41 Prevention and Management of Preterm Parturition HYAGRIV N. SIMHAN, MD, MS | VINCENZO BERGHELLA, MD | JAY D. IAMS, MD Preterm birth is the principal unsolved problem in perinatal to this approach and alternative classification systems that medicine. Nearly 15 million infants were born prematurely in reflect a modern understanding of clinical presentation and 20101—more than 1 in 10 of all births. Over 1 million babies biology. The physiology and pathophysiology of preterm par- die of the consequences of being born too soon: one every 30 turition are discussed in Chapters 6 and 7. This chapter addresses seconds. The majority of premature births—60%—occur in the overall problem of preterm birth, including the epidemiol- south Asia and sub-Saharan Africa. In 2010, the United States ogy and burden of disease for all preterm neonates and specific ranked sixth in the world for the number of babies (517,443) care for the clinical syndrome of preterm labor. pPROM and born preterm. The rate of preterm birth in the United States stillbirth are discussed in Chapters 42 and 45, respectively. rose by more than one-third between 1980 and 2006, even as Newborn and childhood complications of preterm birth are the perinatal and infant mortality rates decreased. In the past discussed in Chapter 73. 10 years, the rate of preterm birth plateaued and, for the first time since 1980, decreased slightly. Advances in care have improved outcomes for preterm infants, but prematurity is still The Problem of Preterm Birth 2 the most common underlying cause of perinatal and infant DEFINITIONS morbidity and mortality3 in developed nations. Consequences of preterm birth for surviving infants extend across the life A birth at less than 245 days after conception (with good ges- course and include neurodevelopmental, respiratory, gastro- tational dating criteria), or at or after 20 and before 37 weeks’ intestinal, and other morbidities. (259 days) gestation from the first day of the last normal men- Preterm birth is a unique condition, defined by time rather strual period, is commonly defined as preterm or premature. than a distinct phenotype or pathology. The duration of preg- Births at or after 37 0/7 weeks are considered to be term, nancy at birth reflects two major correlates of maternal and fetal nomenclature that has been recently revisited to reflect fetal health: (1) whether the birth was occasioned by a normal or an maturity (e.g., 37 weeks: term, near-term, or early term; 39 aberrant pathway, and (2) whether the infant has reached matu- weeks: full term). Infants who weigh less than 2500 g at birth, rity at birth. Infants born at full term after the spontaneous regardless of gestational age, are designated as low birth weight onset of normally progressive labor are most likely to be healthy (LBW). Infants who weigh less than 1500 g are called very low and mature. A process that leads to birth before the fetus has birth weight (VLBW), and those below 1000 g are extremely fully matured suggests that continued pregnancy may carry low birth weight. Preterm and LBW infants have in the past some health risk for the mother or the fetus, or both. Thus, been considered together, but advances in the accuracy of preg- premature parturition may provide a health advantage over nancy dating increasingly allow outcomes related to gestational continued pregnancy for the mother and infant and yet also age to be distinguished from outcomes related to birth weight. may compromise an immature infant’s health. This is important, because perinatal and infant morbidities vary Classifications of preterm birth may advance biologic under- substantially according to age and maturity as well as weight.4 standing, define clinical phenotypes, and aid in designing trials Obstetric data are reported by gestational age. Traditionally and interpreting their data. The most commonly used catego- reported by birth weight, newborn and infant data are increas- ries are based on clinical presentation as either a spontaneous ingly described by gestational age as well.5 or an indicated preterm birth. Spontaneous preterm births are Both lower and upper boundaries of “preterm” are currently preceded by activation of one or more steps of the parturition under scrutiny. The lower boundary between preterm birth and process: cervical ripening, membrane and decidual activation, spontaneous abortion (SAB) was historically based on maternal and coordinated uterine contractility (see Chapter 6). Clinical perception of fetal movement, and it is commonly defined as presentations of spontaneous preterm delivery include preterm 20 weeks’ gestation in the United States, but it varies among labor with intact membranes, preterm premature rupture of states2 and countries. The 