What does "3 cm dilated" mean?

Three centimeters dilated means that the cervix is opened about three centimeters in preparation for birth. Being three centimeters dilated does not mean a woman is in labor, according to About.com; a woman can be up to five centimeters dilated without being in true labor.

True labor happens when contractions are regular and continue to increase in intensity and frequency, according to WebMD. Dilation is not the only part of labor, as the cervix must also efface. Effacement is the thinning of the cervix and is measured as a percentage. For example, a woman's cervix must be 10 centimeters dilated and 100 percent effaced before she begins pushing. Effacement can also begin well before true labor starts.