Rick Santorum’s Daughter’s Diagnosis: What Is Trisomy 18?

Rick Santorum's daughter has trisomy 18: the disorder and its prognosis.

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A supporter of Republican presidential hopeful and former U.S. Senator Rick Santorum wears a pin with a photo of the candidate's daughter, Bella, which reads "Go Dad! Love, Bella," while Santorum gives a speech on Jan. 3 in Johnston, Iowa.

Republican presidential candidate Rick Santorum is facing some tough challenges both on the campaign trail and at home. His daughter Isabella, or “Bella,” 3, was hospitalized over the weekend with pneumonia, a common complication for children who are born with the rare genetic condition known as trisomy 18.

During a call with Florida voters on Sunday, Jan. 29, Santorum said his daughter was recovering: “She without a doubt has turned a corner.”

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Trisomy 18 causes patients to have three copies of chromosome 18 instead of two. The additional DNA can lead to medical and developmental problems affecting the brain, heart and other organs, which make it difficult for most affected infants to survive past the first few weeks. Fifty percent of infants born with the disorder die within the first week of life. Only 5% to 10% of babies born with trisomy 18 survive their first year.

Prenatal testing can detect trisomy 18. Like Down syndrome, which is caused by an extra copy of chromosome 21, the risk of the condition increases with maternal age. Trisomy 18 is three times as common in girls as in boys. Early signs of the genetic condition include a hole or cleft in the baby’s iris, separation of the rectal muscle and hernia.

Children with trisomy 18 have compromised immunity, which makes them especially vulnerable to common infections like colds or flu. In addition, patients often have potentially fatal heart defects and kidney abnormalities.

The extra genes also disturb the exquisitely regulated process of development, leading to mental deficiencies, low birth weight and physical defects such as a small head, underdeveloped fingernails, abnormally shaped chest and feet that are rounded rather than flat.

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The cause of the extra genetic material in trisomy 18 isn’t clear, but it can most likely be traced to random errors in the way DNA is split and copied when egg and sperm cells are formed. When these egg and sperm cells meet during fertilization, the additional genes are passed on to the fetus.

Santorum returned to campaigning on Monday after canceling appearances scheduled for the weekend, but he said his daughter’s condition, which leaves her susceptible to continued infections, was “gut-wrenching.” He said Bella continues to have difficulty breathing.

Santorum’s wife Karen takes care of their daughter full time, prepared for but hoping against the next emergency. In an interview with the Washington Post last year, he said, “You think she’s fine, and then one cold and she’s this close to dying.”

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.

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