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Trisomy 18, the genetic disorder that sent  Rick Santorum's daughter Bella to the hospital Saturday evening, kills about 90 percent of children before or during birth and those that do live past birth suffer serious symptoms.

Children with Trisomy 18, also known as Edwards syndrome, have an extra copy of chromosome 18, which causes symptoms like clenched hands, low birth weight, mental deficiency, small head and jaw and an unusual-shaped chest, according to the National Institutes of Health.

"Most children with Trisomy 18 die in the first three months of life, and only 10 to 20 percent survive past the first year," said Dr. Robert Marion, Chief of Genetics and Developmental Medicine at Children's Hospital at Montefiore and Albert Einstein College of Medicine.

Tests can be done during pregnancy to determine if the child has Trisomy 18. Half of infants with this condition do not survive beyond the first week of life, and the few children who survive to the teenage years have serious medical and developmental problems,  according to the National Institutes of Health.

"Those who survive are almost always girls," said Marion. "The reason for this is that the condition is more lethal in boys, who die intrauterinely."

Dr. Ronald Crystal, chair of genetic medicine at Weill Medical College of Cornell University in New York City, said the disease is "invariably fatal," with survival depending on the severity of symptoms and the quality of care. Bella Santorum, 3, is "already an exception," he said.