My mom is my stepmother. She married my dad when I was four and my sister was two. She fed us when we were sick, got me through my jackass teenage years, and I talk to her nearly every other day still. She's my mom. She had three more children of "her own", so to speak, but my sister and I are just as much her kids as our other siblings. My biological mother and I haven't spoken in nearly a year. God willing, we never will again.
My wife's dad is her stepfather. Her biological father hasn't been in the picture since she was a year old. The man that raised her, though, he's her dad.
Pregnancy is only a tiny, tiny part of raising a child. Sure, it's special and magical, but giving yourself heart and soul to care for an innocent soul who bears no responsibility for the suffering they will have due to whatever choices their biological parents make has its own magic.
I love my kids to death, and I wouldn't trade them for anything, but I do sometimes wonder how it would have been to adopt like we had discussed originally. Instead, I focus on the two that I have plus my little nephew, also growing up fatherless. But those souls that make that sacrifice based on pure love, with no biological impulse at all? Those people qualify as "saints" in a very real and totally non-religious sense.