Conjoined twins are a medical phenomenon resulting from the incomplete splitting of a single fertilized egg. This rare occurrence leads to two individuals who are physically connected at birth and may share internal organs.
The Biological Mechanism of Formation
Conjoined twins are always monozygotic, originating from a single fertilized egg, just like identical twins. The process is theorized to be an incomplete fission of the embryonic disk after fertilization. For typical identical twins, the zygote splits completely within the first twelve days following conception. In conjoined twins, the separation of the inner cell mass occurs later, usually between 13 and 15 days post-fertilization, after embryonic layers have begun to form. This late and incomplete splitting determines the location and extent of the physical connection and shared anatomy, though the exact cause for the incomplete separation remains unknown.
Global Incidence and Statistical Rarity
The estimated occurrence ranges broadly from approximately 1 in 50,000 to 1 in 200,000 live births globally. The overall prevalence at conception is higher, estimated at about 1 in 50,000 pregnancies, but a large percentage result in miscarriage or stillbirth. The condition appears to affect females more frequently than males, often cited as a three-to-one ratio. While rare globally, some studies suggest variances in incidence across different regions, with slightly higher rates noted in areas like southwest Asia and Africa.
Anatomical Classification of Conjoined Twins
Conjoined twins are categorized based on the primary site of their physical connection, with classification terms derived from Greek prefixes describing the joined body part. The most common types involve the chest and abdomen, accounting for the majority of cases.
- Thoracopagus twins are joined face-to-face at the chest and share a common sternum, diaphragm, and upper abdominal wall. This type, along with omphalopagus, represents about 75% of all cases and frequently involves shared vital organs, including the heart.
- Omphalopagus twins are joined at the anterior abdominal wall, typically from the breastbone down to the navel. They often share a liver, parts of the digestive tract, and sometimes the diaphragm, but generally have separate hearts.
- Pygopagus twins are joined back-to-back at the pelvis and sacrum, often sharing the lower gastrointestinal tract and parts of the genitourinary system.
- Ischiopagus twins are joined at a single bony pelvis, which may result in shared lower limbs and a fused lower intestinal tract.
- Craniopagus twins are the least common type, accounting for about 2% to 6% of cases, and are fused at the skull, but not the face or neck. Their brains may or may not be connected.
Survival Rates and Factors Affecting Viability
The prognosis for conjoined twins is generally poor, reflecting the severity of their condition and anatomical complexity. Estimates suggest that between 40% and 60% of pregnancies result in stillbirth, and approximately 35% of those born alive die within the first 24 hours. The extent of shared vital organs is the primary factor determining viability and long-term outcome. Twins who share a single heart have a very low chance of survival, while those with separate vital organs have substantially greater chances following surgical separation. Only a small fraction of live-born conjoined twins, estimated at about 25%, live long enough to be considered candidates for separation surgery.