These things will happen more often than we can imagine. I have an acquaintance who is a midwife, and she had to deliver or help deliver babies at the ER, right after telling the woman she was pregnant!
And it happened to one of my friends, too. She was 40, had 2 teens at home, and no plans for another child. Her gynecologist had even told her she was getting pre-menauposal, so when her period became less regular she did not worry. She didn't have any pregnancy symptom, didn't put on weight... One day, she had weird stomach cramps, so she went to the doctor's and since he couldn't find anything wrong with her, he ended up requesting a sonogram... When she got there, the sonographer quickly ended up seeing a 5 month-old fetus... there was the reason for her cramps!
She was lucky enough to learn the news at that point, so she could prepare for the birth and enjoy the rest of that pregnancy...