How Is a Due Date Calculated?
Your estimated due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP) — a method called Naegele’s rule. This assumes a 28-day cycle with ovulation around day 14, so it estimates roughly 266 days from conception plus the two extra weeks before ovulation actually happens. You can get your estimate instantly with the pregnancy due date calculator, which also supports conception-date, IVF-transfer, and ultrasound-based dating if LMP dating doesn’t apply to you.
It’s worth setting expectations up front: a due date is an estimate, not a prediction. Only about 5% of babies are born on their exact due date, and a full-term pregnancy spans a window of 37 to 42 weeks.
The Four Ways to Calculate a Due Date
Different situations call for different dating methods, and healthcare providers will use whichever is most reliable for your case:
| Method | How It Works | Best For |
|---|---|---|
| LMP (Naegele’s Rule) | First day of last period + 280 days | Regular 28-day cycles, most common method |
| Conception date | Known/estimated conception date + 266 days | If you know your ovulation or conception date |
| IVF transfer date | Transfer date + (261 or 263 days, depending on Day 3 vs Day 5 transfer) | IVF pregnancies, the most precise method available |
| Ultrasound dating | Measured gestational age at the scan, projected forward | Irregular cycles or unknown LMP; most accurate in the first trimester |
The due date calculator supports all four methods — pick whichever matches what you actually know, and it will do the date math for you.
Naegele’s Rule, Step by Step
The most precise version of the rule is simply: LMP + 280 days.
Worked Example
If your last period started on March 10, 2026:
- March 10, 2026 + 280 days = December 15, 2026
- That’s your estimated due date.
The Quick Hand-Calculation Shortcut
Many people learned a faster mental-math version: add 7 days to the LMP, then subtract 3 months (adding a year if needed). For March 10, that’s March 17 → December 17. Notice this lands 2 days later than the precise 280-day count (December 15) — that’s expected, since calendar months don’t all have the same number of days, so the shortcut is only an approximation. For an exact date, use the full 280-day count (or the due date calculator, which always calculates the precise date).
Cycle length matters. Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, your actual ovulation date shifts, which shifts your due date too. The pregnancy calculator lets you adjust for your actual average cycle length instead of assuming 28 days.
Why Ultrasounds Sometimes Change the Due Date
An early ultrasound (typically done at 8–12 weeks) measures the embryo or fetus and estimates gestational age from its size. Because early fetal growth is remarkably consistent across pregnancies, a first-trimester ultrasound is often considered more accurate than LMP dating alone — especially if:
- Your cycles are irregular, so you’re not sure of your actual ovulation date
- You don’t remember the exact date your last period started
- Your cycle length is significantly shorter or longer than 28 days
If the ultrasound estimate differs from the LMP-based due date by more than about a week (in the first trimester), providers typically adjust the due date to match the ultrasound.
How Accurate Are Due Dates, Really?
Due dates are a statistical estimate of the most likely delivery window, not a guarantee:
- Only about 5% of babies arrive on their exact calculated due date.
- Most babies are born somewhere between 39 and 41 weeks.
- A pregnancy is considered full term at 37 weeks — babies born from 37 to 42 weeks are all within the normal range.
- First-time pregnancies statistically trend slightly later than subsequent pregnancies.
Rather than fixating on the single due date, many providers find it more useful to think in terms of a due window spanning a couple of weeks on either side.
Common Due Date Myths
- “Due dates are precise predictions.” In reality, they mark the midpoint of a normal delivery window, not a guaranteed delivery day.
- “Every pregnancy is exactly 40 weeks.” Full-term delivery spans 37–42 weeks; “40 weeks” is simply the average, not a fixed length.
- “An ultrasound is always more accurate than LMP.” This is true mainly in the first trimester — later ultrasounds estimate size (which varies more between babies) rather than gestational age, so they’re less reliable for dating.
- “Second pregnancies always come early.” Timing does trend slightly earlier for subsequent pregnancies on average, but plenty of first pregnancies arrive right on schedule and second pregnancies run late.
Milestones Counted From Your Due Date
Providers track pregnancy in weeks, counted backward from the estimated due date, which is why appointments and screenings cluster around specific windows:
| Milestone | Typical Timing |
|---|---|
| First prenatal visit | Weeks 6–10 |
| Dating ultrasound | Weeks 8–12 |
| End of first trimester | Week 12 |
| Anatomy scan | Weeks 18–22 |
| Glucose screening | Weeks 24–28 |
| Full term begins | Week 37 |
| Estimated due date | Week 40 |
Due Date vs. Days Until Due Date
Once you have your estimated due date, a natural follow-up question is simply “how many days/weeks do I have left?” That’s a straightforward date-math problem — plug your due date and today’s date into the date difference calculator to get the exact day count, which is handy for countdown planning, hospital bag timing, or maternity leave scheduling.
Frequently Asked Questions
What is Naegele’s rule? It’s the standard formula for estimating a due date: add 280 days (40 weeks) to the first day of your last menstrual period, assuming a typical 28-day cycle.
Can my due date change during pregnancy? Yes. If a first-trimester ultrasound suggests a gestational age that differs meaningfully from the LMP-based estimate, your provider may officially adjust your due date. Due dates are rarely changed after the first trimester.
Is IVF due-date calculation different? Yes — IVF due dates are calculated from the embryo transfer date rather than LMP, since the exact fertilization timing is known. This makes IVF dating one of the most precise methods available, typically adding 261 days for a Day 5 transfer or 263 days for a Day 3 transfer.
Do twins or multiples have a different due date? The due date calculation itself is the same, but pregnancies with twins or higher-order multiples statistically tend to deliver earlier than singleton pregnancies, often before 40 weeks.
How early can a due date be estimated? As soon as you know the first day of your last period, or once a dating ultrasound has been performed (usually between 8–12 weeks), which is generally considered the most accurate single estimate.
The Bottom Line
A due date is Naegele’s rule doing simple date math — LMP plus 280 days — refined by ultrasound measurements or known conception/IVF dates when they’re available. It’s a useful planning target, but remember it’s an estimate: full term spans 37–42 weeks, and only 1 in 20 babies actually arrives on the calculated day. Get your personalized estimate with the pregnancy due date calculator, then use the date difference calculator to count down the days once you know your target date.