The short version of my story is that my first was born at 35 weeks, and my early labor was not stereotypical. I called my doctor and was told the standard story--come in when the contractions are a certain time apart or a certain intensity. I waited because I couldn't clearly time my contractions. I went into transition at home. Baby was born less than 45 minutes after I arrived at the hospital. It's a great story to tell, and ended up being a very positive experience once I was through with the fear.
With my second, I was worried the whole time about the same thing happening, especially because everyone says second babies arrive more quickly (and I had an almost 3 year old at home!). I had some great advice from my doctor to share:
If you're not sure, it's better to go to the hospital too early and be sent home than to deliver a baby on the side of the road. I ended up going to the hospital around 36 weeks to be checked because I started having some of my early labor symptoms from the first time. False alarm, but it was a relief!
Trust yourself. In spite of all my worry, when the time actually came with baby 2, I knew. I'd gone through it before, and recognized what was happening earlier. And I was more attuned to it gradually intensifying, even though I couldn't time the contractions again at first. Labor did progress pretty quickly again, so I was glad I decided to go to the hospital when I did, so there was a lot less drama!
And...Rest while you can.