Inadequate power is the most common cause of inadequate labor progress in women delivering for the first time. The powers of labor are determined by how hard the uterus contracts and how hard the mother pushes. Power in the first stage of labor may be assessed by evaluating the labor pattern, consisting of the duration, frequency, and quality of uterine contractions.
Contractions must last long enough, come frequently enough, and be forceful enough to make the cervix dilate and the fetus descend through the birth canal. Exactly how much is enough may vary greatly for individual women and for individual pregnancies. For women in spontaneous labor, contractions are usually two to five minutes apart, last from 30 to 60 seconds, and have moderate strength.
The easiest way to assess the power of labor is to time the frequency and duration of the contractions (from the beginning of one to the beginning of the next). The intensity of the contractions can be estimated by touching the uterus. The relaxed or mildly contracted uterus usually feels about as firm as a cheek, a moderately contracted uterus feels as firm as the end of the nose, and a strongly contracted uterus is as firm as the forehead.
In the hospital, the most common way to assess the frequency and duration of contractions is with a tocodynometer. This device is held on the abdomen, over the uterus, with an elastic belt and contains a button that moves a spring when the uterus contracts. An electric signal then allows the contraction to be recorded as a peak on a computer screen or monitor paper. The tocodynometer measures frequency and duration without measuring intensity. This device may also be limited in its use by how it's placed on the uterus, the size and shape of the mother's abdomen, and the baby's movement. Tocodynometers are usually used in conjunction with a fetal heart rate monitor.
Intrauterine Pressure Catheter (IUPC)
When there is still uncertainty about an adequate labor pattern, the pressure of contractions from within the uterus is measured with an intrauterine pressure catheter (IUPC). The IUPC consists of a fluid-filled piece of soft tubing that is passed through the vagina and cervix, into the uterus. The end of the catheter sits in the amniotic fluid and converts the measured pressure to an electric signal that is traced onto a computer monitor or a piece of paper. These contractions look similar to those measured by a tocodynometer. However, an IUPC measures the frequency, duration, and intensity of contractions. The strength of the contraction is measured from the baseline (when the uterus is relaxed) to the peak of the contraction and is recorded in units-one unit is the amount of pressure it takes to raise a column of mercury one millimeter. Studies have suggested that 200 units worth of contractions every 10 minutes is usually adequate for vaginal delivery after spontaneous labor. An IUPC increases the risk of intraamniotic infection and therefore is not used routinely.