Medical brain notes 42

  INTRAUTERINE GROWTH RESTRICTION   “Fetal growth restriction” describes infants whose weights are much lower than expected. Population-based norms are used to categorize abnormal growth.  A fetus or infantwhose weight is less than the 10th percentile of a specific pop-ulation at a given gestational age is designated as having  intrauterine growth restriction  [ IUGR ] (Table 18.1).Therefore, careful assignment of gestational age is cru-cial to the diagnosis and management of patients with IUGR.   The term  “small for gestational age” (SGA)  is used to describe an infant with a birth weight at the lower extreme of the normal birth weight distribution. In the United States, the most commonly used definition of SGA is a birth weight below the 10th percentile for gestational age. The use of the terms “small for gestational age” (SGA) and “intrauterine growth restriction” has been confusing, and the terms often are used interchangeably. The use of ges...

  Pathophysiology   For a fetus to thrive in utero, an adequate number of fetal cells and cells that differentiate properly are both requisite. In addition, nutrients and oxygen must be available via an adequately functioning uteroplacental unit to allow an increase in the number of cells and in cell size. Early in pregnancy, fetal growth occurs primarily through  cellularhyperplasia,  or cell division, and early-onset IUGR maylead to an irreversible diminution of organ size and, per-haps, function. Early-onset IUGR is also more commonly associated with heritable factors, immunologic abnormal-ities, chronic maternal disease, fetal infection, and multi-ple pregnancies. Later in pregnancy, fetal growth depends increasingly on  cellular hypertrophy  rather than hyper-plasia alone, so that delayed-onset IUGR may also result in decreased cell size, which may be more amenable to restoration of fetal size with adequate nutrition. The nor-mal fetus grows throughout...

  Etiology   IUGR is a descriptive term for a condition that has numerous potential causes.  Determining the specific diagnosis is impor-tant for optimal management. Although a number of causes of IUGR have been recognized, a definite etiology of IUGR cannot be identified in approximately 50% of all cases. In addition, because the utilization of a percentile cut-off of 10% alone will result in a high proportion of false-positives, two-thirds or more of such fetuses categorized as IUGR will be simply constitutionally small and otherwise healthy.   Factors that affect fetal growth are extensive and include maternal, fetal, and placental causes; these are listed in Box 18.1.   MATERNAL FACTORS   Maternal factors include viral infections, such as rubella, varicella, and cytomegalovirus, which are associated with high rates of growth restriction, particularly if infection occurs early in pregnancy. Although these infections may manifest only as mild “flu-like” i...

  Diagnosis   Assessment of gestational age is important in early preg-nancy, because dating becomes increasingly imprecise at later gestational ages. Antenatal recognition of IUGR depends upon the recognition of risk factors and the clinical assessment of uterine size, fol-lowed by biometric measurements.   Physical examination is limited in usefulness in recognizing IUGR or in making a specific diagnosis, but it is an impor-tant screening test for abnormal fetal growth. Maternal size and weight gain throughout pregnancy also have limited value, but access to such information is readily available; a low maternal weight or little or no weight gain during pregnancy may suggest IUGR. Serial measurements of  fundal height  are commonly used as a screening test forIUGR, but have high rates of false-negative and false-positive predictive values. Between 20 and 36 weeks of ges-tation, fundal height should increase approximately 1 cm per week, consistent with gestation...

  Management   The goal of management of a growth-restricted fetus is to deliver the healthiest possible infant at the optimal time. Continued management of pregnancy with IUGR is based on the results of fetal testing.   Serial evaluations of fetal biometry should be performed every 3 or 4 weeks to follow the extent of growth restriction.  Fetal monitoring  is important, and may include fetal move-ment counting, nonstress testing, biophysical profiles, and Doppler studies. There are no specific therapies that have proven beneficial for pregnancies complicated by IUGR.   The fetus should be delivered if the risk of fetal death exceeds that of neonatal death, although in many cases these risks are difficult to assess. For example, a fetus with IUGR with normal anatomic sur-vey, normal amniotic fluid volume, normal Doppler studies, and normal fetal testing may not benefit from early delivery. Conversely, the growth-restricted fetus with serial biometry measure...

  MACROSOMIA   Two terms have been used to define excessive fetal growth.  Fetal macrosomia  is based on weight alone and refers to afetus with an estimated weight of 4000–4500 g or greater.  Large for gestational age (LGA)  generally implies a birthweight >90% for a given gestational age, and is dependent on both weight and gestational age with percentiles gener-ated from population-specific norms (see Table 18.1). By definition, the prevalence of LGA is fixed, but not all neonates at the upper extreme of size are pathologically large. Growth potential, growth rate, and gestational age at onset may be important considerations.   Etiology   Macrosomia, like fetal growth restriction, has multiple potential causes, categorized into fetal or maternal factors (Box 18.2). Similar to fetal growth restriction, fetal factors include the genetic composition or inherent growth poten-tial of the individual, and genetic syndromes such as Beckwith-Wiedemann...