A: You and your fiance must first be married. The natural/biological father's consent is necessary except under limited and specific circumstances as they may exist at the time the Petition for Adoption is filed with the Probate Court.
A: Of course you will heed the services of an experienced, competent family law attorney. As a "legal stranger" to the child, in order to obtain custody you would have to establish to the court's satisfaction that it would be detrimental to the child to be placed with either her father or her mother. This is called parental unsuitability.