105: Intrauterine Growth Restriction (Small for Gestational Age)

I. DEFINITION

The terms intrauterine growth restriction (IUGR) and small for gestational age (SGA) are sometimes used interchangeably. Although related, they are not synonymous. SGA describes an infant whose weight is lower than population norms or lower than a predetermined cutoff weight. Most commonly, SGA infants are defined as having a birthweight below the 10th percentile for gestational age or >2 standard deviations below the mean for gestational age. In contrast, IUGR infants have not attained optimal intrauterine growth.

The ponderal index, arrived at by the following formula, can be used to identify infants whose soft tissue mass is below normal for the stage of skeletal development. A ponderal index <10th percentile may be used to identify IUGR infants. Thus, all IUGR infants may not be SGA, and all SGA infants may not be small as a result of a growth-restrictive process.

1. Symmetric IUGR. (HC = Ht = Wt, all <10%) The head circumference (HC), length (Ht), and weight (Wt) are all proportionately reduced for gestational age. Symmetric IUGR is due to either decreased growth potential of the fetus (congenital infection or genetic disorder) or extrinsic conditions that are active early in pregnancy.

2. Asymmetric IUGR. (HC = Ht < Wt, all <10%) Fetal weight is reduced out of proportion to length and head circumference. The head circumference and length are closer to the expected percentiles for gestational age than is the weight. In these infants, brain growth is usually spared. The usual causes are uteroplacental insufficiency, maternal malnutrition, or extrinsic conditions appearing late in pregnancy.

3. More recently, fetal growth restriction (FGR) is being used to denote impaired fetal growth based on healthy fetal growth standards. An individual baby's growth potential is determined by both maternal and fetal factors. Recent attempts have been made to develop individualized growth charts taking into account maternal physiological characteristics such as race, ethnicity, parity, height, and so on, as well as fetal characteristics like gender.

   Term optimal weight (TOW) is defined as the optimal weight based on fetal weight curves of healthy infants born at term. For an individual pregnancy, fetal growth can be combined with TOW to show the gestation-related optimal growth (GROW) curve. GROW charts adjusted for maternal height, weight, parity, and ethnicity are available at www.gestation.net.

   Estimation of fetal growth velocity with serial measurements may be useful to identify FGR. For example, a fetus with weight >10th percentile may be growth restricted if fetal growth velocity declines. FGR may be early or late, comparable to symmetric and asymmetric IUGR.

   Early FGR is difficult to identify (by measuring crown-rump length), as often the timing of conception is not known. However, slow growth velocity between the first and second trimesters can identify infants at risk for perinatal death before 34 weeks' gestation. The onset of FGR before 34 weeks is associated with sequential changes on Doppler studies that parallel worsening placental function. Typically, umbilical artery Doppler changes precede biophysical profile parameters. Late FGR after 34 weeks' gestational age is more difficult to identify and has less characteristic Doppler changes.

II. INCIDENCE

About 3–10% (up to 15%) of all pregnancies are associated with IUGR, and 20% of stillborn infants are growth retarded. The perinatal mortality rate is 5–20 times higher for growth-retarded fetuses, and serious short- or long-term morbidity is noted in half of the affected surviving infants. IUGR is estimated to be the predominant cause for low birthweight in developing countries. It is estimated that a third of infants with birthweights <2500 g are in fact growth retarded and not premature. Term infants with birthweights <3rd percentile have a higher morbidity and a 10 times higher mortality than appropriate for gestational age infants. In the United States, uteroplacental insufficiency is the leading cause of IUGR. An estimated 10% of cases are secondary to congenital infection. Chromosomal and other genetic disorders are reported in 5–15% of IUGR infants.

III. PATHOPHYSIOLOGY

Fetal growth is influenced by fetal, maternal, and placental factors.

1. Fetal factors

   1. Genetic. Approximately 20% of birthweight ...