I start looking for mine at around 38 weeks, which is why I think I've always been cognizant of the fact I've lost it. It's a big glob of blood tinged mucous...comes out (for me) over the course of a couple of visits to the bathroom. With #1 I lost it and labor began within an hour or two. With #2, I lost it and didn't go into labor for a couple of days. And I was walking around at 4 cm dilated! This time, since I'm so far from the hospital, I'm going to use it as an indicator that it's time to go hang out at my mom's house. If she had her way, I'd be there from 38 weeks on. But I can't imagine anything more disruptive to my mental state or my family life!