A major focus of prenatal care is to determine whether a fetus is at risk for growth restriction and to identify the growth restricted fetus. Fetal growth is important because there is an inverse relationship between the fetal/neonatal weight percentile and adverse perinatal outcome, with the greatest risk at weights below the third percentile for gestational age [1,2]. In addition, fetal growth restriction appears to be an antecedent to some cases of hypertension, hyperlipidemia, coronary heart disease, and diabetes mellitus in the adult (Barker hypothesis). (See "Small for gestational age infant" and "Possible role of low birth weight in the pathogenesis of primary (essential) hypertension".)
Prenatal screening for fetal growth restriction (FGR) in general obstetrical populations involves identifying risk factors for impaired fetal growth and physically assessing fetal size. Clinical suspicion based upon risk factors or physical examination is followed by a detailed sonographic assessment of the fetus, placenta, and amniotic fluid. The most common sonography-based definition of FGR is a weight below the 10th percentile for gestational age, although other definitions employing a variety of criteria have been advocated. This definition is controversial because it does not make a distinction among fetuses who are constitutionally small, growth restricted and small, and growth restricted but not small (see 'Definition' below).
The goal of prenatal detection of fetuses that are small is to reduce the morbidity and mortality associated with this problem by employing some intervention. Although stillbirth rates are higher when FGR is not detected antenatally , there is only low quality evidence that antenatal identification of FGR improves outcome . Unfortunately, significant problems remain in terms of defining the population of growth restricted fetuses at high risk of adverse outcome, accurately identifying these fetuses in utero, and determining interventions to improve outcome. These issues need to be addressed by large multicenter studies employing consistent definitions, randomly assigned interventions, and with long-term follow-up.
The diagnosis of FGR will be reviewed here. The etiology, management, and prognosis of this disorder are discussed separately. (See "Fetal growth restriction: Causes and risk factors" and "Fetal growth restriction: Evaluation and management" and "Small for gestational age infant".)
The most common definition of FGR refers to a weight below the 10th percentile for gestational age , although other definitions employing a variety of criteria have been advocated (eg, <5th percentile, <3rd percentile) [6,7]. This definition is problematic because it does not make a distinction among fetuses who are constitutionally small, small because a pathologic process has kept them from achieving their genetic growth potential, and not small but a pathologic process has kept them from achieving their genetic growth potential. It also requires an appropriate reference standard (table 1). Whether this standard should be based on birth weights across gestation, ultrasound-estimated fetal weights across gestation, or a customized standard, is also controversial . The major criticism of the birth weight reference is that before term it is flawed because babies born preterm are often growth restricted. The ultrasound-based approach is limited by the inaccuracy and imprecision of ultrasound estimated fetal weight.