You can estimate your due date from your last period with one simple calculation: count back 3 months from the first day of your last menstrual period, then add 7 days and 1 year. This is called Naegele’s rule, and it has been used in obstetrics for roughly two centuries. It assumes a 28-day cycle with ovulation on day 14, which does not describe every pregnancy, so the number it gives you is an estimate rather than a fixed date.
How To Calculate Due Date From Last Period 3 Steps
The three steps are quick, but the starting point matters more than most people realize. You begin with the first day of your last menstrual period — the day bleeding started, not the day it ended. That single detail throws off the math for a lot of people.
Here is the sequence:
- Step 1: Write down the first day of your last period.
- Step 2: Count backward 3 months from that date.
- Step 3: Add 7 days, then add 1 year.
An example makes it concrete. Suppose your last period began on June 10, 2025. Count back 3 months to March 10, 2025. Add 7 days to get March 17, 2025. Add 1 year, and your estimated due date is March 17, 2026.
If counting backward crosses into the previous year, the arithmetic still works — you just add the year at the end. Say your last period started on January 5, 2026. Back 3 months is October 5, 2025. Add 7 days gives October 12, 2025. Add 1 year gives October 12, 2026.
One clarification worth knowing: the method is named for Franz Naegele, a German obstetrician, but the rule as it is used today is not quite what he originally described. The version taught in clinics now — subtract 3 months, add 7 days, add 1 year — is the standard form. It is also written as LMP + 280 days, which produces the same answer.
Why Does the Calculation Add 280 Days?
A full-term pregnancy is counted as 280 days, or 40 weeks, from the first day of the last period. That number is a convention, not a biological constant. It comes from the observation that most pregnancies that end in spontaneous labor cluster around that point.
Notice where the count starts. The 280 days begin on the first day of bleeding — roughly two weeks before conception actually happens in a typical cycle. Ovulation usually occurs about midway through the cycle, and fertilization follows within a day or so of ovulation. That means the true biological age of the pregnancy, called gestational age from conception, is about two weeks shorter than the number your due date is based on.
This is why pregnancy is dated from the last period rather than from conception. Most people do not know the exact day they conceived, but they usually remember when their last period started. It gives clinicians a consistent, shared reference point.
The practical result: at your estimated due date, the pregnancy is considered 40 weeks along by the standard count, even though the embryo has existed for closer to 38 weeks.
What If Your Cycle Is Not 28 Days?
Naegele’s rule assumes a 28-day cycle. If your cycles run longer or shorter, the calculation drifts, and the size of the drift depends on how far you are from 28 days.
Here is the logic. Ovulation tends to happen a fairly consistent number of days before the next period, not after the last one. In a 28-day cycle, ovulation lands around day 14. In a 35-day cycle, it lands closer to day 21 — about a week later. A due date calculated by the standard rule would therefore fall roughly a week earlier than the pregnancy actually is.
Short cycles work the same way in reverse. In a 21-day cycle, ovulation is earlier, and the standard calculation would place the due date later than reality.
A common adjustment is to add or subtract the difference from 28 days. For a 35-day cycle, you would add 7 days to the calculated due date. For a 21-day cycle, subtract 7 days. This is a reasonable correction, but it depends on knowing your typical cycle length with some confidence — and on your cycle being regular. If your cycles vary widely, the adjustment becomes unreliable.
For anyone with irregular cycles, the LMP calculation is a starting point at best. Ultrasound dating tends to be more accurate in those cases.
How Accurate Is a Due Date From Your Last Period?
It is an estimate, and the honest framing is that only a small share of babies arrive on their calculated due date. Most births happen in a window around it, not on the day itself.
Several factors push the estimate off:
- Irregular cycles, which make the ovulation timing uncertain.
- Uncertainty about the exact first day of the last period.
- Bleeding early in pregnancy that gets mistaken for a period.
- Natural variation in how long any individual pregnancy lasts.
Because of this, clinicians often refine the estimate with an early ultrasound. Ultrasound dating measures the fetus directly and is generally considered more accurate than the LMP method, particularly when the two disagree. The earlier the ultrasound, the more precise it tends to be for dating purposes, because early fetal growth is more consistent across pregnancies than growth later on.
This is also why an LMP-based due date is not usually treated as the final word. It is a useful first estimate that may be adjusted after imaging.
Key Terms You Will See on Your Chart
Once you calculate a due date, you will encounter a few terms that can be confusing because they sound similar. Here is what they mean.
| Term | What It Means |
|---|---|
| LMP | Last menstrual period — the first day of your most recent period, used as the starting point. |
| EDD | Estimated due date — the date the calculation produces. |
| Gestational age | Weeks counted from the first day of the LMP. |
| Full term | Generally defined as birth between 39 and 40 weeks of gestation, with 37 to 38 weeks considered early term. |
The gestational age is the number you will hear most often at appointments — “you’re 20 weeks” refers to this count from the LMP, not from conception.
When Should You Ask a Clinician Instead of Calculating?
The three-step method is fine for a rough estimate, but certain situations call for a clinician’s input rather than a home calculation.
If your cycles are irregular, if you are unsure when your last period started, or if you have conditions that affect ovulation, the LMP method becomes less reliable. If you became pregnant while using hormonal birth control, or if you have no memory of a clear last period, the starting point itself is uncertain.
In these cases, an early ultrasound is typically used to establish the due date. That is a clinical decision, not something to work out at home.
It is also worth saying plainly: a due date is not a deadline. It is a reference point that helps guide prenatal care, schedule tests, and track growth. Babies arrive on their own timeline, and a due date that shifts by a few days after an ultrasound is normal, not a sign that anything is wrong.
If you want to track your pregnancy week by week, an accurate dating method matters more than the specific date on the calendar. For most people, the LMP calculation is a solid first step — and a clinician or ultrasound can confirm it.
Frequently Asked Questions
How do you calculate your due date from your last period?
Count back 3 months from the first day of your last period, then add 7 days and 1 year. This is Naegele’s rule and gives an estimated due date based on a 28-day cycle.
Is the due date calculated from the first or last day of your period?
It is calculated from the first day of your last period, the day bleeding began. Using the last day instead will push your estimate off by several days.
How accurate is a due date based on your last period?
It is an estimate, and most babies are not born on the exact calculated date. Irregular cycles and uncertainty about the last period reduce accuracy, so an early ultrasound is often used to confirm dating.
What if my cycle is longer or shorter than 28 days?
The standard calculation assumes a 28-day cycle, so longer or shorter cycles shift the estimate. A common adjustment is to add or subtract the difference in days, though this works best for regular cycles.
