Due Date Calculator
Estimate your due date — the day your pregnancy reaches 40 weeks — using whichever date you know. Pick a method, enter that one date, and the calculator returns an estimated due date plus how far along you are today. The four methods (last period, conception, IVF transfer, and ultrasound dating) each use a standard offset, so they can give slightly different dates. Treat every result as an estimate: only about 4% of babies are born on their due date.
Calculate
Default result: Oct 8, 2026
Due Date Calculator · Result
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Estimated due date
lmp × 2026-01-01 × 2026-02-01 × 2026-02-01 × 2026-03-01 × 70 × 2026-04-02
- Weeks along (as of date)
- 13
- Days into the week
- 0
- Trimester
- 1
This calculator provides an estimate for general information only and is not medical advice. Due dates, gestational age, and fertility windows are estimates — babies rarely arrive exactly on the estimated due date, and individual cycles and pregnancies vary. Always confirm dates and any health decisions with your healthcare provider or OB-GYN.
Reviewed by the calculators.dev team · Last updated 2026-06-24
Formula reviewed against Naegele's rule and estimated due date (review), PubMed 33079400; ACOG / American Pregnancy Association due-date guidance
How to calculate
Choose a dating method, then fill in the single date that method needs — the helper text under each field tells you which one. Leave the other date fields as they are; the calculator only reads the field for the method you selected. The result is your estimated due date, and the 'as of' date (today by default) shows your current gestational age.
Last period (Naegele's rule): due date = first day of last period + 280 days. Conception: + 266 days from the conception date (280 − 14). IVF Day-5 (blastocyst): + 261 days from transfer (266 − 5); IVF Day-3 (cleavage): + 263 days (266 − 3). Ultrasound: due date = scan date + (280 − gestational age in days at the scan). Every method assumes a 40-week (280-day) pregnancy measured from the last menstrual period.
Example calculation
From a last period on January 1, 2026, Naegele's rule adds 280 days to give an estimated due date of October 8, 2026. As of April 2, 2026 that pregnancy is 13 weeks 0 days along — the first trimester. Only about 4% of babies arrive on the estimated due date.
- eddDay
- October 8, 2026
- currentGaWeeks
- 13
- currentGaDays
- 0
- trimester
- 1
Assumptions
- Last menstrual period (LMP) method: reads only the 'first day of last period' field and adds 280 days (Naegele's rule). It assumes a regular 28-day cycle with ovulation on day 14; a longer or shorter cycle shifts the true date.
- Conception method: reads only the 'conception date' field and adds 266 days (280 − 14), since dating from conception skips the two-week pre-ovulation offset.
- IVF methods: read only the 'IVF transfer date' field. A Day-5 blastocyst transfer adds 261 days and a Day-3 cleavage transfer adds 263 days, because the embryo is already 5 or 3 days old at transfer.
- Ultrasound method: reads the 'ultrasound date' and 'gestational age at scan' fields and projects forward to 280 days. Early ultrasound dating is often the most accurate when the cycle is irregular.
- All other fields are ignored for the method you choose. The 'as of' date is only used to report how far along you are today and never changes the due date.
- The estimated due date marks 40 completed weeks. Only about 4% of babies are born exactly on it; a normal full-term birth is anywhere from 37 to 42 weeks.
Common mistakes
- Filling in the wrong field for the chosen method. Each method reads only one date — the helper under each field names the method it belongs to. The other fields are ignored.
- Treating the due date as a deadline. It is an estimate of 40 weeks; most babies arrive in the 37-to-42-week window, and only about 4% on the date itself.
- Using the LMP method with an irregular or non-28-day cycle. Naegele's rule assumes a 28-day cycle, so an early ultrasound is usually more reliable when cycles vary.
Frequently asked questions
How is a due date calculated from my last period?
Naegele's rule adds 280 days (40 weeks) to the first day of your last menstrual period. It assumes a regular 28-day cycle, so it can be off by a few days if your cycle is longer or shorter.
Why do the methods give slightly different due dates?
Each method measures from a different point and uses a different offset (280 days from the last period, 266 from conception, 261 or 263 from an IVF transfer). They normally agree within a few days; an early ultrasound is often used to confirm dating.
Will my baby be born on the due date?
Probably not on the exact day — only about 4% of babies are. The due date marks 40 weeks, and a normal full-term birth happens anywhere from 37 to 42 weeks.
Which method is most accurate?
When cycles are irregular, an early ultrasound is usually the most accurate way to date a pregnancy. For regular cycles the last-period method is a good estimate. Your provider will confirm the dating used for your care.