How Your Estimated Due Date (EDD) Is Calculated.
And how you can calculate the probability of going into labor.
A due date estimates when a pregnancy reaches 40 weeks of gestation. The most biologically accurate approach is based on fertilization, since human gestation averages 266 days from ovulation and fertilization.
—> The most precise due date calculator ←—
Methods based on the last menstrual period use assumptions about when ovulation occurred, which introduces error because ovulation timing varies widely. For that reason, due date accuracy depends on how closely the method reflects the true timing of fertilization.
Only 5% of pregnant women deliver oin their due date and there are many factors that determine when you are likely to go into labor. Calculate your own probability HERE.
Methods of Estimating Due Date (Ranked by Precision)
1. Ovulation Date (Most Precise)
When the exact ovulation date is known, the due date is calculated as ovulation + 266 days. This directly reflects the timing of fertilization and avoids assumptions about cycle length or follicular phase variability. It is most reliable when ovulation is confirmed by hormonal testing, ultrasound, or structured cycle tracking. In routine clinical practice, this level of precision is uncommon without active monitoring.
2. IVF Date
In in vitro fertilization, the timing of fertilization or embryo transfer is known, allowing highly accurate dating. For example, with a day 5 embryo transfer, the due date is calculated as transfer date + 261 days. This method closely approximates true embryologic age because conception timing is controlled. IVF dating is therefore considered a clinical reference standard.
3. Last Menstrual Period (LMP)
The LMP method estimates due date as LMP + 280 days, assuming ovulation occurs on day 14 of a 28-day cycle. This assumption is often incorrect due to natural variation in ovulation timing. Cycle irregularity and recall error further reduce reliability. As a result, LMP-based dating can introduce errors of several days or more.
4. First Trimester Ultrasound
First trimester ultrasound estimates gestational age using crown–rump length (CRL). When performed between 7 and 13 weeks, accuracy is approximately ±3 to 5 days. It is particularly useful when LMP is uncertain or inconsistent with clinical findings. However, it remains an indirect estimate and is slightly less precise than known ovulation or IVF timing.
5. Naegele’s Rule (Traditional Calendar Method)
Naegele’s rule estimates the due date from the first day of the last menstrual period by adding 1 year, subtracting 3 months, and adding 7 days. This is a calendar-based shortcut rather than a true day-count from ovulation or fertilization. Because calendar months vary in length and ovulation does not reliably occur on day 14, the method is inherently approximate. It remains widely used because it is simple, not because it is the most accurate.