Intrauterine growth restriction is suggested when the estimated fetal weight falls below the 10th percentile for gestational age. Approximately 70% of so-called IUGR is merely constitutional, although an underlying etiology may be difficult to elucidate in the antepartum period. The incidence of pathologic IUGR is between 4% and 8% of gestations in developed countries and between 6% and 30% in developing countries. Fetuses with IUGR have a 2- to 6-fold increase in perinatal morbidity and mortality. The degree of symmetry present in IUGR may suggest an etiology. In symmetrical IUGR, the fetus is proportionally small, whereas in asymmetrical IUGR, abdominal growth lags behind head circumference. Symmetrical growth restriction implies an early insult such as chemical exposure, infection, or aneuploidy. Asymmetrical growth is more associated with a late pregnancy insult such as placental insufficiency.

The etiology of IUGR includes both maternal and fetal causes:

• Constitutionally small mothers and inadequate weight gain. Women who weigh <100 lb at conception have double the risk for a small-for-gestational age newborn. Inadequate or arrested weight gain after 28 weeks of pregnancy is also associated with IUGR.

• Chronic maternal disease. Multiple medical conditions of the mother, including chronic hypertension, cyanotic heart disease, pregestational diabetes, malnutrition, and collagen vascular disease, can cause growth restriction. Preeclampsia and smoking are associated with IUGR.

• Fetal infection. Viral causes including rubella, cytomegalovirus, hepatitis A, parvovirus B19, varicella, and influenza are among the best known infectious antecedents of IUGR. In addition, bacterial (listeriosis), protozoal (toxoplasmosis), and spirochetal (syphilis) infections may be causative.

• Chromosomal abnormalities. Chromosomal abnormalities, such as trisomy 13 and 18 and Turner syndrome, are often associated with IUGR. Trisomy 21 usually does not cause significant growth restriction.

• Teratogen exposure. Any teratogen can produce fetal growth restriction. Anticonvulsants, tobacco, illicit drugs, and alcohol can impair fetal growth.

• Placental abnormalities. Placental abnormalities that lead to decreased blood flow to the fetus can cause growth restriction.

• Multiple gestation is complicated by growth impairment of at least one fetus in 12% to 47% of cases.

Diagnosis is made by sonographic assessment. Gestational age must be established with certainty, preferably in the first trimester, to assess fetal growth accurately. A lag in fundal height of more than 2 cm from gestational age after 20 weeks should prompt sonographic evaluation.

Management generally depends on gestational age. In general, growth restriction diagnosed in the second trimester, or in the setting of a structural defect, prompts offering amniocentesis or fetal blood sampling for karyotype and viral studies. Even when termination is not considered, the information gained from these tests may be important for parents, obstetricians, and pediatricians planning the delivery and newborn care. Other management includes the following:

• IUGR at or near term: Fetal assessment includes serial fetal growth ultrasounds every 3 to 4 weeks, nonstress testing or biophysical profiles, Doppler studies, and assessment of AFV. For uncomplicated growth restriction in singleton gestation, ACOG recommends induction at 38 to 39 6/7 weeks. The ACOG recommends individualizing delivery timing between 32 and 37 6/7 weeks' gestation with growth restriction complicated by oligohydramnios, abdominal Doppler studies, or maternal comorbidities such as preeclampsia or chronic hypertension.

• IUGR remote from term: Attempt conservative management and fetal testing as discussed in IUGR at or near term. Ensure adequate nutrition and initiate fetal surveillance. Umbilical artery Doppler velocimetry showing elevated systolic-to-diastolic ratio or absent or reversed end-diastolic flow suggests fetal compromise (see chapter 4) and should prompt escalating surveillance or delivery.

• The decision to deliver an IUGR infant remote from term, particularly before 32 weeks' gestation, weighs the risk of preterm birth against continued exposure to the intrauterine environment. Contemporary management combines information from nonstress testing, biophysical profiles, and umbilical artery Dopplers. In pregnancies prior to 32 weeks' gestation, the addition of Doppler information from the ductus venosus can be employed to better identify those pregnancies that can be prolonged, although use of this parameter is not yet incorporated into contemporary guidance. In general, vaginal delivery is not contraindicated, but there is an increased risk of fetal intolerance of labor. Growth-restricted newborns are susceptible to hypothermia and other metabolic abnormalities, such as hypoglycemia. Some data show that fetal growth restriction has long-term negative effects on cognitive function, independent of other variables.