Pregnancy Calculator
Direct answer: Your due date is your last menstrual period (LMP) plus 280 days — 40 weeks. For example, if your LMP was 140 days ago, your estimated due date is about 140 days from now. This is called Naegele’s rule, and it assumes a regular 28-day cycle.
Estimated due date: March 21, 2027.
Estimated conception: around June 28, 2026.
You are about 20 weeks and 0 days pregnant (Second trimester).
How the calculation works (Naegele’s rule)
Step 1: Take the first day of your last menstrual period. Step 2: Add 280 days (40 weeks) to get the estimated due date. Step 3: Estimated conception is about 14 days after the LMP, which assumes ovulation on day 14 of a regular 28-day cycle. Step 4: Your current week is the number of full weeks since the LMP, and the trimester follows from it.
Trimester guide
| Trimester | Weeks | What happens |
|---|---|---|
| First trimester | Weeks 1–13 | The baby’s major organs form; this is when prenatal vitamins and the first clinician visit matter most. |
| Second trimester | Weeks 14–27 | Often called the most comfortable trimester; the anatomy scan usually happens around week 20. |
| Third trimester | Weeks 28–40 | Rapid growth, preparation for birth, and more frequent checkups. |
Worked examples
Example 1: LMP 140 days ago. Due date = LMP + 280 days = about 140 days from now. Estimated conception ≈ 126 days ago.
Example 2: LMP 100 days ago. Due date = LMP + 280 days = about 180 days from now.
Example 3: If you know conception was 126 days ago, add 266 days (38 weeks) instead: conception + 266 days = about 140 days from now — the same result, because pregnancy is counted from the LMP, about two weeks earlier.
How accurate is a due date?
Only a small share of babies arrive exactly on the due date — most arrive within two weeks before or after it. Early ultrasound dating is more accurate than LMP dating, especially with irregular cycles, and your clinician may adjust the date after the first scan. If your cycles are longer or shorter than 28 days, the LMP estimate shifts by roughly the difference.
Tips for a healthy pregnancy
Book your first prenatal visit early, take prenatal vitamins as advised by your clinician, and keep a simple record of symptoms and questions for each visit. A common mistake is relying only on the calculator date — always confirm it with an ultrasound and your clinician’s guidance.
FAQs
How is a pregnancy due date calculated?
By Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period.
What is Naegele’s rule?
A standard estimate that assumes a regular 28-day cycle with ovulation on day 14: due date = LMP + 280 days.
How accurate is a due date calculator?
It is an estimate. Most babies arrive within two weeks before or after the due date, and early ultrasound dating is more accurate, especially with irregular cycles.
What if I know my conception date?
Then add 266 days (38 weeks) to the conception date instead — pregnancy is counted from the LMP, which is about two weeks earlier.
What are the three trimesters?
First: weeks 1–13. Second: weeks 14–27. Third: weeks 28–40.
When should I see a doctor?
As early as possible after a positive test — early prenatal care matters, and your clinician will confirm the due date with an ultrasound.
Disclaimer: This tool provides general information only, not medical advice. Confirm your due date and care plan with your clinician.
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Sources & review
Due-date math on this page follows the standard 40-week pregnancy and Naegele’s rule described by the American College of Obstetricians and Gynecologists (acog.org). Reviewed by the Average Calculator Team. Last reviewed: 1 November 2026. This is general information only, not medical advice — confirm your due date with your clinician.
Last updated: November 1, 2026.