These changes occur in genes essential for development and often disrupt the development of an embryo in its early stages. Because these mutations have very serious effects, they are incompatible with life. It is important to note that genes themselves do not cause diseases. Genetic disorders are caused by mutations that prevent a gene from working properly. For example, when people say that someone carries the cystic fibrosis gene, they are usually referring to a mutated version of the cystic fibrosis transmembrane conductance regulator gene, which causes the disease. All people, including those without cystic fibrosis, have a version of the cystic fibrosis transmembrane conductance regulator gene. Whereas pregnancies with trisomy (a baby who has received an extra chromosome) or monosomy (having a missing chromosome) can end completely. term and result in the birth of a child with health problems, it is also possible that the pregnancy fails or the baby is stillborn due to the chromosomal abnormality. In studies of first-trimester miscarriages, about 60 percent (or more) have chromosomal abnormalities. In studies of stillborn babies, 5 to 10 percent have one chromosome