Using clinical measures, it is important to identiy the shortest anteroposterior diameter through which the fetal head must pass. Before labor, the fetal biparietal diameter averages from 9.5 to as much as 9.8 cm. Therefore, it might prove diicult or even impossible for some fetuses to pass through a pelvic inlet that has an anteroposterior diameter <10 cm. Mengert (1948) and Kaltreider (1952), employing x-ray pelvimetry, demonstrated that the incidence of diicult deliveries rises when either the anteroposterior diameter of the inlet is < 10 cm or the transverse diameter is < 12 cm. As expected, when both diameters are contracted, dystocia rates are much greater than when only one is contracted. Either of these measures is used to consider a pelvis contracted.
he anteroposterior diameter of the inlet, which is the obstetrical conjugate, is commonly approximated by manually measuring the diagonal conjugate, which is approximately 1.5 cm greater. Ascertainment of these measures is described in Chapter 2 (p. 30). herefore, inlet contraction usually is defined as a diagonal conjugate < 11.5 cm.
A small woman is likely to have a small pelvis, but she is also likely to have a small neonate. homs (1937) studied 362 nulliparas and found that the mean birthweight of their ofspring was significantly lower-280 g-in women with a small pelvis than in those with a medium or large pelvis.
as the rise in this pressure above a resting pressure baseline. hese investigators also introduced the concept of Montevideo units to deine uterine activity (Chap. 23, p. 443). With this deinition, uterine performance is the product of contraction intensity in mm Hg multiplied by the number of contractions in a 10-minute span. For example, three contractions in 10 minutes, each of 50 mm Hg intensity, would equal 150 Montevideo units.
During the irst 30 weeks of pregnancy, uterine activity is comparatively quiescent. Contractions are seldom greater than 20 mm Hg, and these have been equated with those irst described by John Braxton Hicks. Uterine activity increases gradually after 30 weeks, and it is noteworthy that these Braxton Hicks contractions also increase in intensity and frequency. Uterine activity is further enhanced during the last weeks of pregnancy. During this phase, the cervix ripens (Chap. 21, p. 409).
According to Caldeyro-Barcia and Poseiro (1960), clinical labor usually commences when uterine activity reaches values between 80 and 120 Montevideo units. his translates into approximately three contractions of 40 mm Hg every 10 minutes. Importantly, no clear-cut division marks labor onset, which is a gradual and progressive transition.
In irst-stage labor, uterine contractions progressively grow in intensity from approximately 25 mm Hg at labor commencement to 50 mm Hg at its end. At the same time, the frequency advances from three to ive contractions per 10 minutes, and uterine baseline tone rises from 8 to 12 mm Hg. Uterine activity is further enhanced during second-stage labor, aided by maternal pushing. Indeed, contraction intensity of 80 to 100 mm Hg is typical, and the uterus contracts as frequently as ive to six times each 10 minutes. Hauth and coworkers (1986) quantiied uterine contraction pressures in 109 women at term who received oxytocin for labor induction or augmentation. Most of these women achieved 200 to 225 Montevideo units, and 40 percent had up to 300 units to efect delivery. The authors suggested that these levels of uterine activity should be sought before consideration of cesarean delivery for presumed dystocia (Chap. 23, p. 443).