Conjoined twins are two babies that are born physically connected to each other.
Conjoined twins develop when an early embryo partially separates to form two individuals. Although two fetuses will develop from this embryo, they will remain physically connected — most often at the chest, pelvis or buttocks. Conjoined twins may also share one or more internal organs.
Most conjoined twins are stillborn or die shortly after birth. Some surviving conjoined twins can be surgically separated. The success of surgery to separate conjoined twins depends on where the twins are joined and how many and which organs are shared, as well as on the experience and skill of the surgical team.
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.
There aren’t any specific signs and symptoms that indicate a woman is carrying conjoined twins. As with other twin pregnancies, the uterus may grow more rapidly than expected, and mothers of twins may also have more fatigue, nausea and vomiting early in the pregnancy.
Conjoined twins are usually classified according to where they’re joined, and there are many ways that conjoined twins may be connected. Some of the more common ways include:
- Joined at the chest. One of the most common of conjoined twins, thoracopagus twins are joined at the chest. They often have a shared heart and may also share one liver and upper intestine.
- Joined near the bellybutton. Omphalopagus twins are joined near the bellybutton. Many omphalopagus twins share the liver, and some share the lower part of the small intestine (ileum) and colon. They generally do not, however, share a heart.
- Joined at the base of the spine. Pygopagus twins are joined at the base of the spine and commonly face away from one another. Some pygopagus twins share the lower gastrointestinal tract, and a few share the genital and urinary organs.
- Joined at the pelvis. Ischiopagus twins are joined at the pelvis. Many ischiopagus twins share the lower gastrointestinal tract, as well as the liver and genital and urinary tract organs. Each twin may have two legs or in some cases, one pair of legs and even a fused leg, though that’s uncommon.
- Joined at the head. Craniopagus twins are joined at the head. Craniopagus twins share a portion of the skull, and possibly brain tissue. This sharing may involve the cerebral cortex — the part of the brain that plays a central role in memory, language and perception.
In rare cases, twins may be asymmetrically conjoined, with one twin smaller and less fully formed than the other (parasitic twins).