An obstetric ultrasound scan uses high-frequency sound waves to create images of the developing fetus and the surrounding uterine environment. Beyond the visual images, the numerical data generated by the sonographer are the foundation for medical assessment, tracking growth, establishing a timeline, and evaluating overall well-being. These measurements quantify the physical and physiological status of the pregnancy at the time of the scan.
Calculating Gestational Age
The measurement of Gestational Age (GA) determines how far along the pregnancy is, typically expressed in weeks and days. While an initial GA is often calculated from the first day of the Last Menstrual Period (LMP), the ultrasound provides a more precise, biometric-based age, especially when the LMP date is uncertain or irregular.
This dating is considered most accurate during the first trimester, primarily through the measurement of the Crown-Rump Length (CRL). The CRL is the distance, measured in millimeters, from the top of the embryonic head to the bottom of the buttocks. Between approximately seven and 13 weeks of gestation, the fetus is growing rapidly and consistently, making the CRL the most reliable measurement for calculating GA.
Once the most accurate Gestational Age is established, the final numerical calculation is the Estimated Due Date (EDD). The EDD is the projected date of birth, set at forty weeks of gestation from the determined GA. This EDD is the reference point against which all subsequent growth measurements will be compared.
Key Measurements of Fetal Growth
After the first trimester, the focus shifts from dating the pregnancy to confirming that the fetus is growing appropriately for its established age. This involves a series of biometric measurements that quantify the size of different body parts.
The head is measured in two ways, starting with the Biparietal Diameter (BPD), which is the distance across the widest part of the skull. This is accompanied by the Head Circumference (HC), a measurement taken around the perimeter of the head. Together, these numbers help assess the development of the fetal brain.
Fetal body size is assessed primarily through the Abdominal Circumference (AC), which measures the perimeter of the abdomen at the level of the liver. This measurement is particularly sensitive to changes in fetal nutrition and is considered a strong indicator of overall fetal growth and estimated weight.
The length of the long bones provides another key metric, most commonly measured as the Femur Length (FL), which is the measurement of the thigh bone. The FL is used to track skeletal growth. These four biometric data points—BPD, HC, AC, and FL—are combined using validated formulas to assess growth consistency and estimate the final weight.
Evaluating the Fetal Environment
The numbers on an ultrasound report also describe the immediate surroundings of the fetus. These measurements relate to the volume of amniotic fluid and the maturity of the placenta.
The fluid volume is quantified using two primary measurements, the Amniotic Fluid Index (AFI) and the Maximum Vertical Pocket (MVP). The AFI is obtained by dividing the uterus into four imaginary quadrants and summing the deepest, unobstructed vertical pocket of fluid in each one. A normal AFI range is generally considered to be between 5 and 24 centimeters.
The MVP, in contrast, involves measuring only the single deepest pocket of amniotic fluid. For this measurement, a normal range is typically defined as 2 to 8 centimeters. If either the AFI or MVP falls below the lower threshold, it may indicate a condition called oligohydramnios, or low fluid, while values exceeding the upper threshold indicate polyhydramnios, or excess fluid.
The placenta, the organ responsible for nutrient and oxygen exchange, is assessed using the Placental Grade, which is a number from 0 to 3 based on its texture and calcification, reflecting its maturity. Grade 0 is the least mature, and Grade III is fully mature. An accelerated progression to Grade III maturity may warrant closer monitoring, as it can sometimes be associated with a reduced efficiency in nutrient delivery.
Interpreting Vital Signs and Overall Health
A direct assessment of well-being is the Fetal Heart Rate (FHR), measured in beats per minute (bpm). The normal range throughout most of the pregnancy is generally considered to be between 110 and 160 bpm.
The culmination of the fetal growth measurements from the second and third trimesters is the Estimated Fetal Weight (EFW). This numerical output provides an estimate of the baby’s weight at the time of the scan. EFW is an estimate, and the actual birth weight may differ.
All measurements are placed on a growth curve to determine the baby’s Percentile. A percentile is a number that indicates where a fetus falls in comparison to others of the same Gestational Age. For instance, a fetus in the 50th percentile is exactly average, while one below the 10th percentile may be considered small for gestational age, and one above the 90th percentile is considered large. This percentile number helps healthcare providers determine if the baby is growing consistently.