If Grandma had an “oops” baby, try and determine if it was really her “oops” baby or whether it was the baby of one of her older daughters. Sometimes the “oops” wasn’t Grandma and Grandpa’s fault. While it’s possible your great-grandma had her tenth child when she was in her very late forties, it’s also possible that it was actually her grandchild.
But don’t conclude it was actually one of her daughter’s babies until you have some evidence. A suspicion alone does not count.