His campaign had released a statement Sunday morning saying Santorum intended to "return to Florida and resume the campaign schedule as soon as is possible." Florida's primary election is Tuesday.
Bella, whose full name is Isabella Maria Santorum, has become a symbol of Santorum's steadfast pro-life stance on the campaign trail.
He has mentioned her in interviews and at events, saying in a video in October: "Bella makes us better."
In a column Santorum wrote for The Inquirer in 2010, he said his daughter's condition requires constant attention and "she is worth every tear."
Trisomy 18 is a relatively common genetic disorder found in one in 3,000 births, according to Victoria Miller, executive director of the Trisomy 18 Foundation, based in Virginia. Most babies die soon after birth or are stillborn, making it "a hidden disorder," Miller said.
Trisomy 18 causes problems throughout the body, involving the brain and heart, as well as respiratory and digestive systems. Weak muscles and other developmental disabilities make it difficult to perform simple tasks - like breathing and swallowing.
Santorum has said that Bella was brought home from the hospital with a prescription for oxygen and that even a cold could prove life-threatening for her.
Only "a small percentage of children" survive past their first year, said Miller, who has no direct knowledge of Bella's medical status. Individual life expectancy depends on how the disorder manifests, she said.
"Every child is different," Miller explained.
