Having missing or extra chromosomes is a condition called aneuploidy. The risk of having a child with an aneuploidy increases as a woman ages. Trisomy is the most common aneuploidy. In trisomy, there is an extra chromosome. A common trisomy is  trisomy 21 (Down syndrome), there are 3 copies of chromosome 21: abnormal features of the face and body, medical problems such as heart defects, and intellectual disability occur. Many children with Down syndrome live to adulthood. trisomy 13 (Patau syndrome) there are 3 copies of chromosome 13: causes serious problems with the brain and heart as well as extra fingers and toes, cleft palate and lip, and other defects. Most infants with trisomy 13 die within the first year of life. trisomy 18 (Edwards syndrome) there are 3 copies of chromosome 18: causes severe intellectual disability and serious physical problems such as a small head, heart defects, and deafness. Most of those affected with trisomy 18 die before birth or within the first month of life.  Monosomy is another type of aneuploidy in which there is a missing chromosome. A common monosomy is Turner syndrome, in which a female has a missing or damaged X chromosome.It causes a webbed neck, short height, and heart problems but does not usually cause developmental delays.