Trisomy 18 has been in the news lately because it's the chromosome abnormality affecting GOP presidential candidate Rick Santorum's daughter, Bella. What is it?

Trisomy 18, also known as Edwards syndrome, is a chromosomal abnormality in which a child has a third copy of chromosome 18 instead of two - one from each parent, according to the National Institutes of Health.

Bella Santorum, 3, has been hospitalized twice during her father's campaign for the presidency, causing him to cancel campaign events to tend to his daughter. 

The first hospitalization was in January, just as her father started to build momentum with a win in the Iowa caucuses, and the second occurred Friday. Bella is out of the hospital, according to the Huffington Post.

Trisomy 18 is considered to be a common type of trisomy syndrome, with about 1 in 5,000 babies born with the abnormality, according to WebMD. Most babies with the condition are female.

Symptoms of the abnormality include clenched hands, low birth weight, mental deficiency and low-set ears, among other signs, according to the NIH. Others include cleft palate and clenched fists, according to WebMd.

Trisomy 18 is usually diagnosed via ultrasound, although observation of a newborn's face and body can reveal signs of the condition.

Prognosis for Trisomy 18 is poor, with half of all infants not reaching their second week of life, according to the NIH. Children with Trisomy 18 who reach adolescence do so with serious medical and developmental problems, the website says.

On his campaign website, Santorum talks about his sweet Bella and includes a video all about his 3-year-old daughter.

Santorum said his family was touched by the tremendous outpouring of support we received for our sweet Bella after she had a recent surgery.