My understanding is that it is quite difficult for Americans to adopt from Mexico. It appears that there is a six month period during which the child must live with the prospective parents before the adoption is final and the child may not leave Mexico during that time.
Mexico Adoption - International, Agencies, Programs, Information, Costs -
Just a note: It might make some feel uncomfortable that your
first thought when you hear of a 16 yo being pregnant is that you can adopt her child. It kinda sounds predatory. I hope that it was just an unfortunate choice of words here.