Along with his brothers, a soldier goes off to war leaving behind his wife and two sons. Six years later he returns to his family after losing both his brothers in action. Something is different though. His wife suspects something, but can’t put her finger on it. She just knows that something is different about her husband. Over the next two years, the family grows by twins (a boy and a girl) and then another girl. Then, in an auto accident, the husband dies and his widow decides that she can now investigate a hunch she has had for some time without upsetting her husband.
That month, she takes all of her children in for their annual checkup and vaccines, and also asks the doctor to check her blood type along with all of the children.
The results, mailed to her (see below) later that week, give her a start as she realizes her hunch was correct.
What was her hunch? How did she arrive at her conclusion?