from happening during pregnancy. Due to the twins being joined, they are frequently delivered via c-sections. An interesting fact is the mortality rate of these twins. Not simply due to their condition, but due to separation procedures – the timing and accuracy, and the neurological, urogenital, circulatory, and musculoskeletal development. Most of the time, the organs that are shared cannot properly function to support 2 lives. The separation of these twins takes a huge amount of time to prepare. There can be no to very few surprises to the surgeons if the surgery is to succeed.

Here are a few ways in which twins can be conjoined:

• Cephalopagus: Conjoined at the top of the head to the belly button, facing each other. (typically not viable)
• Thoracopagus: Most common. Conjoined at the chest and umbilicus and has a very low survival rate/quality of life.
• Omphalopagus: Share a portion of the abdominal wall and gastrointestinal tract. They can be separated and have the best chance at survival.
• Ischiopagus: Joined at the pelvis and facing each other. 
• Parapagus: Fused side-by-side at the pelvis and share most organs. The image below the cell division shows this.
• Craniopagus: Different from cephalopagus, these twins have their own complete body’s yet are only connected at the head. After separation, both twins go on to lead normal lives.

Conjoined twins are an embryological abnormality that occurs during development. Scientists are still working to discover more, therefore more anomalies are still being discovered. As medicine has continued to advance, those in the healthcare field are learning more ways to increase life expectancy and create technologies to help these twins live a longer and more comfortable life. So far we can already do certain separations on viable twins. Who knows what else is possible. 

Bindlish, A., & Sawal, A. (2022, September 24). A detailed description and discussion on conjoined twins. Cureus. https://pmc.ncbi.nlm.nih.gov/articles/PMC9595239/#REF1 

Conjoined twins: Definition, causes & outlook. Cleveland Clinic. (2025, March 19). https://my.clevelandclinic.org/health/diseases/22895-conjoined-twins 

Kaufman, M. H. (2004, July 27). The embryology of conjoined twins – child’s nervous system. SpringerLink. https://link.springer.com/article/10.1007/s00381-004-0985-4