When you carry a child in your body for 9 months, you bond with it, more deeply than you can possibly understand. It's a part of you. Giving that child away for adoption is like severing a limb, only worse. That limb doesn't isn't living and you don't worry if it's hungry, cold, being loved, or being appropriately taken care of.Maybe it's a guy thing (as in I'm not a woman so I'll never understand it) or maybe it's a legal issue I'm not aware of but why would she not be able to place the child for adoption. Forgive my ignorance here but I really don't understand the scenario you're presenting.
Hormones play a BIG part in this. Every instinct in your body is screaming to hold on to that baby, to nurse it, love it, and keep it safe.
That said, these hormones generally don't kick in 'til maybe 5-6 months.
For many women, it's far easier to have an abortion and end the pregnancy BEFORE those hormones begin and before you fall irretrievably in love with that child.