Pregnancy Calculator
Track pregnancy week by week with trimester dates and key milestones.
About Pregnancy Calculator
Gestational age is measured from the first day of the last menstrual period (LMP), not from conception, because LMP is more reliably known. A full-term pregnancy is 40 weeks (280 days) from LMP. The trimesters are weeks 1–12, 13–26 and 27–40. The due date from LMP is calculated using Naegele's rule: add 1 year, subtract 3 months, add 7 days.
Formula
Naegele's rule
EDD = LMP + 1 year − 3 months + 7 days (≡ LMP + 280 days)
- EDD
- Estimated due date
- LMP
- First day of last menstrual period
Worked example
With the default values below, Pregnancy Calculator returns Weeks pregnant of 25. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Calculate fromone of Last menstrual period (LMP), Conception date, Due date (work backwards)
- Last menstrual period (LMP)
- First day of last period
- 2025-01-01
- Conception date
- 2025-01-14
- Due date
- 2025-10-08
- Today's date
- 2025-07-01
Results
- Weeks pregnant
- 25
- Days into current week
- 6
- Current trimester
- 2
- Estimated due date
- 2025-10-08
- Days until due date
- 99
- Progress
- 65 %
- 1st trimester starts
- January 1, 2025
- 2nd trimester starts
- April 2, 2025
- 3rd trimester starts
- July 9, 2025
Step by step
- 1LMP= January 1, 2025
- 2Days since LMP= 181
- 3Gestational age= 25 weeks + 6 days
- 4Trimester= 2
- 5Due date (Naegele's rule)= October 8, 2025
- 6Days until due date= 99
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to whole numbers, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
- This calculator is for informational and educational purposes only. It is not a substitute for prenatal care or advice from a qualified healthcare provider. Always confirm gestational age with your obstetrician or midwife.
- This is an estimate for general information and not professional medical advice. See the full disclaimer for the limits that apply to this kind of calculator.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
This calculator is for informational and educational purposes only. It is not a substitute for prenatal care or advice from a qualified healthcare provider. Always confirm gestational age with your obstetrician or midwife.
Frequently Asked Questions
Why is pregnancy counted from the last period, not conception?
The exact date of conception is usually unknown, whereas the start of the last period is a concrete, remembered date. Gestational age dating from LMP is universal in clinical practice and in this calculator.
When do the trimesters start and end?
First trimester: weeks 1–12 (conception through 12 weeks). Second trimester: weeks 13–26. Third trimester: weeks 27–40. These boundaries vary slightly by source; this calculator uses the most common clinical division.
How accurate is the due date?
Only about 5 % of babies are born exactly on their due date. Most births occur within 2 weeks before or after. An early ultrasound (before 13 weeks) is more accurate than LMP dating when cycles are irregular.