Biological bond

How much stock do you put in the idea that there truly is a physical/psychological connection between a biological mother and child after birth? I want to believe that's true, but I haven't been able to find any scientific evidence of that. I've found a lot on just moms ( like the experiment where kids could pick their mother out of a lineup just by smell) but nothing about birth mothers specifically. The bonds talked about after birth could just be a result of bonding, which birth moms don't get. What do you think?