Discovering your baby is measuring two weeks behind expected size during a prenatal scan can be worrying. This measurement does not necessarily indicate a problem, but it signals that further investigation is needed. “Measuring behind” refers to the estimated fetal weight or specific body measurements falling below the average range for the calculated gestational age. Medical teams use this information as a screening tool to distinguish between a healthy, naturally smaller baby and one whose growth may be restricted by an underlying issue.
Understanding the Difference Between Gestational Age and Size
A measurement discrepancy prompts a review of two factors: the accuracy of the dating and the actual size of the fetus. Gestational age is typically calculated from the first day of the last menstrual period (LMP), but this method is often inaccurate due to ovulation timing variations. A more precise gestational age is established using an early first-trimester ultrasound, where the crown-rump length (CRL) measurement provides the most accurate dating, with a margin of error of about 5 to 7 days.
If the size difference is only about two weeks, the most common explanation is a slight error in the initial dating, especially if the due date was based on LMP. In this scenario, the fetus is actually the correct size for its true age, and the due date may simply be adjusted. Once a reliable gestational age is confirmed, the focus shifts to the fetus’s size, assessed using measurements like head circumference, abdominal circumference, and femur length to calculate an estimated fetal weight (EFW).
The EFW is plotted on a growth chart to determine its percentile. A baby is generally classified as Small for Gestational Age (SGA) if the EFW falls below the 10th percentile for its confirmed age. This classification is purely statistical, meaning 10% of all healthy babies will naturally fall into this range. The medical challenge lies in determining whether the SGA classification is due to a natural, “constitutionally small” size or a pathological condition known as growth restriction.
Primary Medical Reasons for Slowed Fetal Development
When dating is confirmed and the baby’s size remains below the 10th percentile, physicians determine if the fetus is simply small but healthy, or if it has Fetal Growth Restriction (FGR). FGR, also known as Intrauterine Growth Restriction (IUGR), implies a pathological process where the fetus fails to achieve its growth potential. Constitutionally small babies, accounting for over half of all SGA cases, are healthy; they are small due to non-pathological factors like parental height, ethnicity, or the baby’s sex.
The most frequent cause of true FGR involves the placenta, which transfers oxygen and nutrients from the parent to the fetus. If the placenta is not functioning optimally (placental insufficiency), the baby receives inadequate resources, leading to restricted growth. This often results in “asymmetric FGR,” where the fetus redirects blood flow to the brain at the expense of the abdomen, resulting in a small abdominal circumference relative to the head circumference.
Maternal health conditions contribute significantly to placental insufficiency and FGR. Chronic high blood pressure, pre-existing diabetes, or autoimmune disorders like lupus can compromise placental blood flow. Lifestyle factors such as smoking, substance use, or poor maternal nutrition also hinder optimal fetal growth.
Less common causes of FGR relate to the fetus itself, including chromosomal abnormalities or congenital infections. Infections such as cytomegalovirus (CMV) or toxoplasmosis can directly impact fetal cells, affecting early growth. The timing of the growth restriction is significant; problems arising earlier in gestation are more likely to result in a symmetrically small fetus, while placental issues often manifest later, leading to the asymmetric pattern.
Next Steps in Monitoring and Follow-Up Care
Once a fetus is identified as SGA, a detailed monitoring plan is implemented to distinguish a small, healthy baby from one with FGR. The cornerstone of this follow-up is serial ultrasound examinations, typically scheduled every two weeks, to assess growth velocity. These scans track the rate of growth over time, which is a better indicator of health than a single size measurement.
Doppler studies are a major component of follow-up, assessing blood flow in specific fetal vessels. The umbilical artery Doppler measures resistance in the placental circulation; an abnormal reading suggests inefficient placental function. Doppler of the middle cerebral artery, which checks blood flow to the fetal brain, can detect the “brain-sparing” effect that occurs when the fetus attempts to compensate for lack of oxygen or nutrients.
Fetal well-being is monitored using a Non-Stress Test (NST) or a Biophysical Profile (BPP). An NST records the fetal heart rate in response to movement. A BPP combines the NST with ultrasound observations of fetal breathing, body movements, and amniotic fluid volume. These tests provide a comprehensive picture of the baby’s current health status and tolerance for the intrauterine environment.
Management strategies are tailored to the severity of FGR and the gestational age. For mild cases with reassuring monitoring results, the primary intervention is often close surveillance and lifestyle adjustments, such as improved nutrition or rest. If monitoring indicates significant distress or worsening blood flow abnormalities, the medical team may recommend early delivery (usually between 37 and 40 weeks). In severe FGR cases, where the intrauterine environment is riskier than outside, delivery may be planned earlier to prevent complications.