## BACKGROUND

KEY QUESTIONS

• How common is preterm labor?

• What is the pathophysiology of preterm labor?

• How is preterm labor diagnosed?

• What are the risk factors for preterm labor?

• How is preterm labor treated?

• How can we reduce the risk of recurrent preterm birth?

CASE 45-1

A 25-y.o. gravida 1 at 30 weeks gestation presents to L&D with contractions. Upon examination, the patient’s cervix is dilated to 3 cm, with complete effacement and tense, palpable membranes.

Preterm labor is defined as the presence of uterine contractions accompanied by cervical change between 20 and 37 weeks gestation. It is the most common indication for hospital admission among pregnant women and the leading cause of neonatal morbidity and mortality. Neonatal consequences of preterm delivery include both immediate and long-term sequelae, notably respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, and cerebral palsy. Healthcare costs for these complications are estimated to be at least $26 billion per year, or$51,000 per infant.1

It is often difficult to predict whether preterm labor leads to preterm delivery; however, the implications and potential risks of long-term disability cannot be taken lightly. In recent years, 10% of all births in the United States were preterm, accounting for more than 400,000 infants.2 It is important to acknowledge that this number includes both spontaneous and medically indicated deliveries. This chapter focuses on the diagnosis and management of spontaneous preterm labor with intact membranes.

PRACTICE POINT

• Preterm labor is defined as the presence of regular uterine contractions with cervical change between 20 and 37 weeks’ gestation. It is the most common indication for hospital admission and the leading cause of neonatal morbidity and mortality.

## PATHOPHYSIOLOGY

Preterm labor can be described as a pathogenic onset of the physiological process of labor. Labor is multifactorial, encompassing a series of anatomical, biochemical, and endocrine changes in the uterus, cervix, and fetal membranes. This is regulated by the immunologic and endocrine systems, ultimately leading to the delivery of the fetus.

Activation of the myometrial component of labor occurs following the development of gap junctions among myometrial cells through increased expression of connexin 43. This causes transition from uterine contractures, characterized by low-intensity muscular activity lasting several minutes, to uterine contractions of high-intensity activity for brief but frequent episodes. At the histological level, these changes are the result of myometrial hyperplasia and hypertrophy from the increased production of extracellular matrix proteins, and eventually contractile modifications toward the end of gestation.

Another critical component of labor is cervical remodeling, defined as the softening, shortening, and dilation that occurs preceding uterine contractility. The hallmarks of cervical remodeling are changes in the production, modification, and subsequent degradation of components in the extracellular matrix, such as collagen fibers and glycosaminoglycans. The most prominent is cervical dilation, caused by an inflammatory process via a massive influx of ...

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