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It is generally admitted that there exists in the medulla a centre for uterine contractions, which can be stimulated by an excess of carbon dioxide in the blood, by anaemia and the presence of various toxic substances; and it seems highly probable that the frequency of premature labour in cases of renal insufficiency and eclampsia may be due to the action of metabolic poisons upon the centre.

—J. Whitridge Williams (1903)


In this textbook’s first edition, very little was mentioned regarding preterm birth. Indeed, preterm birth was not incorporated as a stand-alone topic until the 13th edition in 1966. And, this content totaled only three sentences that cited use of isoxsuprine as a tocolytic agent. In contrast, present-day research now produces more than 3000 articles published annually. Data derive from study of animal models, translational research, clinical trials, and genetic investigations. Despite efforts, elucidating the biology of human parturition and the subsequent efforts to prevent preterm birth remain elusive (Martin, 2017).


Low birthweight defines neonates who are born too small. Preterm or premature birth describes neonates who are born too early. With respect to gestational age, a newborn may be preterm, term, or postterm. With respect to size, a newborn may be normally grown and appropriate for gestational age; undersized, thus, small for gestational age; or overgrown and consequently, large for gestational age. Small for gestational age categorizes newborns whose birthweight is <10th percentile for gestational age. Other frequently used terms have included fetal-growth restriction or intrauterine growth restriction. The term large for gestational age describes newborns whose birthweight is >90th percentile for gestational age. The term appropriate for gestational age designates newborns whose weight is between the 10th and 90th percentiles.

Thus, neonates born before term can be small or large for gestational age, but still preterm by definition. Low birthweight refers to neonates weighing 1500 to 2500 g; very low birthweight are those between 1000 and 1500 g; and extremely low birthweight refers to those between 500 and 1000 g.

Before the 15th edition of this textbook, a preterm or premature newborn was defined by a birthweight <2500 g. With that edition, preterm neonates were considered to be those delivered before 37 completed weeks, that is, <366/7 weeks (Pritchard, 1976). This definition, which has now been in use for more than 40 years, was first promulgated in 1976 by the World Health Organization (WHO) and the International Federation of Gynecology and Obstetrics (FIGO). The definition derived from a statistical analysis of gestational age distribution at birth (Steer, 2005). Importantly, the denotation lacks a specific functional basis and should be clearly distinguished from the concept of prematurity. Prematurity represents incomplete development of various organ systems at birth. For example, the lungs are particularly affected, leading to the respiratory distress syndrome (RDS) (Chap. 34, Respiratory Distress Syndrome).

In 2013 in the United States, 23,446 infants died in their first year of life, and a third of ...

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