Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes in the uterus. There are several reasons for the "vanishing" fetus, including it being embodied or absorbed by the other fetus, placenta or the mother. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus. Conjoined twins
Conjoined twins (or the once-commonly used term "siamese") are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12 following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart, liver or lungs. Chimerism
A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras.
Crib death – Sudden infant Death Syndrome (SIDS). Sudden infant and perinatal unexplained death: the pathologists viewpoint. Berlin Heidelberg, Germany: Springer. ISBN 978-3-319-08346-9. Joan Hodgman; Toke Hoppenbrouwers (2004). SIDS. Calabasas, Calif: Monte Nido Press. ISBN 978-0-9742663-0-5. Lewak N (2004). "Book Review: SIDS". Arch Pediatr Adolesc Med. 158 (4): 405. doi:10.1001/archpedi.158.4.405. Archived from the original on 17 October 2008. External links

SIDS at Curlie
"Sudden Unexpected Infant Death and Sudden Infant Death Syndrome". Data and Statistics. Center for Disease Control and Prevention. Retrieved 26 March 2017.