Fetal Growth Restriction

Fetal growth restriction (FGR) is a common pregnancy complication worldwide, leading to stillbirth and neonatal mortality and morbidity. FGR is generally defined as the failure of a fetus to achieve its full genetically determined growth potential due to pathologic etiologies, primarily placental dysfunction. The terminology used to classify fetuses and newborns who do not achieve weight within population-based norms is often inconsistent. Historically, FGR has been defined based on fetal size, typically using estimated fetal weight (EFW) or abdominal circumference below the tenth percentile compared to gestational age reference standards. The American College of Obstetricians and Gynecologists (ACOG) employs this criteria for FGR. However, some fetuses are constitutionally small in accordance with their genetic growth potential and are not growth-restricted, while other appropriate for gestational-age fetuses that have not achieved their full growth potential are growth-restricted.

Adding to this confusion is the term small for gestational age (SGA), which is often used synonymously with FGR. ACOG and several other professional societies employ SGA to describe newborns whose birth weight is below the tenth percentile for their gestational age, while others, including the International Federation of Gynecology and Obstetrics (FIGO), use SGA as a preliminary diagnosis to characterize an EFW or birthweight below the tenth percentile. However, most experts utilize the term SGA as a reflection of neonatal size that may or may not be associated with an underlying pathological etiology, whereas FGR is solely due to an antenatal pathologic condition.

Consequently, only using fetal size to diagnose FGR can result in misdiagnoses, as the tenth percentile cutoff does not distinguish between healthy, constitutionally small fetuses and those genuinely affected by growth restriction. Additionally, this definition does not identify growth-restricted fetuses with an EFW above the tenth percentile. Evidence suggests that increased perinatal risks can start from higher EFW percentiles, though fetal mortality is the highest in fetuses with an EFW below the third percentile. Therefore, many experts have recommended implementing the third percentile to identify FGR. In 2016, the International Society of Ultrasound in Obstetrics and Gynecology published a consensus defining diagnostic criteria for FGR not associated with congenital abnormalities. These parameters differed depending on whether FGR was early-onset FGR (<32 weeks gestational age) or late-onset FGR (≥32 weeks gestational age) based on gestational age at diagnosis (see Table. International Society of Ultrasound in Obstetrics and Gynecology Consensus Definition of Fetal Growth Restriction). Early-onset FGR is usually more severe, often associated with preeclampsia, and easier to detect, whereas late-onset FGR is more common, subtle, and harder to diagnose. The 2016 consensus included the following FGR parameters:

Conditions leading to FGR are the disorders inherent to the fetal-placental-maternal unit, fetal malnutrition, and intrauterine space constraints restricting fetal growth, resulting in a significantly increased risk of intrauterine demise, neonatal morbidity, and neonatal death. Diagnostic evaluation of FGR primarily comprises ultrasonographic assessment of fetal biometric measurements and Doppler velocimetry, especially of the umbilical artery. However, the predictive accuracy of these methods can be limited. Management of fetal growth restriction involves careful fetal surveillance, serial fetal growth and amniotic fluid volume assessments, and determining the appropriate timing for delivery to balance the risks of stillbirth and prematurity.