Fetal growth percentiles are a tool healthcare providers use to monitor a baby’s development during pregnancy. These percentiles help assess how a baby’s size compares to others at the same gestational age. By tracking these measurements, medical professionals can identify potential deviations, providing information about the baby’s progress. This monitoring is a routine part of prenatal care.
Understanding Fetal Growth Percentiles
Fetal growth percentiles interpret a baby’s size relative to a reference population at a specific point in gestation. A baby in the 50th percentile is average, meaning 50% of babies at the same gestational age are smaller and 50% are larger. A baby in the 10th percentile is smaller than 90% of babies of the same age, while a baby in the 90th percentile is larger than 90% of their peers.
These percentiles are derived from ultrasound measurements of fetal biometric parameters. Key measurements include head circumference (HC), abdominal circumference (AC), and femur length (FL). These measurements calculate an estimated fetal weight (EFW), which is then plotted on specialized growth charts to determine the percentile. These charts help assess whether the baby is growing as expected.
Typical Percentiles at 28 Weeks
At 28 weeks of gestation, a baby is entering the third trimester, a period characterized by significant growth and weight gain. The average estimated fetal weight at 28 weeks is approximately 2.7 pounds (about 1210 grams) and the average length is around 14.9 inches (37.9 cm).
Healthcare providers consider a fetal growth percentile between the 10th and 90th percentiles to be within the expected range for gestational age. This broad range acknowledges natural variations in growth among healthy babies. Measurements within this spectrum suggest the baby is developing appropriately. While the 50th percentile represents the average, being above or below this median within the typical range is common and usually not a cause for concern.
Factors Influencing Fetal Growth
A baby’s growth and percentile can be influenced by genetics, maternal health, and placental function. Genetic predispositions play a role, as a baby’s size often correlates with the parents’ body size and genetic makeup.
Maternal health conditions can impact fetal growth. Conditions like gestational diabetes can lead to a baby being larger than average, while chronic conditions such as high blood pressure or kidney disease can restrict growth. Lifestyle choices, including nutrition, smoking, alcohol consumption, and drug use, also affect fetal development. Placental health is important, as it delivers oxygen and nutrients to the fetus. Issues like placental insufficiency can restrict fetal growth.
What to Do if Percentiles Are Not Typical
When a baby’s growth percentile falls outside the 10th to 90th range, healthcare providers initiate further investigation. A percentile below the 10th may indicate fetal growth restriction (FGR), meaning the baby is smaller than expected. Conversely, a percentile above the 90th might suggest the baby is large for gestational age (LGA). A single measurement outside the typical range does not automatically signify a problem, as individual growth patterns can vary.
Healthcare providers will consider the overall clinical picture, including trends in growth over time, maternal health, and other risk factors. Additional monitoring may include more frequent ultrasounds to track growth velocity and Doppler ultrasounds to assess blood flow to the baby. Management strategies are tailored to the specific situation and may involve managing underlying maternal conditions or considering the optimal timing for delivery.