Measuring two weeks behind can cause anxiety, but it is a common finding during prenatal care. Fetal measurements estimate the baby’s size and growth trajectory. These initial findings are screening tools, not definitive diagnoses, and a slight discrepancy is often benign. The next steps involve understanding these measurements and determining if the size difference is a simple technicality or suggests the baby is not meeting its full growth potential.
What Does Measuring Behind Mean
A measurement two weeks behind the expected gestational age is typically identified through two methods: Fundal Height (FH) and Ultrasound Biometry. Fundal height is a quick clinical screening tool where the provider measures the distance from the pubic bone to the top of the uterus. After about 20 weeks, this measurement in centimeters should roughly match the number of weeks pregnant, allowing for a variation of plus or minus 2 to 3 centimeters.
If the FH is consistently smaller than expected, or if risk factors are present, the provider orders an ultrasound for a detailed assessment. Ultrasound biometry measures specific parts of the baby, such as the head circumference, abdominal circumference, and femur length, to calculate the Estimated Fetal Weight (EFW). A baby is considered “small for gestational age” (SGA) if the EFW or abdominal circumference falls below the 10th percentile. This means the baby weighs less than 90% of others at the same gestational age. A two-week lag often correlates with this 10th percentile threshold, prompting investigation, but it does not automatically indicate a problem.
Benign Reasons for Inaccurate Measurements
In many cases, a small measurement is due to factors unrelated to an underlying medical issue. The most frequent explanation is incorrect gestational dating, occurring if the initial conception date or last menstrual period was miscalculated. An early ultrasound, typically in the first trimester, provides the most accurate dating, and reviewing those initial scans can often resolve the discrepancy.
The mother’s physical characteristics, or maternal body habitus, can also affect fundal height accuracy. A mother who is tall, thin, or whose uterus is tilted backward may consistently measure small because the top of the uterus is harder to palpate. The baby’s position can also temporarily influence the measurement, such as when the baby is lying deep in the pelvis.
Many fetuses measuring below the 10th percentile are simply “constitutionally small” babies. These babies are naturally small but healthy, following a growth curve influenced by parental size, ethnicity, and genetics. Up to 40% of fetuses diagnosed as small for gestational age are constitutionally small, with no underlying pathology.
When Size Discrepancy Indicates Fetal Growth Restriction
While many small measurements are benign, the discrepancy flags the need to rule out Fetal Growth Restriction (FGR), formerly known as Intrauterine Growth Restriction (IUGR). FGR is a condition where the fetus fails to achieve its genetically determined growth potential due to a pathological process. FGR is a concern because it can increase the risks of stillbirth and other complications.
The most common cause of FGR is placental insufficiency, meaning the placenta does not function efficiently to deliver sufficient oxygen and nutrients. Maternal health conditions, such as high blood pressure, chronic kidney disease, or preeclampsia, can impair placental blood flow. Other less frequent causes include certain infections, like cytomegalovirus (CMV), or fetal chromosomal abnormalities.
FGR is classified into two types: symmetrical and asymmetrical. Symmetrical FGR occurs when the baby’s head and body are proportionally small. This type often begins early in pregnancy and is associated with genetic issues or early infections. Asymmetrical FGR is more common and usually begins later. It is characterized by a smaller abdominal circumference relative to the head circumference, reflecting the body’s attempt to prioritize blood flow and nutrients to the brain, a process known as “brain-sparing.”
Clinical Management and Next Steps
Once a discrepancy is identified, the next steps involve a comprehensive surveillance plan to monitor fetal well-being and growth velocity. The initial step is often serial growth scans, which are repeat ultrasounds performed every two to four weeks to track the rate of growth. Measuring the trend of growth is more telling than a single measurement, helping distinguish a constitutionally small baby from one with FGR.
The healthcare team utilizes specialized fetal surveillance testing to assess the baby’s health. This includes:
- The Non-Stress Test (NST), which monitors the baby’s heart rate in response to movement.
- The Biophysical Profile (BPP), an ultrasound-based test assessing fetal breathing, movement, tone, and amniotic fluid volume.
A specialized ultrasound called a Doppler Flow Study is often performed to measure blood flow in the umbilical cord and sometimes the fetal brain. Abnormal Doppler findings, such as decreased blood flow in the umbilical artery, strongly indicate placental dysfunction and FGR. Management for confirmed FGR involves close observation, and in severe cases, may include maternal rest or a planned early delivery, depending on severity and gestational age.