Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to estimate your due date.
| Method | Reference Point | Days Added |
|---|---|---|
| Last menstrual period (LMP) — Naegele's Rule | First day of last period | 280 days |
| Estimated conception date | Date of ovulation/conception | 266 days |
| Difference between the two methods | Typical 28-day cycle | 14 days |
FAQ
How is the due date calculated?
This calculator uses Naegele's Rule, the standard method: it adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle.
How accurate is this estimate?
It's an estimate, not a guarantee — only a small percentage of babies arrive exactly on their calculated due date. Actual delivery can happen anywhere from a couple of weeks before to two weeks after, and cycle length variations affect accuracy.
What if my cycle isn't 28 days?
This calculator assumes a standard 28-day cycle. If yours is significantly longer or shorter, your actual conception date — and due date — may differ; an ultrasound dating scan from a healthcare provider gives a more personalized estimate.
What are the trimesters?
Pregnancy is typically divided into three trimesters: first trimester (weeks 1-13), second trimester (weeks 14-27), and third trimester (weeks 28 to birth).
What's the difference between an LMP-based and a conception-based due date?
This calculator uses Naegele's Rule, which adds 280 days to the first day of your last menstrual period (LMP). A conception-based estimate instead adds about 266 days to the actual conception date — the two methods typically land about 14 days apart for a standard 28-day cycle.
Can I use this calculator if I know my exact conception date instead of my LMP?
This tool is built around the LMP method, so it's most accurate when you enter the first day of your last period. If you know your conception date instead, your due date is generally about 266 days after that date rather than 280.
What does 'full term' mean?
Full term is generally defined as weeks 39 through 40 of pregnancy. Babies born from week 37 are considered 'early term,' while babies born before week 37 are considered preterm.
About this tool
This calculator estimates a pregnancy due date using Naegele's Rule, based on the first day of your last menstrual period, and shows your current estimated week and trimester. This is an estimate for general reference only, not medical advice — always confirm dates with a healthcare provider.