Moment of Truth: A Child Too Many

I absolutely loved this movie. The only thing I didn't quite understand was the conception process. I know that Bill's sperm was used, but is Patty the biological mother along with being the birth mom?? It seems quite strange because now a days in vitro fertilization is used more often.Since Bill's wife had a hysterectomy then that would mean that she would have no more eggs but they could have also purchased donated eggs, then used Bill's sperm, then implant the embryo into Patti. For me it seems quite strange that they would use Bill's sperm and Patti's egg to make a baby, that would mean that she is the biological mother, I don't understand how she could give up her baby even if it is not her husband's child.