When a healthcare provider suggests a baby is “measuring small,” it causes immediate concern for expectant parents. This diagnosis indicates that the baby’s estimated size falls below a predetermined threshold for the current stage of pregnancy. Medical professionals use precise criteria to determine if the small measurement is a variation of normal or an indication of a potential underlying issue. Understanding the terminology and the difference between a small baby and a growth-restricted baby is the first step in managing the diagnosis.
Understanding the Terminology of Fetal Size
The finding that a baby is measuring small is categorized into two distinct groups: Small for Gestational Age (SGA) and Fetal Growth Restriction (FGR).
Small for Gestational Age (SGA)
SGA is a statistical description based purely on size. A fetus is defined as SGA if its estimated weight or abdominal circumference is below the 10th percentile for its gestational age. Many SGA babies are constitutionally small, meaning they are genetically predisposed to be smaller due to factors like maternal height, weight, or ethnicity, but are otherwise healthy and growing at a normal rate.
Fetal Growth Restriction (FGR)
FGR is a pathological diagnosis, meaning the baby’s growth potential is being restricted by an underlying medical condition. This condition is more serious than SGA. A baby with FGR is not just small; their growth has slowed or stopped due to a problem interfering with the supply of nutrients and oxygen. A baby can be FGR even if their estimated weight is technically above the 10th percentile, provided there is evidence of a pathological restriction. FGR carries a higher risk of adverse outcomes compared to a baby who is SGA but growing appropriately. Doctors use specialized customized growth charts, which factor in the mother’s characteristics, to help distinguish between a healthy, constitutionally small baby and one whose growth is genuinely impaired.
Primary Causes of Restricted Fetal Growth
Fetal Growth Restriction occurs when the baby cannot receive adequate nourishment or oxygen. Causes are typically divided into problems originating from the placenta, the mother, or the fetus itself.
Placental Dysfunction
The most common cause is placental dysfunction, often termed placental insufficiency. This occurs when the placenta does not develop or function well enough to transfer necessary resources to the developing baby. This poor blood flow and nutrient transfer causes the growth to slow, particularly later in the pregnancy.
Maternal Conditions
Maternal health conditions can directly affect placental function and blood flow to the uterus. Conditions like pre-existing high blood pressure, chronic kidney disease, or autoimmune diseases such as lupus increase the risk of FGR. Lifestyle factors, including smoking, alcohol consumption, and poor nutrition, also impair the environment needed for optimal fetal growth.
Fetal Factors
Less common, but still important, are factors originating from the fetus. These tend to cause growth restriction earlier in the pregnancy. These can include chromosomal abnormalities, such as Trisomies 13 or 18, or congenital infections acquired by the mother during pregnancy, such as toxoplasmosis or cytomegalovirus (CMV). Multiple gestations, like twins or triplets, can also lead to FGR because the shared resources of the placenta are distributed among multiple babies.
Diagnostic Tools and Ongoing Monitoring
If a baby is suspected of measuring small, usually after an initial measurement of the mother’s fundal height is low, a formal ultrasound is performed to confirm the size. Ultrasound biometry is the standard method, which involves measuring the baby’s head circumference, abdominal circumference, and femur length to calculate an Estimated Fetal Weight (EFW). Serial ultrasounds, typically performed every two to four weeks, are then used to track the growth velocity. This helps determine if the baby is simply small or if the growth is pathologically restricted.
Doppler Velocimetry
Doppler velocimetry is a vital tool for assessing the severity of FGR, using ultrasound to measure blood flow in specific vessels. Doppler scans of the umbilical artery are particularly informative, as they reflect the resistance to blood flow in the placenta. Increased resistance, indicated by absent or reversed end-diastolic flow, is a sign of worsening placental function and fetal compromise. Doppler is also used to evaluate the fetal circulatory system, particularly the middle cerebral artery (MCA). In response to a lack of oxygen, the baby’s body attempts to protect the brain, a phenomenon called “brain-sparing,” which is detected as decreased resistance in the MCA.
Biophysical Profile and Non-Stress Tests
Healthcare providers also rely on a Biophysical Profile (BPP) and Non-Stress Tests (NSTs) to monitor overall fetal well-being. The BPP combines ultrasound assessment of fetal breathing, movement, tone, and amniotic fluid volume. The NST monitors the baby’s heart rate patterns in relation to its own movements.
Management Strategies and Delivery Timing
The management of confirmed Fetal Growth Restriction relies on careful surveillance, as there is no specific treatment that reverses the underlying pathological process. Expectant management involves increasing the frequency of monitoring to detect signs of worsening condition before serious harm occurs. This surveillance may include weekly or twice-weekly Doppler velocimetry, Non-Stress Tests (NSTs), and Biophysical Profiles (BPPs), depending on the severity of the growth restriction.
Preparing for Preterm Delivery
If early delivery is anticipated, especially before 34 weeks of gestation, the mother is often given a course of antenatal corticosteroids. These injections help accelerate the development and maturity of the baby’s lungs, reducing the risk of respiratory complications associated with prematurity. In extremely preterm cases before 32 weeks, magnesium sulfate may also be administered to provide neuroprotection for the baby’s brain.
Timing Delivery
Determining the optimal timing of delivery is the most difficult decision in managing FGR. This decision balances the risks of staying in the suboptimal intrauterine environment versus the risks associated with prematurity. For FGR with otherwise normal Doppler studies, delivery is typically planned between 38 and 39 weeks. If Doppler studies show signs of severe compromise, such as reversed end-diastolic flow in the umbilical artery, delivery may be necessary much earlier to prevent stillbirth. After delivery, babies born with FGR may require specialized care, but the long-term prognosis is generally favorable.