When a person becomes pregnant, one of the first questions asked is, “When is the baby due?” Determining the estimated timeline of a pregnancy is fundamental to prenatal care, ensuring appropriate monitoring and interventions. Medical documentation uses specific acronyms like EDD and AUA to track this timeline, which can sometimes be confusing for the patient. Understanding these terms, which represent the two main methods for dating a pregnancy, helps clarify the established timeline for care.
Defining the Estimated Due Date (EDD)
The Estimated Due Date (EDD) is the projected date when an uninduced delivery is expected to occur, marking the end of a typical 40-week gestation period. This initial date is traditionally calculated using the first day of the last menstrual period (LMP) as the starting point. The entire pregnancy is counted from this day, even though conception happens about two weeks later.
The standard calculation for the EDD, known as Naegele’s rule, involves adding one year, subtracting three months, and adding seven days to the first day of the LMP. For example, a person with an LMP of March 15th would have an EDD of December 22nd of the same year. This calculation assumes a regular 28-day menstrual cycle with ovulation occurring exactly on day 14.
However, the EDD derived from the LMP is only an estimate, as only about 5% of babies are born on their exact calculated due date. Variations in menstrual cycle length, which affects the timing of ovulation, can make the LMP-based EDD less accurate. This method is most reliable for those who have a certain recall of their LMP and regular cycles.
Determining Age Using Ultrasound (AUA)
The Average Ultrasound Age (AUA) is a gestational age determined by physical measurements of the fetus taken during an ultrasound scan. This method becomes important when the LMP is unknown, uncertain, or the individual has a history of irregular menstrual cycles. The AUA is assigned by comparing the fetal measurements to established growth curves to determine the corresponding age.
The specific measurements used depend on the stage of the pregnancy, with the most accurate dating occurring in the first trimester (up to 13 weeks and 6 days). During this early stage, the Crown-Rump Length (CRL), which measures the length from the top of the head to the bottom of the rump, is the most reliable parameter because variations in fetal size are minimal. The accuracy of the CRL measurement is highest between 7 and 11 weeks of gestation.
As the pregnancy progresses into the second and third trimesters, a multi-parameter assessment is used to calculate the AUA. These measurements include:
- Biparietal Diameter (BPD), which is the width of the fetal head
- Head Circumference (HC)
- Abdominal Circumference (AC)
- Femur Length (FL)
While these later measurements help assess fetal growth and weight, they are less accurate for establishing the initial due date because individual growth rates begin to vary more widely after the first trimester.
Interpreting Differences in Dating Methods
It is common for the initial EDD based on the last menstrual period and the AUA based on ultrasound measurements to yield different dates. Medical guidelines provide thresholds for determining which date takes precedence in establishing the definitive Estimated Due Date. This accurate dating guides clinical care, such as scheduling screening tests and making decisions about labor induction.
If an ultrasound is performed in the first trimester and the AUA differs from the LMP-based EDD by more than 5 to 7 days, the AUA is used to adjust the EDD. This is because the first-trimester ultrasound is considered the most accurate method for dating a pregnancy. If the difference is 5 days or less, the LMP-derived date is maintained.
In the second trimester (between 14 and 22 weeks), a larger discrepancy is allowed before the EDD is changed, specifically if the AUA differs by more than 10 to 14 days. Significant differences between the two dating methods can sometimes indicate more than just an uncertain LMP or irregular cycle. A fetus consistently measuring larger or smaller than expected by the LMP-based EDD may prompt closer monitoring for potential growth issues, such as fetal growth restriction or excessive growth.