About this Pregnancy Calculator
Finding out you are pregnant is exciting, and the first question everyone asks is 'When are you due?' This calculator estimates your delivery date based on the first day of your last period, which is the standard medical method.
Who this calculator is for
- People who want a due-date estimate from the first day of the last menstrual period.
- Expecting parents building a rough pregnancy timeline for trimesters and milestones.
- Anyone comparing LMP-based dating with cycle-length adjustments.
When to use it
- Use this calculator soon after a positive test to estimate due date and gestational age.
- Use it when preparing for appointments or understanding trimester timing.
- Use it to compare standard LMP dating against cycles longer or shorter than 28 days.
How to use this calculator
- Select the first day of your Last Menstrual Period (LMP).
- Adjust Cycle Length if your cycle differs from the average 28 days.
- The result shows your Estimated Due Date and a timeline of important dates.
Key assumptions
- The estimate uses Naegele's Rule and assumes the pregnancy is dated from the first day of the last period.
- Cycle-length adjustment assumes ovulation shifts roughly with the difference from a 28-day cycle.
- Ultrasound dating may change the estimate, especially with irregular cycles or uncertain LMP dates.
Accuracy and testing
Review formula sources, default test cases, assumptions, unit handling, rounding rules, and changelog for this calculator.
How this calculator works
The calculator uses Naegele's Rule: LMP + 280 days (40 weeks). It adjusts for cycle length: if your cycle is longer than 28 days, your due date is pushed later; if shorter, it's earlier.
Inputs explained
Last Menstrual Period (LMP)
The very first day your last period started. This is Day 0 of the pregnancy timeline.
Cycle Length
The number of days between periods. The average is 28 days.
Understanding your results
The Estimated Due Date is when you reach 40 weeks. Only ~5% of babies are born on their exact due date. Most arrive within the two weeks before or after.
Example scenarios
Standard Cycle
LMP Jan 1st. Cycle 28 days. Due Date: Oct 8th.
Long Cycle
LMP Jan 1st. Cycle 35 days. Due Date: Oct 15th (7 days later because ovulation happened later).
Key Features
- Naegele's Rule
- Cycle Adjustment
- Conception Estimate
Tips & Best Practices
- The first ultrasound (dating scan) is usually the most accurate way to confirm the due date.
- Pregnancy is counted from the LMP, meaning you are technically '2 weeks pregnant' when you conceive.
Glossary of Terms
- Gestational Age
- The age of the pregnancy measured from the LMP.
- Trimester
- A period of three months. Pregnancy is divided into three trimesters.