Preterm Labor

Luce A. Kassi

Samantha P. DeAndrade

Audra R. Meadows

OVERVIEW

Preterm labor is a common cause of hospitalization in pregnancy and may lead to preterm birth. Preterm birth is a significant public health challenge, difficult to predict, and the leading cause of infant morbidity and mortality worldwide.¹^,²^,³^,⁴^,⁵ Preterm birth is a live birth prior to 37 weeks’ gestation. Preterm labor is defined as regular contractions accompanied by cervical dilation of greater than or equal to 3 cm, a short cervix, or cervical change on serial examination between 20 and 36 6/7 weeks of gestation⁶ (Table 18.1).

The primary management goal is to reduce the incidence of preterm birth and extend pregnancy to achieve a greater gestational age for babies whose premature birth cannot be prevented. Because more than half of pregnant women hospitalized due to preterm labor deliver at term, appropriate diagnosis and selection of candidates for treatment of preterm labor is essential. It is equally important to prevent unnecessary treatment of women who are not in preterm labor.⁷^,⁸ Patients who deliver preterm are at increased risk for a preterm birth in a subsequent pregnancy.⁹

EPIDEMIOLOGY

Every year, nearly 15 million premature births occur worldwide, with approximately 400,000 preterm births occurring annually in the United States.¹^,⁵ Most cases of preterm births occur in the late preterm period (34 to 36 6/7 weeks), followed by births at 28 to 32 weeks, and the least occur at less than 28 weeks of gestation¹⁰ (Figure 18.1). Racial and ethnic inequities exist and persist among rates of preterm birth.¹ Infants born to mothers of Black race are significantly more likely to be born preterm (Figure 18.2) and die in the first year of life. Presently, preterm birth rates continue to
rise. In 2017, the US preterm birth rate was 9.9%, with increases reported each year for the previous three consecutive years.¹⁰

TABLE 18.1 Clinical Criteria for the Diagnosis of Preterm Labor

Pregnancy between 20 0/7 and 36 6/7 weeks’ gestation
Regular uterine contractions (every 5 min or five in 1 h), PLUS
- Cervical dilation to ≥3 cm OR
- Cervical change in dilation and/or effacement on serial cervical examination OR
- Short cervix or cervical dilation <20 mm (singleton) and <25 mm (twin) on transvaginal ultrasound

Figure 18.1: Preterm birth rates: United States, 2014-2016. (From Martin JA, Osterman MJK. Describing the Increase in Preterm Births in the United States, 2014-2016. NCHS Data Brief, no 312. Hyattsville, MD: National Center for Health Statistics. 2018.

¹Significantly increasing linear trend for 2014-2016 (p < 0.05).

²Significant increase from 2014 and 2015 (p < 0.05). Available at https://www.cdc.gov/nchs/products/databriefs/db312.htm)

Premature infants are at high risk for neonatal complications, long-term developmental and intellectual disabilities, as well as chronic disease. The risk of long-term complications associated with preterm birth strongly correlates with gestational age.¹¹ Short- and long-term complications include respiratory distress syndrome, intraventricular hemorrhage, necrotizing enterocolitis, neurodevelopmental disability (cerebral palsy), chronic respiratory abnormalities (asthma,
bronchopulmonary dysplasia), infections, poor growth, poor hearing, and vision loss due to retinopathy of prematurity—all resulting in recurrent hospitalizations.²^,¹¹

Figure 18.2: Preterm birth rates by race and Hispanic origin of mother: United States, 2014-2016. (From Martin JA and Osterman MJK. Describing the Increase in Preterm Births in the United States, 2014-2016. NCHS Data Brief, no 312. Hyattsville, MD: National Center for Health Statistics. 2018. Available at https://www.cdc.gov/nchs/products/databriefs/db312.htm.)

Beyond health implications, preterm birth is costly. Care of a healthy, term infant ranges from $5000 to $7000, whereas the average cost of care for a preterm infant is roughly $50,000 to $80,000, with costs increasing as gestational age at birth decreases.¹² An Institute of Medicine report estimates medical and societal costs of preterm birth in the United States to be $26.2 billion annually due to hospitalizations and ongoing medical care for mother and baby (to age 5), social services, and lost wages in the workforce; $16.9 billion for healthcare costs alone.² A March of Dimes report estimates that health plans spend $12.7 billion annually due to preterm birth.¹³ Another study found that preterm birth cost employer-sponsored health plans in the United States $6 billion more for preterm infants compared to term infants.¹² These studies approach cost estimates differently yet what is clear is the great expense of preterm newborn care.

RECOGNITION AND MANAGEMENT PRIORITIES

The recognition of women at increased risk for preterm birth as well as women experiencing preterm labor provides an opportunity for emergency medicine practitioners to prevent a preterm birth and extend pregnancy by intervening clinically. Healthcare providers in the emergency department (ED) may be the first point of contact between a patient experiencing preterm labor and the healthcare system. As such, it is important for emergency practitioners to (1) recognize signs and symptoms of preterm labor, (2) treat to extend pregnancy and improve neonatal outcomes, and (3) refer to appropriate perinatal care facilities when needed.

