To calculate your due date, count 280 days (40 weeks) from the first day of your last menstrual period (LMP). This is the method doctors call Naegele’s rule, and you can do it with a calendar and a pencil. This guide walks through each step, shows a worked example, and explains how to adjust the date if your cycles aren’t 28 days long.
Quick Answer: The Due Date Formula
Estimated due date (EDD) = first day of your last period + 280 days. The same result comes from the calendar shortcut: take your LMP, subtract 3 months, add 7 days, then add 1 year. Johns Hopkins Medicine describes the same method and counts the first day of your last normal period as day 1 of the pregnancy.
Want the answer without the math? Use our due date calculator and get your date in seconds.
Step 1: Find the First Day of Your Last Period
Your LMP is the first day of your most recent period, the first day of full flow rather than light spotting. Check a period-tracking app or a calendar if you can’t remember. This date is the starting point for everything that follows.
It may feel odd that pregnancy is counted from a date when you weren’t pregnant yet. Doctors use the LMP because it is usually the one date you can pin down. Ovulation and conception normally happen about two weeks later, so by the time you are “4 weeks pregnant” your baby is only about 2 weeks old. Our guide to how doctors calculate gestational age explains this in detail.
Step 2: Add 280 Days (or Use the 3-Month Shortcut)
You can reach the same date in two ways:
- The direct method: add 280 days to your LMP. A date-add tool or a calendar app makes this quick.
- Naegele’s rule shortcut: subtract 3 months from your LMP, add 7 days, and add 1 year.
Worked example
Suppose your last period began on January 10, 2027.
- Subtract 3 months: October 10, 2026.
- Add 7 days: October 17, 2026.
- Add 1 year: October 17, 2027.
Adding 280 days to January 10, 2027 gives the same answer, October 17, 2027. Because months have different lengths, the shortcut can occasionally land a day or two away from the 280-day count, so the direct method is the more precise of the two.
| First day of last period | Estimated due date (28-day cycle) |
|---|---|
| January 10, 2027 | October 17, 2027 |
| March 31, 2027 | January 5, 2028 |
| December 3, 2026 | September 9, 2027 |
Step 3: Adjust for Your Cycle Length
The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycles are longer or shorter, ovulation usually shifts with them, and so does your due date. As AIMS notes, people with longer cycles tend to have pregnancies that appear a bit longer, and the reverse is true for shorter cycles.
Adjusted due date = LMP + 280 days + (your cycle length − 28).
- 35-day cycle: add 7 days. A January 10, 2027 LMP moves from October 17 to October 24, 2027.
- 24-day cycle: subtract 4 days. The same LMP moves to October 13, 2027.
- 28-day cycle: no change.
To find your cycle length, count from the first day of one period to the day before the next one begins. If your periods are irregular, this adjustment isn’t reliable, and an ultrasound is a better guide. Read more in our article on irregular cycles.
What If You Know Your Ovulation or Conception Date?
If you tracked ovulation or know the day of conception, count forward from that date instead. Conception-based dating is generally more precise than LMP when your cycle is irregular. Add 266 days (38 weeks) to the conception date. For pregnancies from IVF, the embryo transfer date is used, as covered by our IVF due date calculator.
Why Your Doctor May Change Your Due Date
Your LMP-based date is an estimate. At your first ultrasound, usually in the first trimester, your provider compares the baby’s measurements with your LMP date. The American College of Obstetricians and Gynecologists (ACOG) advises changing the due date only when the two differ by more than a set amount for that stage:
| Gestational age at ultrasound | Redate if the difference is more than |
|---|---|
| Up to 8 weeks 6 days | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks and beyond | 21 days |
Once a date is confirmed by ultrasound, it generally stays as your official due date for the rest of the pregnancy. For more on this, see why gestational age gets adjusted.
How Accurate Is a Due Date?
Treat your due date as the middle of a window, not a deadline. Only about 4 to 5 in 100 babies arrive on the estimated date, and about two-thirds are born within a week either side of it. A full-term pregnancy is considered to run from 37 to 42 weeks. ACOG divides this further:
- Early term: 37 weeks 0 days to 38 weeks 6 days
- Full term: 39 weeks 0 days to 40 weeks 6 days
- Late term: 41 weeks 0 days to 41 weeks 6 days
- Postterm: 42 weeks 0 days and beyond
Frequently Asked Questions
How do I calculate my due date from my last period?
Add 280 days to the first day of your last period, or subtract 3 months, add 7 days and add 1 year. Then adjust for your cycle length if it isn’t 28 days.
How many weeks is a full pregnancy?
Pregnancy is counted as 40 weeks from the first day of your last period, which is about 38 weeks after conception.
Is the first day of my period or the last day used?
The first day. Day 1 of full flow is day 1 of your pregnancy timeline.
What if I don’t remember my last period?
Your provider can date the pregnancy by ultrasound, which is the most reliable method in early pregnancy. Our due date guide covers your options.
Can my due date change?
Yes. If an early ultrasound differs from your LMP date by more than the thresholds above, your provider will usually move your due date to the ultrasound date.
Skip the Math
Prefer not to count days? Our due date calculator handles your cycle length automatically. Then follow your pregnancy with our week-by-week guide.
Sources
- Johns Hopkins Medicine: Calculating a Due Date
- ACOG Committee Opinion No. 700: Methods for Estimating the Due Date
- Tommy’s: Pregnancy Due Date Calculator
- AIMS: How accurate is my due date?
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.
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