Preterm laborDr dabbaghi

•  Preterm birth refers to a delivery that occurs before 37 weeks of gestation. It may or may not be preceded by preterm labor.

•Although term pregnancy has been defined as 370/7ths to 416/7ths weeks of gestation, the period 370/7ths to 386/7ths weeks is considered “early term” because neonates born in this gestational age range have higher neonatal morbidity and mortality than infants born at “full term” from 390/7ths to 406/7ths weeks

Classification

Preterm births are described by gestational age, birth weight, and initiating factor.

• Gestational age criteria:World Health Organization: •Moderate to late preterm: 32 to <37 weeks •Very preterm: 28 to <32 weeks •Extremely preterm: <28 weeks Centers for Disease Control and Prevention •Preterm: <37 weeks •Late preterm: 34 to 36 weeks •Early preterm: <34 weeks

●Birth weight criteria:

•Low birth weight (LBW): <2500 grams•Very low birth weight (VLBW): <1500

grams•Extremely low birth weight (ELBW):

<1000 grams

●Initiating factor: spontaneous or iatrogenic (ie, indicated, provider-initiated)

•Spontaneous: 70 to 80 percent, due to preterm labor (40 to 50 percent) or preterm premature rupture of membranes (20 to 30 percent).

•Provider-initiated: 20 to 30 percent, due maternal or fetal issues (eg, preeclampsia, placenta previa, abruptio placenta, fetal growth restriction, multiple gestation). Complications of pregnancy can lead to both spontaneous and provider-initiated preterm births.

Prevalence•  Few countries are able to provide reliable national

preterm birth prevalence data. Worldwide, the preterm birth rate is estimated to be about 11 percent (range 5 percent [parts of Europe] to 18 percent [parts of Africa]), and about 15 million children are born preterm each year (range 12 to 18 million). Of these preterm births, 84 percent occurred at 32 to 36 weeks, 10 percent occurred at 28 to <32 weeks, and 5 percent occurred at <28 weeks.

• In the United States in 2013, 11.4 percent of births were <37 weeks (8 percent at 34 to 36 weeks and 3.4 percent <34 weeks [1.9 percent <32 weeks]).

• An increase in multiple gestations due to assisted reproductive technology is one reason for the increasing prevalence of preterm birth in some countries.

Epidemiology • Preterm birth rates in the United States are

highest among women <20 and >35 years of age and among non-Hispanic black mothers, followed by American Indian or Alaskan Natives, Hispanics, non-Hispanic whites, and Asian or Pacific Islanders. Racial/ethnic disparities in preterm birth rates may reflect biological (genetic) differences and/or variations in socioeconomic, lifestyle, and cultural factors; access to medical care; and environmental/occupational exposures.

Risk factors•  When trying to assess the probability of preterm

labor, it can be helpful to assess the presence or absence of clinical risk factors. Numerous risk factors for spontaneous preterm birth have been reported. Causal associations between most of these risk factors and preterm birth have been difficult to prove because (1) many preterm births occur among women with no risk factors, (2) some obstetrical complications resulting in preterm birth require cofactors to exert their effect, making the chain of causality difficult to document, and (3) an adequate animal model for study of preterm birth does not exist. Risk factors for preterm birth are discussed in detail separately

Significance• Child — Preterm birth is the leading direct cause of neonatal death

(death in the first 28 days of life), and is responsible for 27 percent of neonatal deaths worldwide. The risk of neonatal mortality decreases as gestational age at birth increases, but the relationship is nonlinear. Preterm birth is the second most common cause of death (after pneumonia) in children younger than five years.

• Preterm birth is also a major determinant of short- and long-term morbidity in infants and children. The burden of preterm birth includes neonatal morbidity and long-term sequelae, including neurodevelopmental deficits (eg, cerebral palsy, impaired learning, visual disorders) and an increased risk of a spectrum of diseases in adulthood.

• Maternal — Spontaneous preterm birth has been associated with an increased risk of maternal cardiovascular morbidity and mortality years after the delivery. In a 2015 systematic review and meta-analysis of 10 cohort studies, women who had a spontaneous preterm delivery were at higher risk for the following cardiovascular events when compared with women who delivered at term and followed for 12 to 35 years postpartum

Prevention  Interventions with proven efficacy for prevention of

preterm birth include:●Smoking cessation. ●Progesterone supplementation in asymptomatic women

with previous spontaneous preterm birth or in asymptomatic women with no history of spontaneous preterm birth but a short cervix (ie, ≤20 mm) in the current pregnancy.

●Reduction of multiple gestation by limiting the number of embryo transfers in women undergoing assisted reproductive technology or multifetal pregnancy reduction.

●Avoidance of induction of labor and scheduled cesarean delivery in the absence of medical indications.

●Cerclage in women with previous spontaneous preterm birth and a short cervix in current pregnancy.

