This tends to happen during your third trimester, but can also last a few months before going into labor. It's like your body's way of telling you things are going to start happening. That being said, it's not a surefire sign that labor is coming. With my first, I was nesting from 37 weeks on, and I had to have an induction around 39 weeks due to some complications, so it wasn't a sign of labor for me.