I don't know about you, but my last few weeks of pregnancy felt like they lasted forever. So I'd say it's pretty damn natural to wonder when you'll go into labor and, perhaps more importantly, how you'll know if you're in labor for real. Your doctor or midwife might check to see if your cervix has changed, which can be a sign that your baby is about to arrive, but how many centimeters do you have to be dilated to be in labor and get that delivery show on the road?
According to the American Congress of Obstetricians and Gynecologists (ACOG), during the onset of labor your cervix will dilate — or open — to make space for your baby to be born. You might not even know that this process has begun, unless your obstetrics provider checks or you experience an increase in vaginal discharge. Another sign your cervix is preparing for childbirth is light bleeding, as the mucus plug that blocks your cervix during pregnancy becomes dislodged.
According to the American Pregnancy Association (APA), while every labor is different, you will go through three phases of labor, marked by how dilated your cervix is and how frequent your contractions are. During early labor, your cervix will efface (or thin) and will dilate to about three centimeters. Contractions are generally mild and might be irregular. Unless instructed to by your obstetrics provider, you can probably stay home during this phase.
The next and longest phase is active labor, during which your cervix will dilate from four to seven centimeters, and your contractions will become longer and be more intense. The third and shortest phase is transition, during which your cervix will dilate from eight centimeters to 10 centimeters, and your contractions will be very intense (and might not even give you a break to catch your damn breath).
According to Health Line, and contrary to popular belief, the onset of labor doesn't always correspond with how dilated you are. It's possible to be dilated one or more centimeters for days, or even weeks, prior to labor. It's also possible for your cervix to not dilate at all until the day you deliver. Health Line adds that there are other signs you should look for to know if your baby is going to be born, including feeling your baby drop, losing your mucous plug, having regular contractions, or your water breaking.
According to the Mayo Clinic, your provider might do a cervical exam to check if and how much your cervix has dilated. However, because this information might not be helpful in determining when labor will actually start (and cervical exams can be nothing short of uncomfortable), some providers won't do them routinely unless they're requested. Unfortunately, it seems that every pregnancy and labor will progress differently, which means that, for the most part, it's impossible to predict when your baby will arrive and even if you know how dilated you are.