Conjoined Twins

Hi Everyone!

For my very first blog I have chosen to share some knowledge on conjoined twins. So here goes!

Conjoined twins is an extremely rare phenomenon and occurs in about one in 200,000 births. Around 40-60% are stillborn and only 35% survive longer than one day. It is not known why, but females tend to have higher chances of survivability than their male counterparts despite most conjoined twins in the womb being male.

Courtesy of tvtropes

Question.
How are conjoined twins formed?

Conjoined Twin Formation

Let's start off with non-conjoined twins.

There are two ways twins are formed
1. When two eggs become fertilized (fraternal twins)
2. When one egg divides after fertilization (identical twins)

For conjoined twins one egg becomes fertilized however, the developing embryo in the egg does not fully separate. Conjoined twins are always the same sex because they are genetically identical and tend to be classified by the location at which they are attached.

Conjoined Twin Classification

The most common attachment location is the upper portion of the torso which are called thoracopagus twins. These twins share the same heart making the separation process nearly impossible. About 40% of all conjoined cases are thracopagus twins. The second most common attachment location is from the breast bone to the waist (omphalopagus twins) making up about 33% of all conjoined cases. The rarest type of conjoined twin is craniophaus twins or twins that are attached at the head making up about 2% of all conjoined cases.

History

One of the most famous conjoined twins were Eng and Chang Bunker born in Thailand (called Siam during that time) in 1811 hence where the term Siamese originated. They were well known internationally and eventually settled in the United States where they married two sisters and fathered 21 children. Eng and Chang lived to be 63 years old.

 Eng and Chang Courtesy of jillytracy.com