Pumping???

So I'm only 22 weeks with my first and I'm lactating. My Ob wants me to start pumping already, but everything I've read has said that it could cause preterm labor. I don't want to force our little guy out before he's ready, so I was wondering what experience anyone else has had with this? I already have an appointment with a new Ob to get second opinion, but I don't want to mess anything up.