Chaney and Lagos aren't twins but the bone marrow transplant that saved Lagos' life from cancer had the side effect of making her immune system compatible enough with that of her sister's for her to accept the ovary.
"The reason they were perfect, is because Joy had already gone through the immune suppression that would be necessary for a non-identical transplant," said Dr. Sherman Silber, who performed the surgery.
"There was no question mark about it. We knew there would be no fear of rejection," said Silber, who is also the director of the Infertility Center of St. Louis at St. Luke's Hospital in Chesterfield, Mo.
Chaney says she did not hesitate at the chance to allow her sister to have a natural pregnancy.
"She's my sister," Chaney said. "I think anyone in that same circumstance would do the same thing; just not many people are put into that circumstance. I get to be the lucky one to have all these great experiences."
ABC News' Dan Childs contributed to this report