In some women, forceful uterine contractions that efect delivery begin suddenly. In others, labor initiation is heralded by spontaneous release of a small amount of blood-tinged mucus from the vagina. his extrusion of the mucus plug that had previously illed the cervical canal during pregnancy is referred to as "show" or "bloody show." Its passage indicates that labor is already in progress or likely will ensue in hours to days.
Unique among physiological muscular contractions, those of uterine smooth muscle during labor are painful. Several possible causes have been suggested: (1) hypoxia of the contracted myometrium-such as that with angina pectoris; (2) compression of nerve ganglia in the cervix and lower uterus by contracted interlocking muscle bundles; (3) cervical stretching during dilation; and (4) stretching ofthe peritoneum overlying the fundus.
Of these, compression of nerve ganglia in the cervix and lower uterine segment by the contracting myometrium is an especially attractive hypothesis. Paracervical infiltration with local anesthetic usually produces appreciable pain relief with contractions (Chap. 25, p. 490). Uterine contractions are involuntary and, for the most part, independent of extrauterine control. Neural blockade from epidural analgesia does not diminish their frequency or intensity. In other examples, myometrial contractions in paraplegic women and in women after bilateral lumbar sympathectomy are normal but painless.
Mechanical stretching of the cervix enhances uterine activity in several species, including humans. his phenomenon is the Ferguson relex (Ferguson, 1941). Its exact mechanism is unclear, and release of oxytocin has been suggested but not proven. Manipulation of the cervix and "stripping" the fetal membranes is associated with a rise in blood levels of prostaglandin F2. metabolites.
The interval between contractions narrows gradually from approximately 10 minutes at the onset of irst-stage labor to as little as 1 minute or less in the second stage. Periods ofrelaxation between contractions, however, are essential for fetal welfare. Unremitting contractions compromise uteroplacental blood flow suiciently to cause fetal hypoxemia. In active-phase labor, the duration ofeach contraction ranges from 30 to 90 seconds and averages 1 minute. Contraction intensity varies appreciably during normal labor. Speciically, amnionic fluid pressures generated by contractions during spontaneous labor average 40 mm Hg, but vary from 20 to 60 mm Hg (Chap. 24, p. 479).
Distinct Lower and Upper Uterine Segments. During active labor, the anatomical uterine divisions that were initiated in phase 2 ofparturition become increasingly evident (Figs. 21-11 and 21-12). By abdominal palpation, even before membrane rupture, the two segments can sometimes be diferentiated. he upper segment is firm during contractions, whereas the lower segment is softer, distended, and more passive. This mechanism is imperative because if the entire myometrium, including the lower uterine segment and cervix, were to contract simultaneously and with equal intensity, the net expulsive force would markedly decline. hus, the upper segment contracts, retracts, and expels the fetus. In response to these contractions, the segment -Passive segment--0evel of internal cervical os Cervix Level of external cervical os Vagina
FIGURE 21-12 The uterus at the time ofvaginal delivery. The active upper segment retracts around the presenting part as the fetus descends through the birth canal. In the passive lower segment, there is considerably less myometrial tone.
form a greatly expanded, thinned-out tube through which the fetus can pass.