Pregnancy Due Date Calculator
Knowing your estimated due date (EDD) is one of the first and most important milestones of pregnancy. Healthcare providers use Naegele's rule as the standard method: add 280 days (40 weeks) to the first day of your last menstrual period. This figure assumes a standard 28-day cycle and ovulation on day 14. In practice, cycle length varies between individuals. This calculator adjusts for your actual cycle length by shifting the due date forward or backward relative to the standard 28-day assumption. Along with your estimated due date, the calculator shows the key dates for each trimester, the conception window, and (if you enter a date in the past) how many weeks pregnant you currently are. The result is an estimate, not a guarantee. Ultrasound dating in the first trimester, particularly the crown-rump length measurement at the 12-week scan, is considered more accurate than LMP-based dating and will typically be used by your healthcare provider to confirm or revise your due date.
Formula
EDD = LMP date + 280 days + (cycle length - 28) days
Conception window: LMP + (cycle length - 14) days (approximate)
1st trimester: LMP to LMP + 84 days
2nd trimester: LMP + 85 days to LMP + 182 days
3rd trimester: LMP + 183 days to birth
Pregnancy Due Date Calculator: frequently asked questions
What is Naegele's rule?
Naegele's rule is the standard method for calculating estimated due date (EDD). It adds 280 days (40 weeks) to the first day of the last menstrual period (LMP) for a standard 28-day cycle. This assumes ovulation and conception occur 14 days after the LMP. The rule was developed by German obstetrician Franz Karl Naegele in the early 19th century and remains the clinical standard.
How accurate is the estimated due date?
Only about 4% of babies are born on their estimated due date. Most births occur within two weeks either side of the EDD. The due date is best understood as the midpoint of a normal delivery window rather than a precise date. Ultrasound dating in the first trimester is generally more accurate than Naegele's rule, especially for women with irregular cycles.
How are the trimesters defined?
The first trimester runs from conception (week 1) through week 12. The second trimester covers weeks 13 to 26. The third trimester runs from week 27 until birth. These divisions are approximate and some definitions vary slightly between clinical guidelines.
What if my cycle is longer or shorter than 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. For longer cycles, ovulation and conception are later, so the due date shifts forward by the number of days your cycle exceeds 28. For shorter cycles, the due date shifts backward. This calculator adjusts the due date by adding (cycle length - 28) days to the standard calculation.
Does this calculator replace medical advice?
No. This tool provides an estimate based on your reported last menstrual period. Only your healthcare provider can establish a clinical estimated due date, taking into account ultrasound findings, cycle history, and other clinical factors. Always confirm dates with your doctor or midwife.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.