Welcome to the forum!
Not certain what you are asking exactly. Are you writing a letter to her that will be your first contact with her? Do you mean that she got pregnant from the affair with you? If so, just so you know, that is not uncommon. Many adoptees find that particular situation.
Don't think that fact really has much to do with how to write that first letter though. I think it is probably a fact that your birth mom isn't too proud of though and I do not think that I would mention it in a first contact letter.
Does that answer your question? If not, give me more info and I will try to give you a better response.
