This calculator estimates your due date from the first day of your last menstrual period (LMP) and your usual cycle length. It works best when your cycles are fairly regular and you are confident about your dates. The result is most useful as a planning tool for understanding gestational age, milestone timing, and the general delivery window.
How This Estimate Is Calculated
Most LMP-based due date calculators use Naegele's Rule, which adds 280 days (40 weeks) to the first day of the last menstrual period. If you enter a cycle length that is longer or shorter than 28 days, the estimate shifts to reflect the fact that ovulation may have happened earlier or later than average.
When LMP Dating Is Less Reliable
An LMP estimate is less reliable if your cycles are irregular, you are unsure of the date, you recently stopped hormonal contraception, you are breastfeeding, or you conceived through IVF or another fertility treatment. In these situations, your clinician may rely more heavily on ultrasound dating or treatment dates.
When Your Clinician May Change the Due Date
A first-trimester ultrasound is the most accurate way to confirm pregnancy dating. If the ultrasound measurement differs meaningfully from the LMP-based estimate, your healthcare provider may update your due date. Once a first-trimester due date is established, it usually becomes the date used for prenatal care, growth tracking, and delivery planning.
What This Date Can and Cannot Tell You
A due date is an estimate, not a prediction of the exact day your baby will be born. It helps frame prenatal care and milestones, but it cannot confirm fetal wellbeing, labor timing, or complications. Use it to understand the timeline, then follow your clinician's guidance for the official date used in your records.