Transfer may be necessary in order to match the hospital level to the maternal and neonatal care needed. The available staff and resources impact both perinatal morbidity and mortality.¹⁴ Levels of maternal and neonatal care have separate classification systems. The levels range from Levels I to IV for maternal perinatal centers and Levels I to IV for neonatal perinatal centers as described by the American College of Obstetricians and Gynecologists (ACOG), the Society for Maternal-Fetal Medicine (SMFM), and the American Academy of Pediatrics (AAP).¹⁴^,¹⁵ These tiered systems for maternal and neonatal levels are described in Table 18.2. The centers at the highest level have systems capabilities, staff, and equipment to appropriately manage higher acuity patients. The highest-level centers support the facilities across all the tier levels. Hospitals participate in a regionalized system of perinatal care to direct patient transfers. Emergency medicine providers need to be aware of this system as well as the maternal and neonatal resources at their institutions in order to triage patients appropriately.

PATHOPHYSIOLOGY

Pathways to Preterm Labor and Preterm Birth

Preterm birth may be classified as either spontaneous or induced for obstetric reasons. Spontaneous preterm labor accounts for more than two-thirds of preterm births and may occur with intact amniotic membranes or as a result of preterm prelabor rupture of membranes (PPROM). A complete discussion of PPROM is included in Chapter 19. Factors that trigger spontaneous preterm birth are largely unknown. Available evidence on mechanisms suggest that spontaneous labor with or without PPROM is the result of at least one of four major pathogenic processes leading to a final common pathway of uterine contractions and cervical change: (1) premature activation of the maternal or fetal hypothalamic-pituitary-adrenal axis (HPA); (2) local or systemic infection/inflammatory response; (3) placental-vascular/decidual hemorrhage (abruption); and (4) pathologic uterine overdistension⁷ (Figure 18.3). Examples of conditions related to spontaneous preterm birth along each pathway include: maternal psychological stress (activation of maternal-fetal HPA axis), genitourinary infection (inflammation/infection), placental abruption (decidual hemorrhage), and multiple gestation (uterine distension). Reasons for induced preterm birth include maternal hypertensive disorders (preeclampsia), PPROM (which leads to induced delivery by 34 weeks if spontaneous labor does not occur), and fetal growth restriction.

Risk Factors

There are many factors that lead to an increased risk for preterm labor and preterm birth. Pregnant women with low socioeconomic status, few or no social supports, stressful life events (death, divorce,
domestic violence), Black race, inadequate prenatal care, maternal age (<18 and >40), poor nutrition, medical and obstetric comorbidities (particularly prior preterm birth), short cervix, genitourinary tract infections, and multiple gestation are all at increased risk of preterm labor and delivery.¹⁶ Additionally, behavioral risk factors such as low maternal prepregnancy weight, smoking, substance abuse, and short interpregnancy interval have all been associated with preterm birth.¹⁶

TABLE 18.2 Levels of Neonatal and Maternal Facilities

Level	Neonatal¹⁴	Maternal¹⁵
I	Well Newborn Nursery Infants born at 35-37 weeks of gestation Stabilize infants <35 weeks until transfer	Birth Center and Low risk, term, singleton, vertex pregnancy No cesarean or operative birth Basic Care Facility Birth center plus Cesarean and operative birth
II	Special Care Nursery Infants born ≥32 weeks’ gestation and weighing ≥1500 g Infants convalescing after intensive care Brief mechanical ventilation Stabilize infants <32 wk/<1500 g until transfer	Specialty Care Facility High-risk conditions Transfer from Level I facility
III	NICU Comprehensive care for infants born at all gestational ages and weights	Subspecialty Care Facility Complex maternal medical conditions, obstetric complications, and fetal conditions Transfer from Level II facility Provide perinatal system leadership if no Level IV available
IV	Regional NICU Comprehensive care for infants born at all gestational ages and weights Capability to provide surgical repair of complex congenital or acquired conditions	Regional Perinatal Center All complex maternal medical conditions, obstetric complications, and fetal conditions Perinatal system leadership
Derived from American Academy of Pediatrics Committee on Fetus and Newborn. Levels of neonatal care. Pediatrics. 2012;130:587; Zahn CM, Remick A, Catalano A, Goodman D, Kilpatrick SJ, Menard MK. Levels of maternal care verification pilot: translating guidance into practice. Obstet Gynecol. 2018;132(6):1401-1406. NICU, neonatal intensive care unit.

Prevention

Primary prevention of preterm birth may include public education, preconception care, nutritional supplementation before and during pregnancy, smoking cessation, early and adequate prenatal care, and periodontal care.¹⁶ Prevention strategies initiated during pregnancy that are based on recognition of causal pathways are considered secondary prevention. The ultimate goal of secondary prevention of preterm birth is to delay or prevent early delivery. The initial step in secondary prevention is recognition of women at increased risk either by obstetric history or current risk factors who may qualify for outpatient prophylactic therapy to prevent preterm birth, such as progesterone therapy. Evidence demonstrates positive maternal and neonatal outcomes among women in group prenatal care. This is particularly true among vulnerable populations such as teens, low-income women, and Black women.¹⁷ Positive neonatal outcomes of group prenatal care include a reduction
in preterm birth, neonatal intensive care unit (NICU) admissions, and an increased birth weight for term and preterm infants.¹⁷

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