For the mother, precipitous labor and delivery seldom are accompanied by serious maternal complications if the cervix is efaced appreciably and compliant, if the vagina has been stretched previously, and if the perineum is relaxed. Conversely, vigorous uterine contractions combined with a long, irm cervix and a noncompliant birth canal may lead to uterine rupture or extensive lacerations of the cervix, vagina, vulva, or perineum (Sheiner, 2004). It is in these latter circumstances that amnionic-luid embolism most likely develops (Chap. 41, p. 785). Precipitous labor is frequently followed by uterine atony. he uterus that contracts with unusual vigor bore delivery is likey to be hypotonic ater delivey. In one report of 99 term pregnancies, short labors were more common in multiparas who typically had contractions at intervals less than 2 minutes. Precipitous labors have been linked to cocaine abuse and associated with placental abruption, meconium, postpartum hemorrhage, and low Apgar scores (Mahon, 1994).
For the neonate, adverse perinatal outcomes from rapid labor may be increased considerably for several reasons. The tumultuous uterine contractions, often with negligible intervals of relaxation, prevent appropriate uterine blood flow and fetal oxygenation. Resistance of the birth canal may rarely cause intracranial trauma. Acker and coworkers (1988) reported that Erb or Duchenne brachial palsy was associated with such labors in a third of cases. Finally, during an unattended birth, the newborn may fall to the loor and be injured, or it may need resuscitation that is not immediately available.
As treatment, analgesia is unlikely to modiy these unusually forceful contractions to a signiicant degree. The use of tocolytic agents such as magnesium sulfate or terbutaline is unproven in these circumstances. Use of general anesthesia with agents that impair uterine contractibility such as isoflurane is often excessively heroic. Certainly, any oxytocin being administered should be stopped immediately.
Fetopelvic disproportion arises from diminished pelvic capacity, from abnormal fetal size or presentation, or more usually from both. The pelvic inlet, midpelvis, or pelvic outlet may be contracted solely or in combination. Any contraction of the pelvic diameters that diminishes pelvic capacity can create dystocia during labor. Normal pelvic dimensions are additionally discussed and illustrated in Chapter 2 (p. 30).
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.
The 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). Therefore, inlet contraction usually is defined as a diagonal conjugate < 11.5 cm.