Question: Conjoined Twins FAQ: How Does it Happen?
Answer: Monozygotic-- also known as identical -- twins form when a single fertilized egg splits into two embryos. This split can happen anytime after fertilization; as more time passes, the twins are more likely to share commonalities.
If the split occurs more than twelve days post conception, the embryos may not fully divide and may form with a connection. Or, the "fusion" theory of conjoined twinning suggests that stem cells from one twin adhere to like stem cells from the other twin, fusing together and developing conjoinedly. Like all monozygotic twins, conjoined twins are always same sex (either both boys or both girls). It is estimated that seventy percent of conjoined twins are female.
Conjoined twins, also known as Siamese twins, are a rare type of monozygotic (identical) twins. Find more answers to Frequently Asked Questions About Conjoined Twins.
