Preterm labor

Preterm labor is defined as the presence of uterine contractions of sufficient frequency and intensity to affect progressive effacement and dilation of the cervix prior to term gestation (between 20 and 37 week). Preterm labor precedes almost half of preterm births and preterm birth neonatal mortality in the United States. In addition, preterm birth accounts for 70% of neonatal morbidity, mortality, and health care dollars spent on the neonate, largely due to the 2% of American women delivering very premature infants

The exact mechanism(s) of preterm labor is largely unknown. However, it is believed to include decidual hemorrhage, (e.g., abruption), mechanical factors such as uterine over distension from multiple gestation or polyhydramnios), cervical incompetence (e.g., trauma, cone biopsy) and uterine distortion (e.g., Müllerian duct abnormalities, fibroid uterus). In addition, cervical inflammation (e.g., resulting from bacterial vaginosis [BV], trichomonas), maternal inflammation/ fever (e.g., urinary tract infection), hormonal changes (e.g., mediated by maternal or fetal stress), and uteroplacental insufficiency (e.g., hypertension, insulin-dependent diabetes, drug abuse, smoking, alcohol consumption) Risk factors for preterm birth include demographic characteristics, behavioral factors, and aspects of obstetric history such as previous preterm birth. Demographic factors for preterm labor include nonwhite race, extremes of maternal age (< 17 y or >35 y), low socioeconomic status, and low pre-pregnancy weigh

Among high-risk women with a history of one or more spontaneous preterm births (excluding those with multiple gestation, uterine anomalies, and prior cervical surgeries), 20% of patients demonstrated a cervical length shorter than 25 mm by transvaginal ultrasonography at 22-25 weeks. Among these patients with a short cervix and one previous preterm birth, 37.5% delivered at less than 35 weeks. In contrast, patients with a cervical length longer than 25 mm had a preterm rate (< 35 weeks) of only 10.6%. Cervical length has similarly been demonstrated as the optimal predictor of preterm delivery in low-risk women. In an assessment of low-risk women, short cervical length at 24-28 weeks was detected in 8.5% of women

The risk of neonatal mortality and morbidity

The risk of neonatal mortality and morbidity is low after 34 completed weeks of gestation; although a trial of acute tocolysis may be initiated; aggressive tocolytic therapy is generally not recommended beyond 34 weeks, due to potential maternal complications. Between 24 and 33 weeks’ gestation, benefits of tocolytic therapy are generally accepted to outweigh the risk of maternal and/or fetal complications and these agents should be initiated provided no contraindications exist. Although aggressive tocolysis is not typically used beyond 34 weeks’ gestation, clinicians are advised not to deliver patients at this gestation without indication because of a higher risk of neonatal morbidity in infants born at 34-36 weeks’ gestation compared with deliveries at 37-40 weeks’ gestation

The risk of neonatal mortality decreases as gestational age at birth increases, but the relationship is nonlinear. The burden of preterm birth includes neonatal morbidity and longterm sequelae, including neurodevelopmental deficits (eg, cerebral palsy, impaired learning, visual disorders) and an increased risk of aspectrum of diseases in adulthood.

Therefore, preterm birth continues to be a major determinant of short and long term morbidity in infants and children.

In addition, preterm birth is the second most common cause of death (after pneumonia) in children younger than 5 years.

Prevalence

An estimated 15 million babies are born too early every year. That is more than 1 in 10 babies. Approximately 1 million children die each year due to complications of preterm birth. Many survivors face a lifetime of disability, including learning disabilities and visual and hearing problems.

Globally, prematurity is the leading cause of death in children under the age of 5 years. And in almost all countries with reliable data, preterm birth rates are increasing.

Inequalities in survival rates around the world are stark. In low- income settings, half of the babies born at or below 32 weeks (2 months early) die due to a lack of feasible, cost-effective care, such as warmth, breastfeeding support, and basic care for infections and breathing difficulties. In high-income countries, almost all of these babies survive. Suboptimal use of technology in middle-income settings is causing an increased burden of disability among preterm babies who survive the neonatal period

The rate of preterm birth rose steadily in the United States in the late 20th century,from 9.5% in 1981 to apeak of 12.8% in 2006. Rates of preterm birth have fortunately declined over the last decade. The preterm birth rate reached a nadir of 9.57% in 2014.In 2015,however, the rate of preterm birth was 9.62%—up slightly from the year prior and representing the first time since 2006 that the United States saw an increase in the rate of preterm birth. It is intriguing to note, however, that the PTB rate in the 1980s is not vastly different than the current PTB rate

Of these preterm births, 84% occurred at 32 to 36 weeks, 10 % occurred at 28 to <32 weeks, and 5% occurred at <28 weeks.

Classifications:

Subtypes of preterm birth are variably defined.

By gestational age:

• •World health Organization reported that Moderate preterm:32 to <37 weeks.
• Late preterm:34 0/7ths to 36 0/7ths weeks.
• Very preterm:28 to <32 weeks.
• Extremely preterm:<28 weeks.
• Another classification reported by Preterm: <37 weeks.
• Late preterm: 34 to 36 weeks.
• Early preterm: <34 weeks.
• By birth weight: Low birth weight (LBW): <2500 grams.
• Very low birth weight (VLBW): <1500 grams
• Extremely low birth weight (ELBW): <1000 grams

Pathogenesis of spontaneous preterm birth clinical and laboratory evidence suggest that a number of pathogenic processes can lead to a final common pathway that results in spontaneous preterm labor and delivery.