Fetal growth restriction (FGR)/intrauterine growth restriction (IUGR) and small for gestational age (SGA) are sometimes used interchangeably but are not synonymous. FGR and IUGR are the same conditions where intrinsic fetal pathology exists or when placental support for the fetus is compromised, resulting in fetal hypoxia and undernourishment, and a pathologic restriction of the fetal genetic potential for growth. SGA describes an infant whose weight is lower than the population norms or a predetermined cutoff weight. Most commonly, SGA infants are defined as having a birthweight below the 10th percentile for gestational age and gender, or >2 standard deviations below the mean for gestational age. It can occur following a pathological process (FGR/IUGR) or just represent a small infant based on constitiutional factors (maternal weight, height, ethnicity, parity), known as constitutional SGA. FGR/IUGR infants are not all SGA, and SGA infants are not all FGR. It has been proposed that “SGA should be based on growth percentiles, and FGR be based on evidence of pathologic growth.” This distinction between constitutional SGA and pathologic FGR has important implications for fetal monitoring, risks of perinatal morbidity and mortality, and the optimal timing of delivery. FGR increases the risk of fetal mortality 10- to 20-fold compared to an appropriately grown fetus. Constitutionally small infants are not at an increased risk of perinatal mortality or morbidity.
FGR (as opposed to IUGR) is used more often recently, especially in the obstetric literature. The American College of Obstetrics and Gynecology defines FGR as a fetus with an estimated fetal weight (EFW) <10% of gestational age. It is the condition where infants have not achieved their optimal intrauterine growth potential. FGR infants are further classified as symmetric or asymmetric based on anthropometric measurements and by onset as early or late onset.
Symmetric or asymmetric FGR
Symmetric fetal growth restriction (head circumference, height, and weight all <10th percentile). The head circumference, length, and weight are all proportionately reduced for gestational age. Symmetric FGR is due to either decreased growth potential of the fetus (congenital infection or genetic disorder) or extrinsic conditions that are active very early in pregnancy. Features include: earlier insult in gestation, less common than asymmetric, accounts for approximately 30% of FGR cases, ponderal index (see Section VI.E.4 for definition) normal, malnutrition less pronounced, normal cell size with reduced cell number, and poor prognosis.
Asymmetric fetal growth restriction (more common). Fetal weight is reduced out of proportion to the 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. Common etiologies include uteroplacental insufficiency, maternal malnutrition, and extrinsic conditions appearing late in pregnancy. Features include: occurs later in gestation, more common than symmetric growth restriction (70%–80% of cases), normal cell number but cell size is reduced, low ponderal index, more pronounced malnutrition, and good prognosis.
Early or late onset FGR. Early detection of FGR identifies infants at risk of mortality in utero, but is difficult based on fetal anthropometry (by measuring crown-rump length) alone. 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.
Early onset FGR (<32 weeks’ gestation). Early onset is associated with sequential changes in Doppler studies that parallel worsening placental function. Typically, umbilical artery Doppler changes precede biophysical profile parameters. Early FGR is associated with more severe placental disease, fetal hypoxia and undernutrition, systemic cardiovascular adaptation, increased risks of preeclampsia ...