How to Calculate Your Due Date (and Why It Shifts)
Key takeaways
- To calculate your due date, use Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period.
- Example: a last period starting January 1, 2026 gives a due date of October 8, 2026.
- Only about 5% of babies arrive on their due date — think of it as the middle of a 5-week window (37–42 weeks), not an appointment.
- A first-trimester ultrasound (8–13 weeks) is the most accurate dating method and can revise an LMP-based estimate.
Learning you’re pregnant raises one question immediately: how to calculate due date? The standard answer is Naegele’s rule — 280 days from the first day of your last menstrual period (LMP). It takes ten seconds, and it’s the same math your provider starts with. For instant results, use our due date calculator.
What is Naegele’s rule?
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. Count forward 40 weeks (280 days) from the first day of your last period. The classic shortcut: add 7 days to the LMP date, subtract 3 months, add 1 year. Both give the same answer.
Worked example: LMP = January 1, 2026. Add 280 days → October 8, 2026. (Check the shortcut: Jan 1 + 7 days = Jan 8; minus 3 months = Oct 8; plus 1 year = Oct 8, 2026. Same date.)
The rule is an estimate built on averages: conception usually happens around day 14, but cycles vary, ovulation shifts, and implantation adds its own timeline. That’s why the date it produces is a starting point, not a promise.
Why do only 5% of babies arrive on their due date?
Because human gestation isn’t a fixed timer. Full term spans 37 to 42 weeks — a five-week window — and most births cluster around 39–40 weeks. Studies consistently find only about 1 in 20 babies born on the exact estimated date.
Several things nudge the real date: cycle length (a 35-day cycle means ovulation — and conception — happened later than day 14), uncertain LMP dates, and plain biological variation in how long each pregnancy lasts. First pregnancies also tend to run slightly longer than subsequent ones.
The practical takeaway: prepare for the window, not the day. Have the hospital bag ready by 37 weeks and treat the due date as the midpoint of “any time now” rather than a deadline.
How does ultrasound dating work?
A first-trimester ultrasound — ideally between 8 and 13 weeks — is the most accurate way to date a pregnancy. The technician measures the embryo (crown-rump length), which grows at a remarkably consistent rate in early pregnancy, and converts that measurement to a gestational age.
If the ultrasound date differs from the LMP date by more than about 5–7 days in the first trimester, your provider will usually adopt the ultrasound date instead. Dating gets less precise later: a third-trimester scan can be off by two weeks or more, which is why early dating matters most.
Pinpointing ovulation with our ovulation calculator can also sharpen an LMP-based estimate if you know your cycle well.
How is the due date calculated for IVF?
IVF dating is actually more precise than natural conception, because the embryo’s age is known exactly. For a day-5 blastocyst transfer, add 261 days to the transfer date (266 days from fertilization minus 5 days of embryo age — in practice, most clinics simply add 38 weeks to the transfer date).
For a day-3 transfer, add 263 days. Your clinic will give you the official date — but the principle is the same: known conception date beats estimated ovulation every time.
What happens in each trimester?
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | 1–13 | Major organs form; fatigue and nausea peak for many |
| Second | 14–27 | Energy often returns; anatomy scan around week 20 |
| Third | 28–40 | Rapid growth; weekly checkups near the end |
Track the whole journey week by week with our pregnancy calculator, and see healthy ranges for each stage with our pregnancy weight gain calculator.
This article is for general information only and isn’t medical advice — talk to your doctor before starting a new exercise routine, especially if you have a heart condition. For pregnancy-specific guidance, always follow your own provider’s advice.
When should you call your provider?
Due dates are estimates, but some symptoms aren’t wait-and-see. Call your provider promptly for: vaginal bleeding, fluid leaking from the vagina, severe headache or vision changes, sudden swelling of face or hands, regular contractions before 37 weeks, or noticeably reduced fetal movement after you’ve started feeling kicks.
When in doubt, call. Providers would far rather reassure you about something normal than hear about something serious too late. Save your provider’s after-hours number in your phone now, before you need it.
FAQs
How do I calculate my due date from my last period?
Add 280 days (40 weeks) to the first day of your last menstrual period. For example, a last period on January 1, 2026 gives a due date of October 8, 2026. This is Naegele’s rule — the standard method providers start with, though an early ultrasound may refine it.
Can my due date change?
Yes — it’s common. If a first-trimester ultrasound (8–13 weeks) dates the pregnancy more than about 5–7 days differently from your LMP calculation, your provider will usually switch to the ultrasound date. Later scans are less accurate for dating, so early measurement carries the most weight.
What percentage of babies are born on their due date?
Only about 5%. Full term spans 37–42 weeks, and most babies arrive within a week or two on either side of the estimated date. Think of the due date as the center of a five-week window, and plan to be ready from 37 weeks onward.
How is a due date calculated with irregular periods?
LMP-based math is unreliable with irregular cycles, since ovulation may not fall on day 14. In that case, a first-trimester ultrasound is the gold standard — crown-rump length measurement between 8 and 13 weeks gives the most accurate gestational age regardless of cycle history.
Does the due date change with twins?
The estimated date is calculated the same way, but twins and multiples are more likely to arrive early — the average twin birth is around 35–36 weeks. Your provider will monitor a multiple pregnancy more closely and may plan delivery before 40 weeks.
So: start with Naegele’s rule, confirm with an early ultrasound, and prepare for the window. For broader guidance on staying healthy through pregnancy and beyond, see our healthy living guide.