I’m 37 weeks and 2cm dilated. How long until labor starts?

Once you're 36 or 37 weeks along, your OB provider will likely begin checking your cervix at your prenatal appointments to see if there's any progression or change as the pregnancy gets closer to the end. And ideally, in the days to weeks leading up to the delivery of the baby, the cervix will begin to "ripen" (that's an actual medical term that we use), and it's caused by the release of hormones called prostaglandins that act on the cervix and cause it to thin out, move forward in the woman's pelvis, and actually soften in consistency, and the woman might actually dilate a little bit too. And then later on after the woman starts having regular and intense contractions, a ripe cervix should respond to those intense contractions by dilating or opening up. And the true definition of labor is cervical dilation from hour to hour.
So in other words, lets say that the ripening got your cervix to 1 1/2 centimeters, and then you start having regular and intense contractions, and you change from 1 1/2 to 2, to 2 1/2, to 3, to 4, to 5, to 6, to 7, and 8, and 9, and 10 - that's when you're in labor. But it's possible to have a lot of contractions and your cervix may not be changing as a result of those contractions, and you get sent home from the hospital. That's actually very normal too, and there's no way to know if your cervix is dilating as a result of your contractions without going in and being checked out. And it's never a wasted trip. You don't ever have to worry about taking "the walk of shame" (which I've heard a lot of people call it), because that trip helps you to know what's going on and gives you experience so you know when to come in again in the future.
So you had a specific question about your cervical exam when your doctor checked you, and you want to know how long it's going to be until labor starts. And really, no matter what your cervical exam is, whether you were 1 when the doctor checked you or whether you were 5, there really is no good way to tell. There are women who can walk around at 5 centimeters for weeks. And then there are women who are 1 centimeter in the doctor's office in the morning when they go for their appointment and they end up delivering their baby later that night. So really, the best sign of labor you have is regular and intense contractions, so if you are over 37 weeks and you begin contracting every 45 minutes, let it go on for about an hour or 2 as long as you're not having any vaginal bleeding, you're feeling your baby move in between contractions, and you don't feel like your water is broken. And then once you've been contracting that regularly for a couple of hours, then go to the hospital and see if your cervix is changing as a result of those contractions.
But there are reasons (independent of contractions) to go to the hospital like I alluded to before, so if you have any vaginal bleeding, if you feel like your water is broken, or you're not feeling your baby move as much as usual, those are all reasons to go in (in and of themselves), whether you're contracting or not. If you have any specific questions or concerns, it's always a good idea to call your doctor, because they can ask you more specific questions, get an idea for what's going on, and then determine if you need to be seen or not. Good luck with everything, and if you have more questions in the future for me, feel free to ask them on our Facebook page at / intermountainmoms , and recommend us to your friends and family too.