An ultrasound picture functions as a concise medical report containing specific data points used by healthcare providers to monitor the progression of a pregnancy. These images display fetal anatomy alongside a block of technical information. This data block includes precise calculations and measurements fundamental for tracking fetal growth and development across the trimesters. Understanding this technical information allows a parent to gain insight into the health assessment being performed.
Locating the Estimated Due Date
The estimated due date (EDD) is typically not displayed as a large, obvious date on the ultrasound picture. Instead, it is embedded within the alphanumeric data block that the machine automatically generates. This data block is generally positioned in a consistent, non-image area, often appearing in the header, the footer, or occasionally in a vertical column along the right-hand side.
The EDD is usually identified by the abbreviation “EDD” or sometimes “EDB” (Estimated Date of Birth). You may also see “EDD USG” to clarify that the date was calculated using ultrasound measurements, rather than the date based on the last menstrual period (LMP). This date is a calculation performed instantly by the machine’s software, derived from physical measurements taken during the scan. The software uses established growth charts to correlate fetal size to a corresponding gestational age, projecting the expected date of delivery.
Decoding the Ultrasound Abbreviations
The EDD is a product of several technical measurements, each represented by a specific abbreviation on the ultrasound report. The primary measurement used in the first trimester is the Crown-Rump Length (CRL). The CRL measures the longest length of the embryo or fetus from the top of the head to the bottom of the torso and is the most accurate indicator of gestational age during this early period.
For second and third-trimester scans, the focus shifts to other biometric parameters as the fetus is larger. These measurements include the Biparietal Diameter (BPD), which is the distance between the two sides of the fetal head, and the Head Circumference (HC). The length of the long bones is also measured, particularly the Femur Length (FL).
The machine’s software uses a combination of these measurements to calculate the Gestational Age (GA), which appears on the image as a measure of weeks and days, such as “GA 12w3d”. The BPD, HC, and FL measurements are crucial for determining the GA in later pregnancy stages, as they provide an assessment of overall fetal size and growth rate. These biometric data points feed directly into the final EDD calculation printed on the image.
Comparing Ultrasound Date vs. Last Menstrual Period Date
A pregnant person often has two different estimated due dates: one calculated from the first day of the Last Menstrual Period (LMP) and one calculated by the ultrasound machine. The LMP method assumes a regular 28-day cycle and ovulation on day 14, but this can be inaccurate if cycles are irregular or the date of the last period is uncertain.
The date established by the first-trimester ultrasound is considered the gold standard for dating a pregnancy because early fetal growth is highly consistent. Scans performed before 13 weeks are highly accurate, estimating the due date within a window of five to seven days. If the ultrasound date differs significantly from the LMP date (usually by more than seven days in the first trimester), the ultrasound date will override the LMP date to ensure accurate pregnancy management.
As the pregnancy progresses into the second and third trimesters, the accuracy of using an ultrasound to set a new due date decreases because fetal growth rates begin to vary naturally. For instance, a third-trimester scan may have an error margin of up to three weeks for dating. Therefore, later ultrasounds are primarily used to monitor growth and development, and healthcare providers rarely adjust the EDD established during the first trimester.