Labor

Basics


Labor denotes the sequence of physiologic occurrences that result in a fetus being transported from the uterus through the birth canal

Description

  • Labor brings about changes in the cervix to allow passage of fetus through birth canal
  • Synchronous, coordinated contractions of the uterus
  • Contractions progress in magnitude, duration, and frequency to produce dilation of the cervix and ultimate delivery
  • Labor is divided into 3 stages:
    • Stage 1 (cervical stage): From onset of uterine contractions to full dilation of cervix
    • Stage 1 is further divided into latent and active phases:
      • In the latent phase, uterine contraction with little change in cervical dilation or effacement; contractions are mild, short (<45 s), and irregular
      • This is followed by the active phase, which begins around time of cervical dilation of 3–4 cm; contractions are strong, regular (every 2–3 min), and last longer (>45 s)
    • Stage 2: From onset of complete cervical dilation to time of delivery of infant
    • Stage 3: From time of delivery of baby to time of placental delivery
  • Total duration of labor varies with each woman
  • Generally, lengths of 1st and 2nd stages of labor are significantly longer for nulliparous woman:
    • Nulliparous: Mean length for 1st stage of labor is 14.4 hr and for 2nd stage of labor is 1 hr
    • Parous: Mean length of 1st stage of labor is 7.7 hr and for 2nd stage of labor is 0.2 hr
  • Length of 2nd stage of labor is greatly influenced by “3 Ps”:
    • Passenger (infant size and presentation)
    • Passageway (size of bony pelvis and soft tissues)
    • Powers (uterine contractions)
  • Problems with any of these 3 Ps can cause abnormal progression of labor:
    • Fetal malposition, uterine dysfunction, cephalopelvic disproportion
  • False labor (Braxton Hicks contractions):
    • Irregular, nonsynchronous contractions of uterus several weeks to days before onset of true labor, and do not cause cervical dilation

Etiology

  • Premature labor occurs in 8–10% of pregnancies
  • 30–40% of premature labor is caused by uterine, cervical, or urinary tract infections
  • Premature rupture of membranes is defined as rupture of amniotic/chorionic membranes at least 2 hr before onset of labor in patient before 37 wk gestation:
    • This occurs in only 3% of pregnancies but accounts for 30–40% of all premature births

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