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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