The estimated due date (EDD) is a projection of the day a pregnancy will reach 40 weeks, but it remains an estimate, not a fixed deadline. Only about five percent of babies are born on this specific date. Understanding the methods used to set this date and the factors that influence its accuracy can help address concerns about its validity.
Methods Used to Determine Your Due Date
Healthcare providers primarily use two methods to establish an initial Estimated Date of Delivery. The first relies on the date of your Last Menstrual Period (LMP). This calculation adds 280 days, or 40 weeks, to the first day of your last period, assuming a regular 28-day cycle with ovulation occurring on day 14.
The second, and more accurate, method involves using an early first-trimester ultrasound. This scan measures the Crown-Rump Length (CRL), which is the length of the embryo or fetus from the top of the head to the bottom of the buttocks. Fetal growth up to 13 weeks and 6 days is highly uniform across all pregnancies. This standardization allows the CRL measurement to determine the gestational age with a margin of error of only about five to seven days. If the date derived from the early ultrasound differs significantly from the LMP date, the ultrasound date is used to set the final EDD.
Why Due Date Estimates Are Often Inaccurate
The most common source of inaccuracy stems from the assumptions built into the LMP-based calculation. The 40-week formula only works if a person has a textbook 28-day cycle and ovulates precisely in the middle. Irregular menstrual cycles cause the actual date of conception to shift, which can make the LMP date incorrect by one to three weeks.
A late-performed ultrasound will not provide a reliable dating estimate. While first-trimester measurements are highly accurate due to uniform early growth, the accuracy of an ultrasound-derived date diminishes rapidly in the second and third trimeters. By the third trimester, the margin of error can be as wide as three weeks because fetuses begin to grow at varying rates. An ultrasound performed later in pregnancy is primarily used to monitor growth, not to establish a new gestational age.
Uncertainty about the exact date of the last period is another frequent cause of discrepancy. Many individuals cannot recall the precise start date of their last period, or they may have experienced implantation bleeding mistaken for a light period. This uncertainty can introduce significant error into the initial EDD calculation.
Clinical Relevance of Accurate Gestational Timing
Accurate gestational timing dictates the entire schedule of prenatal medical management. The EDD is used to ensure that time-sensitive prenatal screenings are performed within their appropriate windows. For instance, certain genetic tests, like the quadruple marker screen, require precise gestational age to accurately interpret hormone and protein levels.
The EDD is also the baseline measurement against which a baby’s growth is continuously compared. This timing is necessary to diagnose conditions like Intrauterine Growth Restriction (IUGR), where a fetus is not growing as expected, or macrosomia, which is excessive growth. Without a dependable EDD, providers cannot determine if a baby is truly small or large for its age, or simply due to an incorrect date.
An accurate date becomes important late in pregnancy for managing post-term pregnancy. Pregnancies extending beyond 41 or 42 weeks carry increased risks for both the mother and the baby. A reliable EDD allows the care team to make informed decisions about when to recommend monitoring or induction of labor.
How to Request a Due Date Reassessment
If you believe your due date is wrong, the first step is to gather all relevant information prior to speaking with your healthcare provider.
Gathering Information
You should document the first day of your last menstrual period, the typical length of your menstrual cycle, and the dates and gestational ages noted on any early ultrasound reports. Having this specific data will facilitate a productive conversation with your care team.
Provider Review
Clearly articulate your concerns to your provider, referencing the data you have collected. They can re-evaluate your initial dating based on established clinical guidelines, such as those from the American College of Obstetricians and Gynecologists. A provider is most likely to consider changing the date if the early ultrasound dating differs from the LMP date by more than a week in the first trimester.
If you are already past the first trimester, your provider will be reluctant to change the EDD. Once the date has been established by an accurate first-trimester ultrasound, it is rarely altered, as later changes can introduce new inaccuracies and complicate future care decisions.