Preterm Labor


Preterm deliveries have decreased in recent years due to early diagnosis, pharmacologic interventions, and better monitoring technique. Preterm delivery causes significant morbidity and mortality. The American College of Obstetric and Gynecology has given specific definitions. Once the diagnosis is established by physical and pelvic examination, then bed rest, intravenous hydration, and tocolytic therapy are initiated. Commonly used tocolytic agent include magnesium sulfate, β 2 -adrenergic agonists, prostaglandin synthetase inhibitors, and calcium-channel blockers. Atosiban is an oxytocin antagonist used in Europe. Despite aggressive tocolytic therapy, labor often progresses. Route of delivery is decided based upon gestational age, weight of fetus, fetal heart rate tracing, and fetal presentation. Neuroaxial anesthesia is almost universally preferred over general anesthesia.


β 2 -adrenergic agonists, atosiban, calcium-channel blockers, low birth weight, magnesium sulfate, preterm infant, preterm labor, prostaglandin synthetase inhibitors, tocolytic agent, very low birth weight


Case Synopsis

A 28-year-old woman, gravida 2, para 1, with an estimated gestational age of 28 weeks presents with a new onset of regular uterine contractions. She is admitted to the hospital for intravenous hydration and tocolytic therapy.


The author wishes to thank Dr. Craig Palmer for his contribution to the previous edition of this chapter.

Problem Analysis


Centers for Disease Control and Prevention (CDC) data from 2014 reveal preterm deliveries of 9.57%. Data from 2012 reported 450,000 preterm births; that is 1:9 for every live birth in the United States. Preterm birth is defined as birth before 37 weeks of pregnancy. Preterm-related death accounted for 35% of all infant deaths in 2010 and is a leading cause of neurologic disabilities in children. Some problems include breathing problems, feeding difficulties, cerebral palsy, developmental delay, and vision and hearing impairment ( Box 73.1 ). Current obstetric practices aim at delaying such delivery as much as possible.

BOX 73.1

  • Breathing problems

  • Feeding difficulties

  • Cerebral palsy

  • Developmental delay

  • Vision problems

  • Hearing impairment

Sequel of Prematurity


The American Congress of Obstetricians and Gynecologists (ACOG) definition is as follows:

  • Preterm labor is defined as regular uterine contractions occurring at least once every 10 minutes and resulting in cervical dilation or effacement before 37 weeks’ gestation ( Table 73.1 ).

    TABLE 73.1

    Definition of Commonly Used Terms in Preterm Labor

    Term Definition
    Preterm labor Regular uterine contraction every 10 minutes leading to changes in cervical examination at 20–37 weeks
    Preterm infant Infant born before 37 weeks’ gestation
    LBW infant Weight less than 2500 g at birth
    VLBW infant Weight less than 1500 g at birth
    PROM Rupture of membranes before initiation of labor
    Pre-PROM PROM before 37 weeks’ gestation

    LBW, Low birth weight; PROM, premature rupture of membranes; VLBW, very low birth weight.

  • A preterm infant is any infant delivered before 37 weeks’ gestation.

  • Any infant weighing less than 2500 or 1500 g at birth is a low-birth-weight (LBW) or very-low-birth-weight (VLBW) infant, respectively, regardless of gestational age.

  • At 29 weeks’ gestation, more than 90% of estimated fetal weights are less than 1500 g.

  • Although survival at 23 weeks of gestational age may be as high as 25%, most of these survivors have significant long-term neurologic impairment.

  • Delay of delivery from a gestational age of 23 to 31 weeks improves neonatal survival rate from just over 25% to 96%.

Risk Assessment

The initial assessment of a patient with preterm labor consists of a thorough physical examination to eliminate treatable medical conditions that may have precipitated labor and a pelvic examination to rule out premature rupture of membranes. Bed rest, intravenous hydration, continuous fetal heart rate monitoring, and tocography are almost universally indicated. Bed rest and hydration alone are effective in a large number of patients. If these conservative measures are ineffective, ultrasonography, and occasionally amniocentesis, is undertaken to establish gestational age and fetal maturity. Lecithin/sphingomyelin (L/S) ratio is seen as a marker of fetal maturity. L/S ratio greater than 2 is a marker of fetal lung maturity.

Once the diagnosis is established, the obstetrician must decide whether to institute pharmacologic tocolytic therapy. This decision is based on the estimated gestational age, fetal weight, and the presence or absence of a reassuring fetal heart rate. In general, a gestational age between 20 and 34 weeks and a fetal weight of less than 2500 g in the presence of a reassuring fetal heart rate trace and absence of fetal distress are indications for tocolytic therapy. Sometimes waiting to finish the two doses of dexamethasone or betamethasone therapy 24 hours apart for lung maturity is warranted.

The tocolytic agents currently in use have the potential for significant interactions with commonly used anesthetic agents. In addition, prematurity may have implications for the route of delivery (whether vaginal or abdominal).


To understand the mechanisms of pharmacologic intervention to stop preterm labor, physiology of uterine contraction and role of calcium is important.

Although the processes that initiate labor are incompletely understood, much is known about the physiology of uterine contractions. The myometrium contains myosin and actin filaments that generate contractile force. Pacemaker cells within the myometrium are capable of initiating spontaneous contractile activity, which spreads throughout the myometrium via gap junctions between cells.

Calcium plays a critical role. Before contraction, intracellular calcium concentration increases due to release of calcium from the sarcoplasmic reticulum or flux across the sarcolemma. Calcium interacts with calmodulin, activating myosin light-chain kinase. The activated myosin light-chain kinase phosphorylates myosin, which then binds with actin. Adenosine triphosphate (ATP) is hydrolyzed by myosin adenosine triphosphatase (ATPase), resulting in movement of the actin-myosin elements and myometrial contraction. A reduction in intracellular calcium concentration, or dephosphorylation of myosin, inhibits the actin-myosin interaction, causing relaxation ( Box 73.2 ).

Feb 18, 2019 | Posted by in ANESTHESIA | Comments Off on Preterm Labor
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