Pregnancy Due Date Calculator
Calculate your estimated due date (EDD) from your last menstrual period (LMP) using Naegele's rule — the same method used by doctors and midwives.
Calculate Your Due Date
Estimated Due Date (EDD)
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This is an estimate. Your provider will confirm dating with an ultrasound.
How is the due date calculated?
This calculator uses Naegele's rule, the standard clinical method for estimating a due date from the last menstrual period. The rule adds 280 days (40 weeks) to the first day of the last menstrual period.
The formula assumes a 28-day cycle with ovulation on day 14. If your cycle is significantly longer or shorter than 28 days, your healthcare provider may adjust the estimated due date — particularly after an early ultrasound scan, which is considered more accurate than LMP dating.
The three trimesters
- First trimester: Weeks 1–13 (LMP + 0–91 days). The embryo develops major organs. Most miscarriages occur in this period.
- Second trimester: Weeks 14–27 (LMP + 92–189 days). The fetus grows rapidly and movement becomes noticeable.
- Third trimester: Weeks 28–40 (LMP + 190–280 days). The baby gains weight and prepares for birth.
How accurate is a due date?
A calculated due date is an estimate — only about 4–5% of babies are born on their exact due date. The normal range for birth is 37–42 weeks of gestation. An early ultrasound (ideally before 14 weeks) is the most accurate method of dating a pregnancy and may revise the LMP-based estimate.
Due Date Calculator FAQ
How is the due date calculated?
This calculator uses Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle. Your provider may adjust via ultrasound dating.
How accurate is the due date calculator?
The date is an estimate. Only about 5% of babies are born on their exact due date. Most births happen within 2 weeks either side. An early ultrasound gives the most accurate dating.
When does the first trimester end?
The first trimester ends at the end of week 13 (91 days after LMP). The second trimester runs from week 14 to week 27, and the third from week 28 to delivery.