In pregnancy, the uterus, which is a muscle, becomes much larger. From an organ behind your pubic bone of 100 grams, your uterus grows to your rib cage and weighs more than a kilo! Think of your uterus as a balloon and your cervix is the valve. Contractions are regular contractions of the uterus. As a result of this contraction, the head presses on your cervix and it starts to pass. Expiry is therefore the shortening of the cervix. The cervix is closed during pregnancy, 4 inches long, and firm like the tip of your nose. If your cervix is flat, dilation occurs. | We don't know why the contractions start. The hormone oxytocin is responsible for the contractions, but how the production of that hormone is controlled is unknown. Your body helps you tolerate the pain better by producing the body's own painkillers such as the hormone endorphins. These dampen the pain a bit and make you a bit drowsy and introverted. Your cervix opens more and more and eventually (when you can no longer feel it), you are fully dilated. The contractions also change and increase in intensity as dilation progresses.  | Read more about the different stages of labor |