Preterm Birth

Public Health Impact

Preterm birth is associated with increased infant and childhood morbidity such as neurodevelopmental deficits and behavioral problems. Several adult diseases, such as diabetes, hypertension, and cardiovascular disease, are more likely to occur among preterm infants. Preterm birth is also associated with increased mortality with two thirds of perinatal deaths occurring among preterm infants. Although preterm delivery is associated with birth defects and other causes of mortality, one third of these deaths have been shown to be directly attributable to preterm delivery.

Preterm births can exact a considerable toll on health care systems since most premature babies [Page 836]require extensive neonatal and postneonatal medical care. In the United States, the disease burden associated with preterm deliveries was estimated at $26 billion a year. The annual cost of neonatal care alone was estimated at $1 billion for preterm births occurring in Canada (excluding costs associated with long-term medical care).

Mechanisms

Preterm birth can occur via at least four major pathophysiologic pathways that may work independently or simultaneously. These include the following:

• 1.
  inflammation and infection associated with maternal and fetal cytokine response (∼ 40% of preterm births);
• 2.
  maternal/fetal stress and the production of placental and fetal-membrane derived corticotropin-releasing hormone, which in turn enhances placental estrogen and stimulates fetal cortisol production (∼ 25% of preterm births);
• 3.
  abruption or decidual hemorrhage with thrombininduced protease expression and disturbances in uterine tone (∼ 25% of preterm births); and 4. mechanical stretch due to multifetal pregnancy or polyhydramnios-induced uterine or cervical distention (∼ 10% of preterm births).

These pathways result in activation of the uterine myometrium which can initiate a preterm delivery through uterine contractions, cervical dilation, and premature rupture of the membranes.

Risk Factors

Preterm delivery is a multifactorial outcome in which the cause is unknown in nearly half of preterm births (i.e., idiopathic). Nonidiopathic preterm births can be classified by clinical subtypes, including spontaneous preterm labor, premature membrane rupture, and induction of labor or cesarean section triggered by maternal or fetal indications (e.g., hypertensive disorders of pregnancy, cervical incompetence). Risk factors for preterm birth identified in epidemiological studies include young and old maternal age, African American race, smoking, alcohol use, drug use, nutritional deficiency, poverty/neighborhood factors, stress/anxiety, inadequate prenatal care, inadequate weight gain during pregnancy, hypertension, uterine bleeding, short interconceptual interval, and previous preterm delivery. Systemic maternal infections such as pneumonia and periodontal disease have been associated with preterm delivery in epidemiological studies. Genital tract infections such as bacterial vaginosis have also been linked with an increased risk of preterm delivery. Associations have also been reported in epidemiological studies between risk of preterm birth and various environmental and occupational exposures, including pesticides, organochlorinated compounds (e.g., 1,1-dichloro-2, 2-bis(p-chlorophenyl)ethylene) and air pollutants (e.g., sulfur dioxide and particulate matter < 10 µm).

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