Once the cervix is fully dilated, the baby goes through an elaborate dance, called the “mechanisms of labor”, to negotiate and descend through the mother’s pelvic bones. Usually, this process of descent takes 30 minutes to 2 hours. During this period, most women feel a tremendous urge to push the baby out, and do just that.
Protractions and arrests of dilatation happen much more commonly with first-time mothers than with experienced mothers. It’s as though the uterus is inexperienced during the first labor. As many as 30% of first-time mothers will have a protraction or arrest of the progress of labor.
If you have been contracting regularly for a period of 20 hours (14 hours if you’ve had a baby before) without a measurable change in your cervix, your midwife or doctor may make a diagnosis of “prolonged latent phase”. Under these difficult circumstances “patience” should be the guiding principle. Overly aggressive treatment (by rupturing the membranes and/or using Pitocin) may lead to an unnecessary c-section.
The uterus is essentially a hollow muscle. When this muscle contracts it transmits its force in the direction of the vagina. Thus, anything inside the uterus is pushed down and out. Problems involving this force are some of the most common causes of problems during labor. Efficient contraction of the uterus is dependent on several factors. As stated above (The Body Prepares for Labor) the uterus must be properly primed for efficient labor.
Generally, problems with the passage involve restrictions in the size of the hole through the bony part of the pelvis. Although the muscles and ligaments inside the pelvis may present some obstacle, this “soft tissue” (as opposed to bone) will almost always yield to the powers of the uterus.
Size and position of the baby are the factors involved in problems with the passenger. Simply speaking, the passenger’s size must be somewhat smaller than the hole through the bones of the pelvis (the passage). Nature provides some mechanisms (both maternal and fetal) to provide for tight fits. The soft bones of the fetal head are able to “mold” and “overlap” without noticable damage to the baby.
What is “normal labor”? That’s a tough question. The answer probably lies within the answer to another question, “What is a normal human being?” In an effort to define normal labor, an overly simplistic model has emerged. The model is good enough, however, to frame the issue. However, remember, when scientists look for ways to describe the events of large populations, they think about “averages” (or “means”). And when scientists describe “means”, they talk about “variance from the mean”. Variance from the mean is the allowance that is made to explain the individual variations to “normal” (or average, or the mean) which are deemed close enough.