Cervical dilation refers to the opening of the cervix, the muscular lower part of the uterus, in preparation for childbirth. This opening is measured in centimeters, starting at zero and progressing to the ten centimeters required for a baby to pass through. The timing of initial dilation is highly variable. Some individuals may experience a small amount of dilation for several weeks before labor begins, while others remain completely closed until contractions start rapidly changing the cervix.
Cervical Changes Before Labor Begins
The body prepares for delivery through cervical ripening, a gradual process that precedes measurable dilation. During the final weeks of pregnancy, the cervix transitions from a firm, long structure to one that is softer, shorter, and thinner. This thinning is called effacement and is necessary for efficient dilation.
Effacement is measured in percentages, where zero percent indicates a long, thick cervix and one hundred percent means the cervix is fully thinned out. For first-time mothers, the cervix typically thins out significantly before it begins to dilate, often over several weeks. Those who have given birth previously may experience dilation and effacement occurring simultaneously.
Before true labor begins, the cervix may move from a closed state to one or two centimeters dilated over the course of a month or more. These gradual changes are sometimes accompanied by irregular, mild Braxton Hicks contractions, which help soften the tissue. This early opening confirms the body is preparing for birth but does not indicate when labor will start.
Defining the Stages of Dilation
Cervical dilation is the primary measurement used to track progress during the first stage of labor, which is divided into two main phases, progressing from zero to ten centimeters (full dilation). This framework helps health care providers distinguish between the slow, early work and the more intense, predictable part of labor.
The first phase is the Latent Phase, where the cervix dilates from zero up to six centimeters. This phase is characterized by contractions that may be irregular or mild. It is typically the longest and most unpredictable part of labor, often lasting many hours or even a couple of days, especially for first-time mothers.
The Active Phase begins when the cervix reaches six centimeters of dilation and continues until ten centimeters. Once six centimeters is reached, the rate of dilation usually speeds up significantly, and contractions become stronger, longer, and more regular. Dilation is expected to progress at a more predictable rate during this phase, often around one to one and a half centimeters per hour.
Fetal Station
Another measurement tracked alongside dilation is the Fetal Station, which describes how far the baby’s head has descended into the pelvis. This measurement is tracked relative to the mother’s ischial spines. A zero station means the baby’s head is level with these spines, while negative numbers indicate the head is higher up. The baby typically descends further into the pelvis as the cervix dilates.
Physical Signs of Early Cervical Change
While dilation is a clinical measurement, there are physical signs that often indicate the cervix is beginning to change. One common sign is the loss of the mucus plug, a collection of thick, jelly-like mucus that seals the cervical opening during pregnancy. As the cervix softens and dilates slightly, this plug can be expelled.
The mucus plug may be clear, yellowish, or slightly pink or brown. Losing it signals that the cervix is undergoing preparatory changes, but it is not an indicator of immediate labor. Some people lose the plug days or weeks before contractions begin. In some cases, the plug is lost in pieces, or it may not be noticed at all.
Another related sign is the appearance of “bloody show,” which is pink or blood-tinged mucus discharge. This occurs when tiny blood vessels in the cervix rupture as it begins to stretch and open. The bloody show confirms that cervical change is progressing, but it does not mean active labor is imminent.
When Dilation Requires Medical Attention
Knowing when to proceed to the hospital or birthing center depends less on the centimeter measurement and more on the pattern of contractions. Medical guidelines focus on the predictable progression of the active phase to avoid arriving too early, which can sometimes slow the natural progression of labor.
A common guideline for timing contractions is the 5-1-1 rule (or the similar 4-1-1 rule). The 5-1-1 rule suggests seeking medical attention when contractions are approximately five minutes apart, each lasting at least one full minute, and this pattern has been consistent for at least one hour. This consistency signals the transition to the established, active phase of labor.
Always contact a health care provider if the amniotic sac ruptures (“water breaking”), regardless of the contraction pattern. Similarly, any bright red vaginal bleeding, rather than the bloody show, warrants immediate medical evaluation.