Don't : "It happens all the time"
in the moment of deep grief and despair this is the last thing she needs to hear. However common it may be please don't make it seem as though her baby was "just like anyone elses" because he or she wasn't which brings me to the "Do"...
Do : "Your baby mattered & was special"
Don't : "Did you happen to..." ( fill in the blank implying it was something she did to cause it)
Do : Ask if there is any way you can help or if she needs anything.
Just listen. to whatever she has to say.
basically what i'm trying to say is just be there for her. However that may look
Don't : "You're young you'll get pregnant again!"
Do : "No matter what anyone says, You are a Mother."
Don't: "You didn't even know him or her yet"
Do : "It's Okay to NOT be okay"
Don't : "It was just a clump of cells" / "It wasn't even a baby yet"
Please just don't.