How come Edward could hear Renesmee's thoughts before she was born?

Well, in my opinion, it's a pro-life thing. Since, Stephenie Meyer is a Mormon, and Mormons are usually pro-life, this could possibly be bringing a pro-life message as well as a message like, "The fetus loves you, so don't kill it, even if it's going to kill you."

A fetus does think, actually. They have brains.

Babies don't just suddenly start thinking as soon as they come out of their mother. Their brains develop in the womb. Some people even have actual memories of still being inside their mother.

Edward didn't start hearing her until her brain developed and she started thinking.

Actually, according to linkbabies can understand what's going on around them, even to the point where they can show detestation to certain sounds.

Nessie wasn't a normal baby (hence why she sprouted four years in the space of a few months/weeks). Smeyer's logic was (probably) that Nessie's fast development started in the womb. It, in a way, makes sense that she would pick up words, understand their meaning, and be able to form coherent sentences. It was supposed to show how different she was, how fast she grew and understood things.

I dont know...But if my father told me he could hear my thoughts...I would tell him to get help.

Welcome to the World of Twilight.

It's half human, half vampire. It grows at a rapid speed. By the time Edward heard the baby, it must have had thoughts already. It's strong also because it's half vampire.