Fetal Growth Restriction, or FGR, is a condition that occurs during pregnancy where a fetus does not grow at the expected rate. It is defined as an estimated fetal weight that falls below the 10th percentile for its gestational age, meaning the baby is smaller than 90% of other babies at the same week of pregnancy. This indicates the fetus is not achieving its genetically determined growth potential. The condition can develop at any point during pregnancy, affecting the infant’s overall size and organ development.
Differentiating FGR from Small for Gestational Age
The terms Fetal Growth Restriction (FGR) and Small for Gestational Age (SGA) are often used interchangeably, but they describe different situations. SGA is a broad term for a baby whose weight is below the 10th percentile for their gestational age at birth. Many SGA babies are healthy; they are just constitutionally small due to genetics, similar to how some adults are naturally shorter.
In contrast, FGR implies a pathological process is preventing the fetus from reaching its growth potential. While all FGR babies are also SGA, not all SGA babies have FGR. This distinction is important because FGR is diagnosed when there are signs of a reduced growth rate, suggesting an issue with the pregnancy environment, and is associated with a higher risk of complications.
Potential Causes of Fetal Growth Restriction
The causes of FGR fall into three main categories: maternal, placental, and fetal. Maternal health conditions like high blood pressure (preeclampsia), diabetes, and kidney disease can affect blood flow to the fetus. Lifestyle factors such as poor nutrition, smoking, alcohol consumption, or substance use during pregnancy can also impede fetal growth.
Problems with the placenta are the most common cause of FGR. The placenta transfers oxygen and nutrients from the mother to the fetus, and if it doesn’t function correctly—a condition known as placental insufficiency—it cannot support normal development. This can result from poor blood flow to the placenta or issues like a placental abruption, where it separates from the uterine wall.
Fetal factors can also lead to growth restriction, including genetic or chromosomal abnormalities. Infections contracted by the mother and passed to the fetus, such as cytomegalovirus (CMV) or toxoplasmosis, can be a cause. Pregnancies involving multiples, like twins or triplets, carry a higher likelihood of FGR because the placenta may not adequately support more than one fetus.
Diagnosis and Monitoring During Pregnancy
Diagnosis and monitoring begin with accurately establishing the baby’s gestational age from the last menstrual period and a first-trimester ultrasound. Throughout the pregnancy, a series of ultrasounds is the primary tool for tracking growth. These scans allow healthcare providers to measure the fetus’s head, abdomen, and femur to estimate its weight and track it against the expected growth curve.
When FGR is suspected, an umbilical artery Doppler ultrasound is a key diagnostic method. This specialized scan measures blood flow through the umbilical cord, providing a direct assessment of placental function. Abnormal blood flow can indicate the placenta is not delivering enough oxygen and nutrients.
Once FGR is diagnosed, monitoring becomes more frequent. This includes non-stress tests (NSTs), which monitor the fetal heart rate in response to movement. Another tool is the biophysical profile (BPP), which combines an NST with an ultrasound to assess fetal breathing, movement, muscle tone, and the amount of amniotic fluid, as low levels can be an indicator of FGR.
Medical Management and Delivery Considerations
The management of FGR focuses on closely monitoring the fetus, as there is no treatment to reverse the condition during pregnancy. The goal is to balance the risks of delivering a premature baby with the risks of the fetus remaining in a growth-restricting environment. Management strategies may include recommendations for the mother, such as increased rest and nutritional counseling.
The timing of delivery is a central part of the plan. If monitoring reveals the fetus is in distress or if growth has stalled, an early delivery may be recommended. This decision involves weighing the baby’s gestational age against the severity of the growth restriction and Doppler ultrasound findings.
This often leads to a planned early delivery, sometimes well before the 37-week mark. A Cesarean section (C-section) may be advised because growth-restricted infants can be more vulnerable to the stresses of labor. A C-section can be a safer option to avoid potential distress caused by insufficient oxygen during contractions.
Postnatal Care for the Infant
After birth, infants who experienced FGR may require specialized care in a Neonatal Intensive Care Unit (NICU). Their small size can make it difficult to maintain body temperature, so they are placed in an incubator to stay warm. These infants also face a higher risk of low blood sugar (hypoglycemia) and may need support with feeding to ensure they receive adequate nutrition for growth.