But, what is he to that fetus? Biologically, he is the father, isn't he? When the fetus is born, how does it suddenly become his? What was that fetus to him BEFORE it was born? The fetus does not change DNA when born.
The woman is biologically the mother, isn't she? What else can she be to the fetus? A host? What kind, though? The fetus IS biologically hers, so what does that make her to that fetus?
The word "expecting" does not make sense. They ARE expecting the BIRTH of their child however. The being inside the woman DOES exist. It is HERE in a sense that it is still inside the woman. It is alive. It shares both the parents DNA.
Many people also say they are CARRYING their child. Well, of course, because their child is still in them.
I believe the woman is the mother of that fetus, just as she is the mother of her infant, toddler, adolescent, etc. Fetus is the name of a stage of development. Fetus means "unborn offspring".