Chapter 26

### I. Intensive and convalescent care

1. Definition

1. Birthweight is a measure of fetal growth in pregnancy but is imprecise because it represents fetal growth at only one time point, at the end of gestation. Birthweight provides limited insight into the fetal growth pattern throughout gestation.

2. Low birthweight (LBW) is defined as a birthweight less than 2500 g but does not take gestational age into consideration. Thus, some babies who are classified as LBW may in fact be normally grown because they are preterm. Similarly the descriptor of very low birthweight (VLBW), defined as less than 1500 g at birth, will include a number of babies of older gestation with poor growth in utero.

3. Intrauterine growth restriction (IUGR) is the failure to attain optimal intrauterine growth, whereas small for gestational age (SGA) describes an infant whose weight is lower than population norms (3rd, 5th, or 10th percentiles).

4. SGA infants are most often defined as having a birthweight below the 10th percentile for gestational age.

5. The ponderal index, defined below, can be used to identify infants whose mass is below normal for their linear development, ie, a measure of “thinness.” A ponderal index <10th percentile has been used to identify IUGR infants at birth.

$Bithweight×100Crown−Heel Length3$

6. Thus, all IUGR infants may not be SGA, and all SGA infants may not be small as a result of fetal growth restriction.

7. Ultimately, crossing of growth centiles in utero on multiple ultrasound assessments is the only true measure of IUGR, but such information is rarely available and so the proxies described above are used.

8. IUGR may be further thought of as symmetrical or asymmetrical.

1. Symmetric IUGR occurs when head circumference, height, and weight are all proportionately reduced (<10%) for gestational age. This is due to either decreased growth potential of the fetus (normal genetic variation or genetic disorders) or extrinsic conditions that are active early in pregnancy (metabolic conditions, congenital infection, early nutritional insults, or adverse in utero conditions).

2. Asymmetric IUGR occurs when the head circumference and length are closer to the expected percentiles for gestational age than is the weight. In these infants, there is some degree of sparing of brain growth relative to body weight. The usual causes are uteroplacental insufficiency, maternal undernutrition or malnutrition, or extrinsic conditions applied late in pregnancy.

2. Incidence

1. LBW affects about 15% worldwide whereas 3% to 10% of all pregnancies are associated with IUGR.

2. Estimation of IUGR may vary based on the neonatal or fetal growth charts used to define IUGR, with some evidence that individualized centiles may be even more predictive.

3. Perinatal mortality rates are four to eight times higher for IUGR babies and ∼20% to 80% of stillborn infants are IUGR. The latter are due to the individual causes of growth restriction in an IUGR pregnancy and a failure of the fetal adaptive responses of redistribution of fetal blood flow to key organs leading to a decrease in fetal growth rate (Figure 26-1) leading to an increased risk of fetal mortality.

3. Pathophysiology

1. In utero mechanisms

1. IUGR in the human fetus can be caused by factors that are of maternal, placental, or fetal origin (Figure 26-1) including living at high altitude, maternal undernutrition, hyperthermia, drug and alcohol abuse, placentation abnormalities, intrauterine ...

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