1. Are identical twins who develop with a single placenta from a single fertilized ovum.
2. Are always the same sex and race.
3. Are more often female than male, at a ratio of 3:1.
4. Occur as often as once in every 40,000 births but only once in every 200,000 live births.
4. Are more likely to occur in India or Africa than in China or the United States.
5. May be caused by any number of factors, being influenced by genetic and environmental conditions. It is presently thought that these factors are responsible for the failure of twins to separate after the 13th day after fertilization.
6. Conjoined twins can be artificially generated in amphibians by constricting the embryo so that two embryos form, one on each side of the constriction.
7. There are no documented cases of conjoined triplets or quadruplets.
8. Up until 1990, surgical separation of conjoined twins has been attempted 167 times
9. Conjoined twins have lived from a few minutes to 63 years.
10. The first recorded conjoined twins were in England in 1100 A.D.
