How to Calculate Your Pregnancy Due Date (and Why It's Rarely Exact)
Every pregnant person hears the same question within days of a positive test: "When are you due?" The honest answer is that a due date is an estimate, not a promise. Only about 5% of babies arrive on their exact calculated due date. Still, knowing that date matters — it shapes your prenatal appointment schedule, your screening windows, and your own planning.
This guide shows you the two most common ways doctors estimate a due date, why the number can shift after an ultrasound, and how to get your own estimate in seconds.
Method 1: Counting from Your Last Menstrual Period (LMP)
The standard method used by most doctors and midwives is called Naegele's Rule. It assumes a 28-day cycle and ovulation around day 14.
- Take the first day of your last menstrual period.
- Add 1 year.
- Subtract 3 months.
- Add 7 days.
Example: if your last period started on March 10, adding a year gives
March 10 (next year), subtracting 3 months gives December 10,
and adding 7 days gives an estimated due date of December 17.
This method assumes a textbook 28-day cycle. If your cycles run longer or shorter, the estimate can be off by several days.
Why the 28-Day Assumption Matters
A pregnancy is typically counted as 40 weeks (280 days) from the LMP, even though conception usually happens around week 2. That's why the "40 weeks" you hear about actually includes roughly two weeks before you were pregnant at all — it's a dating convention, not a literal countdown from conception.
Method 2: Ultrasound Dating
If your cycles are irregular, or you're not sure of your LMP, an early ultrasound (usually between 8–13 weeks) is more accurate. The sonographer measures the fetus — most often the crown-rump length — and compares it to average growth charts to estimate gestational age.
| Method | Typical accuracy window | Best used when |
|---|---|---|
| LMP (Naegele's Rule) | ± 1–2 weeks | Regular 28-day cycles, known LMP date |
| First-trimester ultrasound | ± 3–5 days | Irregular cycles or unknown LMP |
| Second/third-trimester ultrasound | ± 1–3 weeks | Late-stage dating (less precise) |
If a first-trimester ultrasound date and your LMP date disagree by more than about a week, most providers will use the ultrasound date going forward.
What the Due Date Actually Tells You
- Full term is generally considered 39–40 weeks.
- Early term is 37–38 weeks.
- Late term is 41 weeks.
- Post-term is 42 weeks or beyond, and providers usually discuss induction options around this point.
In practice, going into labor anywhere from 3 weeks before to 2 weeks after the due date is considered completely normal.
Try It Yourself
Doing this math by hand works, but it's easy to miscount days across month boundaries. Our Pregnancy Due Date Calculator does it instantly — just enter your last period date (or conception date, if known) and it returns your estimated due date along with your current week of pregnancy.
Frequently Asked Questions
Can my due date change during pregnancy?
Yes. It's common for a due date to shift slightly after a dating ultrasound, especially if your cycles are irregular. Once set by an early ultrasound, the date usually stays fixed for the rest of the pregnancy.
Is a due date the same as a delivery date?
No. It's a statistical estimate based on average pregnancy length. Only a small percentage of babies are born on the exact calculated date.
What if I don't remember my last period date?
That's normal and common. An early ultrasound is the most reliable way to date the pregnancy when the LMP is unknown or cycles are irregular.
Useful Toolbita Tools
- Pregnancy Due Date Calculator — get your estimated due date and current week instantly.
- Water Intake Calculator — check your recommended daily water intake, which increases during pregnancy.
- Body Fat Calculator — track body composition changes before and after pregnancy.
- Age Calculator — handy for quickly working out ages and important dates around your due date.