Most parents would do anything to keep their children safe.
But what can parents do when their child is sick with a deadly disease and requires a bone marrow transplant?
Many in this situation are turning to a controversial procedure in which they choose to have another baby genetically selected from embryos created outside the womb to save the life of their sick child.
The following are several stories of families who made the choice to have a genetically selected baby to save their child's life.
Elisabeth was dying of a disease called Fanconi anemia that wasn't letting her body make healthy blood.
"She had very little time left," her father, John, told ABC News.
Elisabeth needed a bone marrow transplant.
"Without treatment she would have died," Elisabeth's mom, Martina, said.
The best option for Elisabeth's survival were stem cells from a close genetic match. And that's what Michael delivered just in time by being born.
"I held the umbilical cord, and [the doctor] milked it into a small, little cup that I was holding, so we could gather this 1 or 2 ounces of precious blood," John said.
But the fact that the blood from Michael's cord was able to save his sister is no accident because Michael was no accident.
He was genetically selected for birth from more than 40 embryos his parents conceived outside the womb using a process called preimplantation genetic diagnosis.
Dr. John Wagner, director of the Division of Hematology-Oncology and Blood and Marrow Transplantation at the University of Minnesota, pioneered the procedure in which a mother is given fertility drugs to make an increased amount of eggs to better the odds of creating an embryo with the right DNA.
Michael was chosen for life over more than 40 other potential siblings because he had what his family needed. The "unchosen" remain in deep freeze or were donated for research.
John Hartmann acknowledges the ethics of choosing which embryo will be born can be debated.
"If you have a dying child, I'm not saying it's OK to do anything, but I think it's OK to use reasonable ethical measures to save your child's life," he said of his family's decision.
But Michael is loved just for being Michael, though his parents know that if they hadn't had Michael, they'd no longer have Elisabeth.
John and Lisa Nash knew something was wrong as soon as their daughter, Molly, was born. Before they even held their newborn baby, doctors whisked her away.
Molly had no thumbs, no hip sockets, two holes in her heart and was deaf in one ear.
She was afflicted with the same disease as Elisabeth Hartmann, Fanconi anemia. In Molly's case, the disease would likely lead to bone marrow failure and leukemia. She was not expected to live past the age of 10.
Caring for Molly grew increasingly difficult for her parents. Her disease caused severe digestive problems for Molly, who had trouble eating without vomiting. Doctors had to put a feeding tube in her and perform multiple surgeries on her hands to make them functional.
When Molly was about 3½ years old, her bone marrow started to fail. A bone marrow transplant seemed impossible when a matching donor could not be found.
"We were so brokenhearted, and I really was scared that Molly might not live," Lisa told ABC News' "20/20" in 2001.