You get admitted if you are 4cm or more. They might stop your labor if you are before 37 weeks too.
They usually base it off of your contractions though. They advise to come in when you reach active labor, which is when your contractions are 5 minutes apart. If they aren't, and you're still in early stages of labor, theyll most likely send you home. Hope this helps! I took a class at my hospital taught by nurses and labor&delivery experts so that's how I know. Goodluck! :)