For your baby to be born the cervix (comprised of firm muscle that forms a strong base at the bottom of the uterus) needs to stretch and soften so that it can open, or dilate, and your baby can pass out of the uterus and into the vagina. Toward the end of pregnancy, substances in your blood called prostaglandins start to soften the cervix so that it becomes more malleable. Your baby also gets into "position" to make his way out.
While you are pregnant, your cervix is usually around 2 to 3 cm long. In late pregnancy or early labor, Braxton Hicks' practice contractions start to shorten the cervix, a process known as effacement. Most women have a cervix that has shortened to 1 cm during the very early stages of labor. This is also referred to as 50 percent effaced. As the cervix continues to shorten, the cervix is gradually drawn up by the uterus, and by the time it is 100 percent effaced, the cervix will have started to open.
Once your cervix is stretched and softened, it begins to open, or dilate, so that your baby can pass through into the vagina to be born. Regular contractions cause the cervix to dilate, and in first labors the cervix dilates at an average of 1 cm per hour; this rate is often faster for subsequent labors. Here, at 2 cm dilation, the cervix has shortened and is beginning to open. Contractions may still be irregular.
Dilation -- 6 cm
At 6 cm dilation, you are in active labor. Your contractions will be more frequent, regular, and stronger.
Dilation -- 10 cm
At 10 cm dilation, you are fully dilated. Contractions may be almost continuous and you are nearly ready to start pushing your baby out.
The "station" refers to the position of your baby's head in relation to your pelvis. This is recorded as a number between -5 and +5. Zero station means the head is "engaged" and has entered the vaginal canal within the pelvic bones. A negative number (-5 to 0) means that the head isn't engaged in the pelvis. A positive number (0 to +4) means that your baby's head is moving down the pelvis and +5 means your baby is crowning (being born). Ideally, you should not push until the head is engaged in the pelvis, even if you're fully dilated.