The physical process of childbirth requires a significant transformation of the reproductive anatomy to allow the passage of the baby. At the heart of this change is cervical dilation, the widening of the opening of the uterus. This progression is systematically monitored by healthcare providers to track labor and ensure the safety of both the person giving birth and the baby. The first stage of labor is defined by the journey from a closed cervix to a fully open one.
Defining Dilation and Effacement
Cervical dilation is the opening of the cervix, the muscular, lower part of the uterus that connects to the vagina. This opening is measured in centimeters (cm), progressing from zero to the ten centimeters required for a vaginal birth. When the cervix is closed, it is firm and long, acting as a protective barrier during pregnancy. Rhythmic, involuntary uterine contractions exert downward pressure, gradually pulling the cervix open.
Dilation is accompanied by effacement, which is the thinning and shortening of the cervix. Effacement is measured in percentages, where zero percent means the cervix is thick and long, and one hundred percent means it has thinned completely. For the baby to pass through the birth canal, the cervix must be one hundred percent effaced and ten centimeters dilated.
These two changes, dilation and effacement, are often collectively referred to as cervical ripening. In first-time pregnancies, the cervix typically effaces before it dilates significantly. However, in people who have given birth before, dilation may begin concurrently with effacement, or even slightly before it. The difference between a closed cervix and a fully dilated one represents a major anatomical transformation.
How Dilation is Measured
The assessment of cervical dilation is a routine procedure during labor to gauge its progression. This measurement is most commonly performed through a digital cervical examination, which involves the healthcare provider inserting two gloved fingers into the vagina. By feeling the rim of the cervix, they manually estimate the diameter of the opening.
The measurement is a subjective approximation, relying on the practitioner’s training and the width of their fingers to assess the distance in centimeters. For instance, a cervix that allows one finger to pass is typically estimated as one centimeter, while the full stretch of two fingers across the opening is usually around seven to eight centimeters. Small variations can sometimes occur between different practitioners.
A healthcare provider, such as a doctor, midwife, or nurse, will request consent to perform this check. During active labor, these examinations are generally performed periodically, often every few hours, or when there is a significant change in the pattern of contractions. The results of the examination provide the medical team with vital data to inform decisions about care and to determine when the person is ready to enter the next phase of birth.
The Phases of Labor Progression
The first stage of labor is traditionally divided into three sequential phases based on the degree of dilation. This structured progression provides a framework for understanding the expected timeline and intensity of the birthing process. The initial phase is known as the latent phase, encompassing dilation from zero to approximately six centimeters.
During the latent phase, contractions are typically mild, short, and irregular, sometimes spaced up to twenty minutes apart. This phase is generally the longest and the least predictable, often lasting many hours or even days, particularly for a first labor. The primary work of the body during this time is the softening and effacement of the cervix.
The active phase of labor begins once the cervix has dilated to six centimeters and continues until full dilation at ten centimeters. This phase is defined by a marked acceleration in the rate of dilation. Contractions become noticeably stronger, longer, and much closer together, usually occurring every three to five minutes.
A predictable rate of progress is expected in the active phase, with the cervix typically dilating at least one centimeter per hour. Failure to meet this rate may prompt the healthcare team to consider interventions to encourage progression. The final part of the active phase is the transition phase, occurring as dilation moves from eight to ten centimeters. This phase is the shortest but often the most intense and challenging period of the entire labor. Contractions become overwhelmingly strong, potentially overlapping with very little rest time. This final push to ten centimeters signals the end of the first stage of labor.
What Happens After Full Dilation
The moment the cervix reaches ten centimeters, it is considered fully dilated, marking the conclusion of the first stage of labor. At this point, the cervix is completely retracted, meaning no rim of tissue blocks the baby’s pathway. The body is now ready to begin the second stage of labor, which culminates in the birth of the baby.
Full dilation permits the baby, specifically the head, to descend fully into the pelvic floor and into the birth canal. This descent often creates an intense, involuntary reflex known as the urge to push, which feels similar to the overwhelming need to have a bowel movement. The second stage is focused on maternal expulsive efforts.
The duration of the second stage is highly variable, lasting from a few minutes to several hours. The healthcare team will coach the person on effective pushing techniques and monitor the baby’s position and heart rate until the final delivery.