Q.: "Is there any reason why twins are conjoined ? Is there any way to prevent it before birth?"
As with all types of monozygotic twins, there is no clearly identified reason to explain why some fertilized eggs split, developing into two indiviuals. Nor is there any explanation as to why the split would be delayed or incomplete, resulting in conjoined twins. Thus, there is no known way to prevent conjoined twinning.
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. (If you have a question that is not answered here, please e-mail me: multiples.guide@about.com.)

