A diagnosis that your baby is “measuring small” can be unsettling during pregnancy, raising immediate concerns about the baby’s health. This finding signals that your healthcare provider needs to investigate further to determine the cause and ensure the best possible outcome. Understanding the difference between a naturally small baby and one with restricted growth is the first step in addressing this situation. This article provides information about the definitions, causes, assessments, and management strategies for restricted fetal growth.
Understanding What “Measuring Small” Means
The initial finding that a baby is measuring small is based on a size measurement falling below a specific threshold for that stage of pregnancy. Medically, a fetus is considered Small for Gestational Age (SGA) if its estimated fetal weight (EFW) is below the 10th percentile for its current gestational week. This means the baby weighs less than 90% of babies at the same age. SGA is a statistical designation based purely on size.
Not all babies who are SGA have a problem; many are simply constitutionally small due to normal genetic variation. However, when the small size is due to a pathological process preventing the fetus from reaching its growth potential, it is defined as Fetal Growth Restriction (FGR), also known as Intrauterine Growth Restriction (IUGR). FGR carries higher risks for complications compared to a healthy, constitutionally small baby.
The distinction between a healthy small baby and one with FGR is often made by looking at the pattern of growth restriction. Symmetric restriction means all parts of the baby, including the head, are proportionally small, often associated with a problem that began early in the pregnancy. Conversely, asymmetric restriction is more common and develops later, showing a relatively normal head size but a smaller abdominal circumference. This asymmetric pattern suggests a “brain-sparing” effect, where the baby prioritizes nutrient delivery to the brain over other organs, reflected in the reduced abdominal measurement.
The estimated fetal weight is calculated using measurements including the biparietal diameter (BPD), head circumference, abdominal circumference (AC), and femur length (FL). Before an ultrasound, a provider may suspect an issue if the fundal height—the distance from the pubic bone to the top of the uterus—measures more than three centimeters less than the number of weeks of pregnancy.
Primary Causes of Restricted Fetal Growth
The reasons a baby’s growth may be restricted are grouped into problems stemming from the placenta, the mother, or the fetus itself. The most frequent underlying cause is a problem with the placenta, accounting for a significant percentage of cases. The placenta transfers nutrients and oxygen from the mother to the baby. When it fails to function adequately—a condition called placental insufficiency—the baby is starved of the resources needed for growth.
Maternal health conditions are a common factor that can directly impact placental function. Chronic high blood pressure, preeclampsia, kidney disease, and autoimmune disorders can impair blood flow to the uterus, restricting the baby’s supply. Lifestyle factors also play a role, including poor maternal nutrition, substance use (smoking, alcohol consumption), and the use of certain medications. Additionally, maternal infections during pregnancy, such as those included in the TORCH panel, can infect the fetus and limit its growth.
Factors directly related to the fetus or the pregnancy itself are less common. Fetal chromosomal abnormalities or genetic syndromes are responsible for a smaller percentage of FGR cases, particularly when the restriction is symmetric and begins early. Structural birth defects, such as heart defects, can also limit growth. In multiple gestations, such as twins, the fetuses may compete for limited resources, sometimes resulting in restricted growth for one or both babies.
Monitoring and Diagnostic Assessments
Once a baby is identified as measuring small, detailed assessments are performed to differentiate between a healthy SGA baby and one with FGR. The primary tool is the specialized ultrasound, which allows for serial measurement of the baby’s growth parameters, usually every two to four weeks. This repeated imaging tracks the growth velocity to determine if the baby is simply small or if its growth has slowed or stopped, indicating true restriction.
A Doppler velocimetry study is a non-invasive ultrasound that assesses blood flow in specific vessels, providing insight into the placenta’s function and the baby’s adaptation. Providers check the umbilical artery to assess the resistance of blood flow from the baby to the placenta. They also examine the middle cerebral artery, as increased blood flow here can signal the brain-sparing effect, indicating the baby is redirecting blood to its head in response to placental insufficiency.
Other tests monitor the baby’s overall well-being. A Non-Stress Test (NST) monitors the baby’s heart rate in response to its movements, looking for accelerations that indicate good oxygenation. The Biophysical Profile (BPP) combines the NST with ultrasound observations of fetal breathing movements, body movements, muscle tone, and amniotic fluid volume. If structural anomalies or genetic issues are suspected, diagnostic procedures like amniocentesis for chromosomal microarray analysis may be offered.
Management Strategies and Delivery Timing
There is no specific treatment to reverse the underlying cause of placental insufficiency, the most common reason for FGR. Management focuses on optimizing the uterine environment and close surveillance to prevent complications. Improving maternal nutrition, ensuring adequate hydration, and recommending rest are common initial conservative measures, though their effectiveness in reversing severe FGR is limited.
A key intervention when preterm delivery is possible is the administration of antenatal corticosteroids, such as betamethasone. These injections accelerate the maturation of the baby’s lungs and other organs, reducing the risk of severe respiratory distress if the baby is born prematurely. Magnesium sulfate may also be given if delivery is anticipated before 32 weeks, as it provides neuroprotection for the baby’s brain.
The most important decision in managing FGR is determining the optimal time for delivery. This involves balancing the risks of continuing the pregnancy in a potentially hostile intrauterine environment versus the risks associated with prematurity. As long as surveillance tests show the baby is tolerating the conditions well, the pregnancy is typically allowed to continue for further maturation.
Signs of fetal deterioration, such as severely abnormal Doppler flow or concerning heart rate patterns, often prompt an earlier delivery recommendation. Delivery timing ranges from close to term (38 to 39 weeks) for mild cases to much earlier for severe cases with absent or reversed end-diastolic flow in the umbilical artery. The mode of delivery, whether vaginal or Cesarean section, is determined by the severity of the growth restriction and the baby’s ability to tolerate the stress of labor.