What is the Due Date Calculator?
The Due Date Calculator estimates an expected delivery date (EDD) from the first day of the last menstrual period (LMP). It is useful when you know your cycle start date and want a calendar estimate before or alongside prenatal dating. For a broader progress view after you have an LMP, use the Pregnancy Calculator.
How does the Due Date Calculator work?
For a 28-day cycle, obstetric dating convention places the EDD 280 days after the first day of the LMP. If your average cycle is longer or shorter, this calculator shifts the estimate by the difference from 28 days. If pregnancy resulted from assisted reproduction, use the IVF Due Date Calculator instead of relying on an LMP estimate.
What formula or rule does it use?
EDD = LMP + 280 days + (cycle length − 28 days). The dynamic “How this answer was calculated” panel substitutes your entered values into that rule after a valid calculation.
How should you use this calculator?
Choose the first day of your last menstrual period and enter your usual cycle length. A regular cycle improves the usefulness of a calendar estimate. Once prenatal care establishes an EDD, the Pregnancy Week Calculator can convert that date into week-and-day timing.
How should you understand the result?
The main result is an estimated due date, not a guaranteed birth date. The result also shows an estimated conception date and any cycle-length adjustment so you can see why the date differs from an unadjusted 280-day estimate.
What are the limitations?
LMP dating assumes the entered menstrual date is correct and that cycle timing is reasonably representative. Irregular cycles, uncertain LMP, delayed ovulation, and early ultrasound findings can change the best obstetric estimate. ACOG notes that first-trimester ultrasound is the most accurate ultrasound method to establish or confirm gestational age.
Who can use this calculator?
This tool is useful for adults who know the first day of the last menstrual period and want an educational pregnancy-date estimate. It is not a substitute for prenatal dating, ultrasound interpretation, or clinical decisions. You can also map the estimate into trimester timing with the Pregnancy Trimester Calculator.
Frequently asked questions about Due Date
Sources and methodology
Methodology and interpretation are grounded in the following professional or government references.