If I understand the question it is asking what blood type the baby likely will have. All I can think of is if someone needs an organ, blood or tissue match for donation or transplant. But, people who need it for that purpose I can’t imagine would be asking on askvillem because something pretty serious medically would be going on and a doctor or geneticist would be consulting with them about the chances a new baby would be a match.
If the person is wondering if there were adopted themselves.
Someone mentioned Rh factor, which is a concern if the mother is Rh negative, but usually not a concern with a first pregnancy. This sort of thing would be discussed with a doctor also, so I doubt the questions are about Rh factor.
There are some people who believe your blood type determines some other traits and they believe in eating a diet associated with their blood type.
If they are asking about paternity I don’t see why they would not ask it with wanting to rule a possibilty out. Like if one or both parent is AB you can’t have an O baby. Two A parents can have an A or O baby, but impossible to have a B or AB baby. The basics rule possibilites out more than it can give a conclusive answer what the baby’s blood type will be.