• infants born between 37 weeks 0 days and 38 weeks 6 days gestation experience morbidities that are associated with prematurity compared to births at 39 weeks 0 days through 40 weeks 6 days when infant mortality is lower than at any other time in human gestation.” Those births 370/7 weeks through 386/7 weeks are now defined as early term and those 39 weeks 0 days through 40 weeks 6 days are defined as term.

•A consequence of defining optimal pregnancy outcomes to occur at 39 weeks has been the concern that some births before 39 weeks may be unnecessary, that is, intentional deliveries not based on medical indications.

•Undoubtedly, unnecessary obstetrical interventions are occurring before 38 or 39 weeks’ gestation, but the magnitude of this problem is unclear at this time.

MORBIDITY IN PRETERM INFANTS•Various morbidities, largely due to organ

system immaturity, are significantly increased in infants born before 37 weeks’ gestation compared with those delivered at term

MORBIDITY IN PRETERM INFANTS•For approximately 40 years, complications

in infants born before 34 weeks have been the primary focus. Only recently (2005) have late preterm infants—34 to 36 weeks—gained attention because of their increased morbidity.

MORBIDITY IN PRETERM INFANTS• Attention has also been given to increasingly

small preterm infants—very low birthweight and extremely low birthweight. These very small infants suffer disproportionately not only the immediate complications of prematurity but also long-term sequelae such as neurodevelopmental disability. Indeed, live births once considered “abortuses” because they weighed < 500 g are now classified as live births.

Mortality and Morbidity at the

Extremes of Prematurity

Threshold of Viability

•Births before 26 weeks are generally considered at the current threshold of viability, and these preterm infants pose various complex medical, social, and ethical considerations.

Threshold of Viability• Infants now considered to be at the threshold of

viability are those born at 22, 23, 24, or 25 weeks (American College of Obstetricians and Gynecologists, 2012a,b). These infants have been described as fragile and vulnerable because of their immature organ systems. Moreover, they are at high risk for brain injury from hypoxic-ischemic injury and sepsis. In this setting, hypoxia and sepsis start a cascade of events that lead to brain hemorrhage, white-matter injury that causes periventricular leukomalacia, and poor subsequent brain growth eventuating in neurodevelopmental impairment (Chap. 34, p. 656). Because active brain development normally occurs throughout the second and third trimesters, those infants born at 22 to 25 weeks are believed especially vulnerable to brain injury.

Threshold of Viability

Threshold of Viability

Threshold of Viability

•Cesarean delivery at the threshold of viability is controversial. For example, if the fetus-infant is perceived to be too immature for aggressive support, then cesarean delivery for common indications such as breech presentation or nonreassuring fetal heart rate patterns might be preempted. This aside, national data clearly show a high frequency of cesarean delivery for these small infants.

Threshold of Viability

Threshold of Viability at Parkland Hospital

• Policies were developed in conjunction with the Neonatology Service. We must emphasize that the decision not to perform cesarean delivery does not necessarily imply that the fetus is “nonviable” or “written off.” Neonatologists are consulted before delivery, and there is discussion of survival and morbidity with the woman and her family. A neonatologist attends each delivery and determines subsequent management. From an obstetrical standpoint, all fetal indications for cesarean delivery in more advanced pregnancies are practiced in women at 25 weeks. Cesarean delivery is not offered for fetal indications at 23 weeks. At 24 weeks, cesarean delivery is not offered unless fetal weight is estimated at 750 g or greater. Aggressive obstetrical management is practiced in cases of growth restriction.

Late Preterm Birth

•Infants between 34 and 36 weeks account for more than 70 percent of all preterm births. These are the fastest increasing and largest proportion of singleton preterm births in the United States. Thus, increased attention is being given to determining optimal obstetrical and neonatal management of late preterm birth.

Late Preterm Birth

•Approximately 80 percent of these were due to idiopathic spontaneous preterm labor or prematurely ruptured membranes (Fig. 42-7). Complications such as hypertension or placental accidents were implicated in the other 20 percent of cases.

Late Preterm Birth

•Neonatal mortality rates were significantly increased in each late preterm week compared with those at 39 weeks as the referent and higher neonatal mortality rates for late preterm infants.

Late Preterm Birth

Late Preterm Birth

Late Preterm Birth

•the frequency of respiratory morbidity decreased by approximately 50 percent per week from 34 to 37 completed weeks.

Late Preterm Birth

•Increased rates of adverse neurodevelopment have also been found in late preterm infants compared with term newborns

Late Preterm Birth

•These findings suggest that the health-care focus on prematurity should be expanded to include late preterm births.

•Even so, because approximately 80 percent of these women begin labor spontaneously —similar to births before 34 weeks—attempts to interrupt preterm labor have not been satisfactory.