The third trimester often brings a focus on fetal growth, and the ultrasound appointment around 28 weeks is a significant check-in on the baby’s development. This monitoring is a key part of prenatal care, providing data on how the fetus is thriving in the final months before birth. When a healthcare provider mentions a “percentile,” it can be confusing or trigger concern about whether the baby is the correct size. Understanding what this number represents helps demystify the monitoring process and focuses attention on what truly matters for the baby’s health.
The Standard Range for Fetal Growth
Fetal growth percentiles are a statistical tool used to compare a baby’s size to that of others at the same gestational age. The percentile indicates the percentage of babies who are smaller than the one being measured. For example, a baby at the 50th percentile means that 50% of babies at 28 weeks are estimated to be smaller, and 50% are estimated to be larger.
The goal for a growing fetus is to be within the normal range, which is broadly defined as between the 10th and 90th percentile. A baby in the 50th percentile is considered average, but any percentile within this span suggests a healthy rate of growth. The primary concern is not the single number itself, but whether the baby is maintaining a consistent growth curve over time.
How Healthcare Providers Measure Growth
The percentile number is derived from the Estimated Fetal Weight (EFW), which requires several precise measurements taken during an ultrasound examination. These measurements, known as fetal biometry, are performed during the late second or third trimester. The sonographer measures four primary areas of the fetus to calculate the EFW:
- Biparietal Diameter (BPD), the width of the head.
- Head Circumference (HC).
- Abdominal Circumference (AC), which is indicative of the baby’s nutritional status.
- Femur Length (FL), the longest bone in the body.
These four data points are then entered into a standardized formula to compute the EFW in grams. The EFW is then plotted against established population growth charts specific to the baby’s gestational age to determine the percentile. The EFW is an estimate, not an exact measurement, carrying a margin of error that can be up to 10 to 15%. This variability is why healthcare providers focus on trends over multiple scans rather than reacting strongly to a single isolated measurement.
Implications of Low Percentile Measurements
When the estimated fetal weight falls below the 10th percentile for the gestational age, the baby is classified as Small for Gestational Age (SGA). While many SGA babies are simply healthy but constitutionally small, falling below this threshold prompts closer monitoring to determine if the baby is experiencing Fetal Growth Restriction (FGR). FGR is a condition where the fetus is unable to reach its full growth potential, often due to issues with placental function. Placental insufficiency, where the placenta cannot deliver adequate nutrients and oxygen, is a common cause of FGR.
When FGR is suspected, surveillance is intensified to assess the baby’s well-being and the efficiency of the placenta. Monitoring typically involves repeating the EFW ultrasound every two to four weeks to check the growth velocity. Healthcare providers also use specialized ultrasound techniques called Doppler flow studies, which assess blood flow in specific vessels. These monitoring protocols are enacted to ensure the baby is delivered at the optimal time, balancing the risks of prematurity against the risks of remaining in a suboptimal intrauterine environment.
Implications of High Percentile Measurements
Conversely, an estimated fetal weight above the 90th percentile classifies the baby as Large for Gestational Age (LGA). This finding is particularly relevant around 28 weeks, as it may signal a risk for excessive growth. Though some LGA babies are simply genetically large, this measurement can point toward underlying maternal metabolic conditions that need management. The most common cause of LGA is poorly controlled gestational diabetes, which is often screened for around this time in pregnancy.
High levels of glucose in the maternal bloodstream cross the placenta, causing the baby’s pancreas to produce extra insulin, which acts as a growth hormone. This excess growth tends to distribute fat around the baby’s abdomen, which is reflected in a large Abdominal Circumference measurement on the ultrasound. A significantly large baby introduces specific risks related to the birthing process, including shoulder dystocia and an increased likelihood of needing a Cesarean section. Monitoring involves careful management of maternal blood sugar levels to mitigate delivery complications associated with excessive size.