period or from an improved obstetrical estimate, derived in part using sonography. (Modified with permission from Duryea EL, Hawkins JS, Mcintire DO, et al: A revised Recently, Sovio and colleageus (2015) demonbirth weight reference for the United States. Obstet Gynecol. 2014 Jul;124(1 ):16-22.) strated that growth velocity of the abdominal circumference in the lowest decile distinguishes SGA newborns who sufer increased morbidity. Conversely, an excessive fetalgrowth velocity, particularly ofthe abdominal circumferencewhich may be correlated with increased hepatic blood low-is associated with an overgrown neonate (American College of Obstetricians and Gynecologists, 2016a).
Several conditions or disorders can adversely afect the normal growth of a fetus. It is important clinically to distinguish between fetal-growth restriction and constitutional low birth weight. u
Fetal-Growth Disorders 100 -Morbidity 175 90 -Mortality) 150U) 80D �S s· C 70 > 0 100g0 60 30 0-E �� 75 50 -30 � u 0S 20 0E 259J) 10 >1010987654321<1 Birthweightpercentile FIGURE 44-3 Relationship between birthweight percentile and perinatal mortality and morbidity rates in 1560 small-for-gestationalage fetuses. A progressive increase in both mortality and morbidity rates is observed as birthweight percentile decreases.. (Data from Manning, 1995.) • Symmetrical versus Asymmetrical Growth Restriction Campbell and Thoms (1977) described the use of the sonographically determined head-to-abdomen circumerence ratio (HCIA) to diferentiate growth-restricted fetuses. Those who were symmetrical were proportionately small, and those who were asymmetrical had disproportionately lagging abdominal growth compared with head growth. The onset or etiology of a particular fetal insult is hypothetically linked to either type of growth restriction. In the instance ofsymmetricalgrowth restriction, an early insult could result in a relative decrease in cell number and size. For example, early global insults such as those from chemical exposure, viral infection, or cellular maldevelopment with aneuploidy may cause a proportionate reduction of both head and body size. Asymmetricalgrowth restriction might follow a later pregnancy insult such as placental insuiciency from hypertension. In this variation, resultant diminished glucose transfer and hepatic storage would primarily afect cell size and not number. Thereby, fetal abdominal circumferencewhich relects liver size-would be reduced. Brain Sparing Such somatic growth restriction is proposed to result from preferential shunting of oxygen and nutrients to the brain. This allows normal brain and head growth, that is-brain sparing. Accordingly, the ratio of brain weight to liver weight during the last 12 weeks-usually about 3 to I-may be increased to 5 to 1 or more in severely growth-restricted infants. Because of brainsparing efects, asymmetrical fetuses were thought to be preferentially protected from the full efects ofgrowth restriction. Considerable evidence has since accrued that fetal growth patterns are much more complex. For example, fetuses with 847 .�..,40