The third trimester, beginning around 28 weeks, marks a period of rapid fetal development. Prenatal care at this stage frequently involves growth scans, which healthcare providers use to monitor the baby’s progression. These measurements are converted into percentiles, offering a statistical comparison of the baby’s size against the average for its specific gestational age. Understanding these percentile numbers helps parents feel informed, as this monitoring system ensures the baby is thriving and identifies potential variations in growth trajectory early on.
How Fetal Growth Percentiles Are Calculated
Fetal growth percentiles are generated by comparing a baby’s physical measurements to a large reference population of other babies at the same point in gestation. The foundation of this calculation is the Estimated Fetal Weight (EFW), which is an estimate derived from ultrasound biometrics, not a direct measurement. The healthcare provider measures specific parts of the fetus, including the head circumference (HC), the abdominal circumference (AC), and the femur length (FL).
These individual measurements are mathematically combined using established formulas, such as the Hadlock equation, to calculate the EFW. The resulting EFW is then plotted on a specialized growth chart to determine its percentile. For example, a baby at the 50th percentile means their estimated weight is greater than 50 out of every 100 babies at that same gestational age.
The 50th percentile represents the statistical average for a baby at that particular week of pregnancy. While the EFW provides a single number, the percentile summarizes how the baby’s size compares to the general population. This process allows medical professionals to assess the baby’s size relative to its peers and track the rate of growth over time.
The Expected Percentile Range at 28 Weeks
The primary focus of a growth scan at 28 weeks is to confirm that the baby’s growth is following a healthy and consistent pattern. For most babies, the expected range for the percentile falls between the 10th and the 90th percentile. Falling anywhere within this broad range is considered healthy growth and indicates that the baby is developing as expected for its gestational age.
At 28 weeks, the estimated fetal weight for a baby at the 50th percentile, or the average, is approximately 1.2 kilograms (about 2.7 pounds). This is an estimate, and a baby’s weight can fluctuate within the normal range due to factors like genetics, maternal build, and ethnicity. A baby measuring at the 25th percentile, for instance, is simply smaller than the average but is still growing appropriately.
Variations within the 10th to 90th percentile range are common and often reflect a baby’s unique constitutional size. Some babies are genetically predisposed to be smaller or larger, similar to how height varies in the adult population. The consistency of the growth trajectory over several scans is often more informative than a single percentile number.
When Percentiles Indicate Closer Monitoring
Measurements that fall outside the 10th to 90th percentile range prompt healthcare providers to initiate closer monitoring to rule out underlying issues. If the Estimated Fetal Weight (EFW) is below the 10th percentile, the baby is classified as Small for Gestational Age (SGA). While many SGA babies are constitutionally small and healthy, this finding requires further investigation, such as an ultrasound assessment of the umbilical artery blood flow using Doppler studies.
A diagnosis of Fetal Growth Restriction (FGR), formerly known as Intrauterine Growth Restriction (IUGR), is made when the small size is accompanied by evidence of a pathological process, such as placental insufficiency. This may be indicated by abnormal Doppler results or reduced amniotic fluid volume. Monitoring for FGR typically involves more frequent ultrasounds, often two or more times per week, to assess fetal well-being and determine the optimal time for delivery.
Conversely, an EFW above the 90th percentile classifies the baby as Large for Gestational Age (LGA). A baby measuring LGA may be at a higher risk of complications like shoulder dystocia during delivery and neonatal hypoglycemia after birth. The most common cause of LGA is maternal diabetes, including gestational diabetes.
Closer monitoring for LGA usually involves screening the mother for undiagnosed glucose intolerance. If the baby is significantly large, further assessments are necessary to determine the appropriate timing and mode of delivery. The goal in both SGA and LGA cases is to distinguish between a healthy, constitutionally small or large baby and one whose growth is compromised, ensuring timely medical intervention.