Due Date Calculator
When will your baby arrive? Get an estimated due date from your last period or conception date — plus how far along you are today.
Estimate your due date
Most cycles are 21–35 days. Longer cycles shift the due date later.
If you tracked ovulation or had fertility treatment, this can be more precise than the period date.
An estimate, not an appointment — only about 5% of babies arrive on their due date. Confirm with your healthcare provider.
How Due Dates Are Calculated (Naegele's Rule)
Almost every due date ever calculated uses Naegele's rule — named for the German obstetrician Franz Karl Naegele, who published it in the early 1800s. The recipe:
- Start from the first day of your last menstrual period (LMP).
- Add 280 days (40 weeks). The shortcut version: add 7 days, subtract 3 months, add 1 year.
- If your cycle isn't 28 days, shift by the difference: a 32-day cycle moves the date 4 days later, because ovulation (and conception) happened later.
Why count from the period instead of conception? Because the LMP is a date most people can recall or look up, while the exact moment of conception is usually unknown. Pregnancy is therefore dated about two weeks "older" than the embryo actually is — at the moment of conception, you're already considered 2 weeks pregnant.
If you know the conception date (from ovulation tracking or fertility treatment), the math is even simpler: conception + 266 days (38 weeks).
Worked Example
Last period started January 10, 2026 · 28-day cycle
Step 1 — Naegele's shortcut: Jan 10 + 7 days = Jan 17; minus 3 months = Oct 17; plus 1 year → October 17, 2026.
Step 2 — check with day count: Jan 10 + 280 days = October 17, 2026. ✔
If her cycle were 32 days: add 4 more days → October 21, 2026, because ovulation happened around day 18 instead of day 14.
How Accurate Are Due Date Estimates?
Honest answer: not very, and that's normal. Only about 5% of babies are born on their estimated due date. Most arrive within the two weeks before or after it — which is exactly why doctors talk about a "due month" more than a due day.
The gold standard for dating is a first-trimester ultrasound (up to 13 weeks 6 days). The American College of Obstetricians and Gynecologists (ACOG) considers it the most accurate method: if the scan disagrees with the LMP-based date by more than 5–7 days (depending on timing), the due date is officially changed to the ultrasound date. After the first trimester, dating gets fuzzier, not sharper — so an early scan is worth more than any calculator.
What Can Shift Your Due Date
- Irregular or long cycles — the biggest source of error. Naegele's rule assumes ovulation on day 14; real ovulation varies widely.
- Uncertain LMP — ACOG notes only about half of women accurately recall their last period date. Check a calendar or period-tracking app.
- Early ultrasound — can move the date by a week or more in either direction, and it overrules the LMP estimate.
- Twins or more — multiples often arrive earlier; your provider will monitor more closely and may plan differently.
- First vs later pregnancies — first babies arrive slightly later on average, though the difference is small.
What to Do Next
- Book prenatal care early — ideally as soon as you know you're pregnant. Early care is one of the strongest predictors of healthy outcomes.
- Ask about a dating ultrasound in the first trimester to confirm (or correct) the estimate.
- Start folic acid (400–800 mcg daily) if you haven't — it prevents neural tube defects, and it matters most in the earliest weeks.
- Skip the countdown stress. Mark the due month on your calendar, have your bag ready from week 37, and let the baby pick the day.
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Frequently Asked Questions
How is a due date calculated?
With Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for cycle length. If you know the conception date, add 266 days (38 weeks) instead. This calculator does both.
How accurate is Naegele's rule?
It is a good starting estimate, not a promise — only about 5% of babies arrive on their due date, with most coming within two weeks either side. A first-trimester ultrasound is the most accurate dating method and can officially revise the estimate.
What if I don't know my last period date?
Check a calendar or period-tracking app — ACOG notes only about half of women recall it accurately. If you truly don't know, an early ultrasound is the best way to date the pregnancy; your provider can also estimate from a known conception date.
Can my due date change later?
Yes. If a first-trimester ultrasound disagrees with the period-based date by more than 5–7 days, ACOG guidelines say the ultrasound date becomes the official one. Later scans don't re-date the pregnancy — they check growth instead.
When will I feel the baby move?
Most people first feel flutters ("quickening") between weeks 16 and 25 — earlier in later pregnancies, later in first ones. Regular, countable movements usually establish by week 28. If you notice a big drop in movement late in pregnancy, contact your provider promptly.