Fetal Growth Restriction

Excerpt

Fetal growth restriction (FGR) affects about 3% to 7% of all pregnancies. FGR is a condition in which the fetus fails to attain the growth potential determined by the genetic makeup. Ultrasonography-estimated fetal weight (EFW) of less than the 10th percentile for the specific gestational age (GA) is required for the diagnosis of FGR. Some fetuses are constitutionally small and at less than the 10th percentile in weight for GA in accordance with their genetic growth potential. They are not growth-restricted and may be appropriately characterized as small for gestational age fetuses. Conditions leading to FGR are the disorders inherent to the fetal-placental-maternal unit, fetal undernutrition, and intrauterine space constraints restricting fetal growth. FGR is a fetal pathology that can result in significant short-term and long-term complications and adversely impact the quality of life.

Classification

The severity of FGR is determined by the EFW.

1. EFW between 3rd and 9th percentile - moderate FGR

2. EFW less than the 3rd percentile - severe FGR

Based on additional fetal biometric parameters, such as head circumference (HC), abdominal circumference (AC), femur length (FL), and biparietal diameter (BD), FGR can be categorized as symmetrical and asymmetrical. In symmetrical FGR, all growth parameters are proportionally reduced, whereas, in asymmetrical FGR, classically, the abdominal circumference is reduced below 10 percentile, while other measurements are relatively preserved and may be within normal limits.

Symmetrical FGR

This group constitutes 20% to 30% of all FGR cases. Poor placental function is a well-established cause of FGR. Adverse intrauterine conditions beginning in the early pregnancy (first trimester) that may cause fetal nutrient restriction, such as smoking, cocaine use, chronic hypertension, anemia, and chronic diabetes mellitus, may result in symmetrical FGR. Chromosome anomalies, such as aneuploidy, are a major cause of symmetrical FGR. TORCH infection (Toxoplasma gondii, cytomegalovirus, herpes simplex virus, varicella-zoster virus, Treponema, and others) contracted prenatally are present in 5% to 15% of cases with FGR and form an important group. Depending on the time and duration of occurrence, severe fetal malnutrition can cause either symmetrical or asymmetrical FGR.

Asymmetrical FGR

In asymmetrical FGR, which constitutes about 70% to 80% of all FGR cases, the timing of intrauterine insult is in the late second or third trimester of pregnancy. The growth restriction is disproportionate, with relative preservation of head circumference (fetal brain) and reduced abdominal circumference (fetal liver), resulting in an increased brain to the liver ratio (BLR). Preeclampsia is a well-recognized cause of asymmetrical FGR. This condition, identified in about 8% of pregnancies in Western countries, generally develops after 20 weeks of gestation and is characterized by hypertension and proteinuria. Chronic hypertension leads to placental vascular remodeling, vascular sclerosis, and ischemia, thus impeding blood flow to the fetus. As a result, the fetal liver glycogen and body adipose tissues are diminished while the brain continues to grow normally with a preferential blood supply.