Due Date Calculator
Calculate your estimated due date (EDD) and pregnancy milestones from your last menstrual period (LMP).
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How to use this calculator
Add 280 days to the first day of your last menstrual period. This assumes a 28-day cycle.
- 1
Enter the first day of your last menstrual period (LMP).
- 2
Select your average menstrual cycle length.
- 3
The calculator shows your estimated due date, current pregnancy week, and key milestones.
Frequently asked questions
How accurate is this due date calculation?
Only about 5% of babies are born on their exact due date. The EDD is an estimate — most births occur within 2 weeks either side. An ultrasound in the first trimester gives a more precise date.
What is Naegele's rule?
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period to estimate the due date. It assumes a 28-day cycle and ovulation on day 14. The calculator adjusts for different cycle lengths.
What are the three trimesters?
First trimester: weeks 1–12 (organ formation). Second trimester: weeks 13–26 (growth, movement begins). Third trimester: weeks 27–40 (rapid growth, lung development). Full-term is 39–40 weeks.
What if my cycle is not 28 days?
This calculator adjusts for cycle length. For cycles shorter than 28 days, ovulation occurs earlier, moving the due date earlier. For longer cycles, it moves later.
Pregnancy due date calculation
How due dates are calculated
Due dates are estimated by adding 280 days (40 weeks) to the first day of the last menstrual period. This assumes the standard 28-day cycle with ovulation on day 14. The 40 weeks includes the 2 weeks before conception — so actual fetal age is approximately 38 weeks at delivery.
Pregnancy milestones
Week 4: missed period, positive test. Week 6–8: heartbeat detectable. Week 10–12: nuchal scan. Week 18–20: anatomy scan. Week 24: viability threshold. Week 37: early term. Week 39–40: full term. Week 42: post-term.
What affects the due date accuracy?
Irregular cycles, not remembering the LMP date, and individual variation in ovulation timing all affect accuracy. A first-trimester ultrasound (measuring crown-rump length) provides the most accurate gestational age estimate and is used clinically to set or confirm the EDD.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →