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.
Bella Santorum was diagnosed with Trisomy 18 several hours after her birth. Santorum has spoken about the agony that followed, including arguments he and his wife, Karen, had with doctors who wanted the family to allow their youngest daughter to die. They ultimately entrusted Bella's care with Thane A. Blinman, a surgeon at CHOP. Blinman has had good results with several other Trisomy 18 patients, Santorum has said.
At a conservative forum in November, Santorum related several serious health scares involving Bella, including when she, at 5 months old, stopped breathing while he was caring for her. His wife, a nurse, resuscitated Bella, and she later recovered.
Santorum said he was motivated to run for president in part to repeal the federal health-reform law conservative critics have dubbed "Obamacare," saying it would make it harder for children like Bella to get treatments.
He also told CNN's Christiane Amanpour that Bella has inspired him to "make sure we have a country that respects her life."
Santorum has largely kept his daughter off his campaign schedules, preferring her to stay home with his wife. But Bella did join him for a few days leading up to Iowa's straw poll in August, and she joined her family in Charleston, S.C., earlier this month for that state's primary.
Bella is not the first of the Santorums' children with serious health challenges.
In 1996, son Gabriel was diagnosed with a fatal birth defect while still in utero.
The couple decided to proceed with the pregnancy, and Karen developed a life-threatening infection when she was 20 weeks pregnant, forcing an early delivery.
Gabriel died two hours after birth.
Their experience is chronicled in Karen Santorum's 1998 book, "Letters to Gabriel: The True Story of Gabriel Michael Santorum."
On Sunday, Santorum's oldest daughter, Elizabeth, was campaigning in his stead - though the candidate still had two evening "tele-town hall" events with Florida and Minnesota voters that were scheduled to go forward that night.
This article contains information from The Associated Press.
Contact staff writer Anthony Campisi at 215-854-5015, acampisi@phillynews.com or @campisia on Twitter.
