Educational content, not medical advice. Clinical review of this page is in progress.
Both methods aim to land on the same 40-week estimate, but they start counting from different points and suit different situations.
LMP-Based Due Date
Adds 280 days to the first day of your last period. Works well for regular 28-day cycles, but can be off if ovulation happened earlier or later than day 14.
Conception-Based Due Date
Adds 266 days to your known or estimated conception date. More accurate when you know your ovulation date precisely (e.g., via OPK, BBT, or fertility treatment).
Which Should You Use?
If you tracked ovulation closely, conception-based dating is likely more accurate. Otherwise, LMP-based dating (refined by an early ultrasound) is the standard approach.
The Math Behind Each Method
LMP method: add 280 days (40 weeks) to the first day of your last menstrual period. Conception method: add 266 days (38 weeks) to the date of conception. If your cycles are about 28 days and you ovulated on day 14, the two methods land on the same date. They diverge when ovulation happens earlier or later than day 14.
Adjusting for Cycle Length
Many calculators let you enter your average cycle length. A 35-day cycle, for example, usually means ovulation about a week later than in a 28-day cycle, so the LMP-only date would be about a week too early. Our Due Date Calculator accounts for this.
When Conception Date Is the Better Starting Point
If you tracked ovulation carefully, used ovulation predictor kits, or conceived through IVF or insemination, the conception or transfer date is usually more reliable than the last period. Try the Conception Calculator or, for IVF, the IVF Due Date Calculator.
What Your Provider Will Do
Your provider may confirm dating with an early ultrasound and may keep or change the due date accordingly. Learn how in How Doctors Calculate Gestational Age. Remember that a due date is a target, and most babies are born within a couple of weeks either side.
Related Reading
- Gestational Age vs. Fetal Age — what each term means
- How Doctors Calculate Gestational Age — LMP, ultrasound and adjustments
- Gestational Age FAQs — quick answers to common questions
- Traditional vs. IVF Dating — how the starting point changes
Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.
Frequently Asked Questions
How far is the due date from conception?
A due date falls about 266 days (38 weeks) after conception, or 280 days (40 weeks) after the first day of your last period.
Why do the two methods give different dates?
LMP-based dating assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the dates can differ by several days.
How many babies arrive on their due date?
Only about 4 to 5 percent. A due date is an estimate, and healthy babies are born in the weeks before and after it.
Related Resources
- Due Date Calculator
- Conception Calculator
- IVF Due Date Calculator
- How Doctors Calculate Gestational Age
- The Complete Due Date Guide
Sources and Further Reading
This page is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your OB-GYN or midwife about your own pregnancy.