When a healthcare provider mentions your baby is “measuring behind,” they are referring to a discrepancy between the calculated age of the pregnancy and the size of the fetus as determined by ultrasound measurements. This means the estimated fetal weight or specific biometric measurements, such as the head circumference or femur length, are smaller than what is statistically expected for the current gestational week. A two-week difference is a relatively common finding in prenatal care. A smaller measurement does not automatically indicate a serious problem, and the next steps involve careful investigation to determine the exact cause.
How Gestational Age is Calculated
The calculation of gestational age, which determines your due date, starts with two primary methods. The most common initial method is counting from the first day of the last menstrual period (LMP), which assumes a standard 28-day cycle.
The second, and often more accurate, method involves an early ultrasound, ideally performed before 12 weeks of gestation. During this first-trimester scan, the technician measures the crown-rump length (CRL). The CRL measurement is considered the most precise way to establish the due date. The “measuring behind” situation arises when a later ultrasound suggests the fetus is significantly smaller than the age previously established by the LMP or the initial CRL measurement.
Non-Fetal Reasons for Discrepancy
A frequent explanation for a fetus measuring two weeks behind is a simple dating error. The LMP method relies on the assumption of a predictable 28-day cycle, which is not true for all individuals. Irregular or longer menstrual cycles can mean that ovulation and conception occurred later than the standard calculation predicts.
This variability in ovulation timing directly affects the actual age of the pregnancy, making the LMP-based due date inaccurate. If the initial gestational age was set solely by LMP, a two-week difference can often be attributed to this normal biological variation and delayed conception.
Fetal Causes of Restricted Growth
When dating error is ruled out, the focus shifts to whether the baby is genuinely small for its age. This falls into two distinct categories: Small for Gestational Age (SGA) and Fetal Growth Restriction (FGR).
Small for Gestational Age (SGA)
SGA means the baby’s estimated fetal weight is below the 10th percentile for that gestational week. Many SGA babies are “constitutionally small,” meaning they are naturally small but healthy. This is often due to parental genetics and body type, and these babies typically have an excellent prognosis.
Fetal Growth Restriction (FGR)
FGR, previously known as Intrauterine Growth Restriction (IUGR), indicates a pathological process where the baby is not growing at its full potential. This is most often due to issues with the placenta limiting the delivery of nutrients and oxygen. FGR is characterized by two patterns: symmetric restriction, where all parts of the baby are proportionally small, and asymmetric restriction. Asymmetric restriction, sometimes called “brain-sparing,” occurs when the head size is relatively normal while the abdominal circumference is disproportionately small.
Ongoing Monitoring and Management
Once a baby is noted to be measuring behind, the medical team initiates a surveillance plan to distinguish between healthy SGA and at-risk FGR. This management relies heavily on serial ultrasounds, performed every two to four weeks to track the baby’s growth velocity. The rate of growth is a significant indicator of whether the small size is normal or pathological.
A crucial monitoring tool is the Doppler flow study, which assesses blood flow through the umbilical artery and other fetal vessels to evaluate placental function. Abnormal Doppler readings are a strong indication of FGR caused by placental insufficiency. Additional tests, such as the non-stress test (NST) and the biophysical profile (BPP), are used to assess the baby’s immediate well-being. The management plan is tailored based on the severity of the findings and the specific diagnosis.