Fetal biometry is a standard practice during pregnancy that uses ultrasound to take specific measurements of the developing fetus. This routine assessment helps healthcare providers understand how a baby is growing and developing inside the womb. It provides valuable information to support prenatal care and ensure a healthy pregnancy journey. Understanding these measurements offers reassurance and insights into the baby’s progress.
The Core Fetal Measurements
Fetal biometry involves several precise measurements. The biparietal diameter (BPD) gauges the distance between the two sides of the baby’s head, specifically between the parietal bones of the skull.
Head circumference (HC) is taken around the entire head, assessing head size and monitoring brain development. Abdominal circumference (AC) provides insight into the baby’s nutritional status and internal organ development.
Femur length (FL) measures the longest bone in the thigh, assessing skeletal development and overall length. These four measurements—BPD, HC, AC, and FL—are the primary parameters in fetal biometry.
Establishing Gestational Age and Due Date
Fetal biometry accurately determines gestational age and estimated due date. First-trimester measurements, especially Crown-Rump Length (CRL), are highly accurate. CRL measures the length from the baby’s head to its bottom and is most reliable for dating up to 13 weeks and 6 days of gestation.
After the first trimester, between 14 and 22 weeks of gestation, a combination of BPD, HC, AC, and FL measurements becomes accurate for dating. For singleton pregnancies, mid-trimester measurements can predict gestational age with an accuracy of approximately ±7-10 days. This early, precise due date creates a baseline for subsequent growth assessments.
Monitoring Fetal Growth and Weight
Beyond initial dating, fetal biometry monitors ongoing growth later in pregnancy. Healthcare providers use mid-second and third-trimester measurements to track development over time. These serial measurements assess growth velocity, the rate at which the fetus is growing.
Abdominal circumference (AC) and femur length (FL) are valuable for estimating the baby’s weight. Combining these and other measurements generates an Estimated Fetal Weight (EFW). EFW is plotted on growth charts, helping ensure the fetus follows an expected growth curve for its gestational age.
Interpreting Atypical Results
Fetal biometry results are often presented as percentiles, indicating how a baby’s measurements compare to others at the same gestational age. For example, a 10th percentile measurement means 90% of babies at that age are larger. Measurements between the 10th and 90th percentiles are generally considered normal.
Measurements consistently below the 10th percentile can indicate Intrauterine Growth Restriction (IUGR), suggesting the baby is smaller than expected. Conversely, measurements consistently above the 90th percentile might point to macrosomia, meaning the baby is larger than expected. Macrosomia can be associated with conditions like gestational diabetes.
A single measurement outside the average range does not automatically signify a problem. Healthcare providers consider the overall trend of measurements over multiple ultrasounds, along with factors like parental size and medical history, before diagnosis. Further monitoring and testing may be suggested to understand the baby’s complete development.