20-week boundary is challenged by membranes (pPROM), preterm cervical effacement or insuffi- data showing that pregnancies ending between 16 and 20 weeks ciency, and some instances of uterine bleeding of uncertain have pathophysiology similar to that of births at 20 to 26 weeks, origin. Indicated preterm births are medically caused or initiated and that that pathophysiology confers a similarly increased and are actively undertaken in response to maternal or fetal risk for preterm birth between 16 and 36 weeks in future compromise. This categorization scheme has fallen under scru- pregnancies,6 regardless of whether the fetus was liveborn or tiny in recent years. In this chapter, we address some limitations stillborn.7,8 679 Downloaded for Rodrigo Terra ([email protected]) at Clinica Alemana de Santiago - JCon from ClinicalKey.com by Elsevier on October 19, 2018. For personal use only. No other uses without permission. Copyright ©2018. Elsevier Inc. All rights reserved. 680 PART 4 Disorders at the Maternal-Fetal Interface TABLE Gestational Age Terminology 41.1 Description Gestational Age (wk) < 28 Preterm <37 28−31 Late preterm 34 0/7 to 36 6/7 32−33 Term 37 0/7 to 41 6/7 34−36 Early term 37 0/7 to 38 6/7 Full term 39 0/7 to 41 6/7 37−38 Postterm ≥42 39 From Fleischman AR, Oinuma M, Clark SL. Rethinking the definition 40−41 of “term pregnancy.” Obstet Gynecol. 2010;116:136–139. ≥42 14 Figure 41.2 Chart showing relative frequency of births by gesta- Preterm Low birth weight tional age intervals in weeks. (Data from Martin JA, Hamilton BE, 13 Ventura SJ, et al. Births: final data for 2010. Natl Vital Stat Rep. 2012;61:1–72.) 12 11 10 Percent 9 8 < 28 7 28−31 0 32−33 1990 1995 2000 2006 2011 34−36 SOURCE: CDC/NCHS, National Vital Statistics System. Figure 41.1 Rates of liveborn infants who were preterm (<37 weeks) and low birth weight (<2.5 kg), United States, 1990–2011. (From Martin JA, Hamilton BE, Ventura SJ, et al. Births: final data for 2010. Natl Vital Stat Rep. 2012;61:1–72.) Figure 41.3 Gestational age distribution of all preterm births (PTBs). Of all PTBs, 16% occur before 32 weeks’ gestation, 13% occur at 32 to 33 weeks, and 71% occur at 34 to 36 weeks. (Data from Martin Similarly, the disadvantages of an upper boundary based on JA, Hamilton BE, Ventura SJ, et al. Births: final data for 2010. Natl Vital age rather than maturity have become increasingly apparent. Stat Rep. 2012;61:1–72.) Fetal and neonatal maturation are not complete at 37 weeks’ gestation. Infants born at 37 and even 38 weeks display clinical features of immaturity more commonly than those born at 39 More than 70% of preterm births occur between 34 and 36 weeks, and they often suffer related short- and long-term weeks’ gestation (Fig. 41.3). Although these late preterm infants morbidity.9-11 Current practice acknowledges the less-than- experience significant morbidity, most16 perinatal mortality and complete maturity of 37- to 38-week infants by adoption of the serious morbidity occur among the 16% of preterm infants categories shown in Table 41.1. (<3.5% of all births) who are born before 32 weeks’ gestation, commonly called very preterm births (see Fig. 41.3). More than INCIDENCE OF PRETERM BIRTH half (52%) of all infant mortality occurs among infants born before 32 weeks’ gestation.16 Births before 37 weeks in the United States increased annu- ally from 9.4% in 1980 to a peak of 12.8% in 2006. The rate Ascertainment of Preterm Birth and has since fallen each year from 11.99% in 201012 (Figs. 41.1 Low Birth Weight and 41.2) to just under 9.6% in 2015.13 This recent decrease Reported rates of preterm birth vary according to the gesta- is due to many factors. One is certainly a previously inflated tional age boundaries chosen and whether and when prenatal rate due to the use of incorrect gestational age based on LMP. ultrasound was employed. Definitions of preterm birth vary Gestational age is now calculated by the Centers for Disease internationally and within the United States.2 The lower bound- Control and Prevention birth data analysts based on ultra- ary of gestational age is 20 weeks in most of the United States, sound dating. Other reasons include reduced teenage birth rate but it varies from 20 to 24 weeks in other countries.17,18 The and fewer higher-order multiple births. Additionally, a public definition of LBW is universally accepted as a birth weight of policy shift to prevent non–medically indicated births at less less than 2500 g, but the lower boundary ranges from 350 to than 39 weeks’ gestation and smoking bans in several states 500 g, is variably applied, and is often affected by cultural and have been associated with the reduced rate of preterm births. religious beliefs and whether the infant shows signs of life.