Due Date Calculator

A due date calculator adds 280 days, or 40 weeks, to the first day of your last menstrual period. If your last period started on June 10, 2026, the estimated due date is Wednesday, March 17, 2027, and on November 1, 2026 you are 20 weeks 4 days along. With a 32-day cycle instead of 28, the estimate moves 4 days later, to March 21, 2027.

Every result here is an estimate based on the dates you enter, not medical advice. Only your clinician can confirm your due date, usually with an early ultrasound.

How to use the due date calculator

1. Pick the date you are surest of: Last period, Conception date, IVF transfer or Ultrasound.
2. Enter the date. For the last period, also enter your usual cycle length (leave 28 if you are not sure). For IVF, choose a day-3 or day-5 embryo. For an ultrasound, copy the weeks and days from the scan report.
3. Read your estimated due date, how many weeks and days pregnant you are today, your trimester, the days to go and a conception estimate. The progress bar shows where you are in the 40 weeks.
4. Check the timeline for the dates of key weeks, and open Show the math with your dates for every step. Change Show progress on to look ahead.

How the due date is calculated: the 280-day rule

By convention, the estimated due date is 280 days after the first day of the last menstrual period. That is the rule in ACOG Committee Opinion 700, the guidance obstetricians in the US follow. It assumes a regular 28-day cycle with ovulation on day 14, so conception happens about 2 weeks into the count. So the first two “weeks of pregnancy” come before conception.

For the example above: from June 10, 2026, there are 20 days left in June. July through February add 31 + 31 + 30 + 31 + 30 + 31 + 31 + 28 = 243 days, for a running total of 263. Seventeen more days reach March 17, 2027, which is day 280.

Naegele’s rule, the calendar shortcut

Naegele’s rule gets close without counting days: add 7 days to the first day of the last period, subtract 3 months and add 1 year. June 10, 2026 becomes June 17, then March 17, 2027, the same answer as the 280-day count.

The shortcut is not always exact, because months have different lengths. For a period that started on May 1, 2026, Naegele gives February 8, 2027, but 280 days gives February 5, 2027. The calculator uses the 280-day count.

Adjusting for a longer or shorter cycle

Ovulation usually comes about 14 days before the next period, not 14 days after the last one. NIH’s StatPearls review notes that Naegele’s rule ignores cycle length and describes a version that adds the difference. The adjustment is simple: due date = last period + 280 days + (cycle length − 28).

• 32-day cycle: ovulation around day 18, so add 4 days: June 10, 2026 + 284 days = March 21, 2027.
• 24-day cycle: ovulation around day 10, so subtract 4 days: June 10, 2026 + 276 days = March 13, 2027.

Due date from conception, IVF transfer or ultrasound

Each method finds the same date, 40 weeks 0 days, from a different known starting point.

Conception: 280 days minus the 14 days before ovulation leaves 266 days. Conception day itself counts as 2 weeks 0 days.

IVF: ACOG says that for pregnancies conceived with assisted reproductive technology, the IVF dates should set the due date, because they are known exactly. A day-5 embryo was fertilized 5 days before transfer, so the transfer day is 2 weeks 5 days, and 266 − 5 = 261 days remain. For a day-3 embryo, 266 − 3 = 263.

Ultrasound: a scan report gives the pregnancy’s age, such as 9 weeks 2 days. That is 9 × 7 + 2 = 65 days, so 280 − 65 = 215 days are left. August 14, 2026 + 215 days = March 17, 2027. ACOG calls an ultrasound in the first trimester, up to 13 weeks 6 days, the most accurate way to set or confirm the due date.

The tool’s conception estimate is ovulation on day (cycle − 14), June 24, 2026 in the example. The fertile window is about 6 days, the 5 days before ovulation plus ovulation day (NICHD, womenshealth.gov): June 19 to 24.

How far along am I? Weeks, days and trimesters

Gestational age is the number of days since the first day of the last period (or since the matching start date for the other methods), written as weeks plus days. From June 10 to November 1, 2026 is 144 days. 144 ÷ 7 = 20 remainder 4, so you are 20 weeks 4 days pregnant, often written 20w 4d. Days to go: 280 − 144 = 136.

These boundaries follow ACOG (first trimester up to 13 weeks 6 days) and MedlinePlus (second trimester from week 14, third from week 28). NICHD’s fact sheet groups the weeks slightly differently (1 to 12, 13 to 28, 29 to 40), so you may see a week’s difference between sources.

Term ranges around the due date

A due date is one day inside a range of weeks when birth is considered term. NICHD, with ACOG and SMFM, names those weeks:

NICHD classes babies born before 37 weeks as preterm.

When an ultrasound changes the due date

If you know your last period and also have an ultrasound, the two dates rarely agree exactly. ACOG gives clinicians a guide for when the difference is big enough to change the due date to the ultrasound date. It depends on when the scan was done:

Your clinician makes this decision. If your scan and your period give different dates, ask which one your care team is using.

Common mistakes when working out a due date

• Using the last day of the period. The count starts on the first day of bleeding.
• Counting from conception and adding 280 days. From conception, add 266 days; 280 is for the last period.
• Adding 9 calendar months. Nine months from June 10 is March 10, a week early. Naegele’s rule adds 9 months and 7 days.
• Using the wrong IVF number. Day-5 transfers use 261 days and day-3 transfers 263 days; the embryo’s age at transfer is the difference.

Sources

Checked November 2026.

• ACOG, AIUM and SMFM: Committee Opinion No. 700, Methods for Estimating the Due Date
• SMFM: Committee Opinion No. 700 summary
• NIH, NCBI Bookshelf (StatPearls): Estimated Date of Delivery
• NICHD: Know Your Terms (early, full, late and postterm)
• NICHD: Pregnancy fact sheet
• NICHD: 6 Things to Know About Conception
• Office on Women’s Health (womenshealth.gov): Ovulation calculator and fertile window
• MedlinePlus: Fetal development
• MedlinePlus: Prenatal care in your second trimester
• MedlinePlus: Prenatal care in your third trimester

Due date practice quiz: 10 questions

Test yourself with 10 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How is a due date calculated?

Add 280 days (40 weeks) to the first day of your last menstrual period. For a period that started June 10, 2026, that is March 17, 2027. From a known conception date, add 266 days instead; after an IVF transfer, add 261 days for a day-5 embryo or 263 for a day-3 embryo.

How accurate is a due date calculator?

It is as accurate as the dates you enter and the assumptions behind them. A date from your last period assumes regular cycles and a known ovulation day. ACOG considers a first-trimester ultrasound the most accurate way to date a pregnancy, and IVF dates are exact.

Can my due date change?

It can, early on, if an ultrasound and your last period disagree by more than ACOG's limits for that stage. After that, ACOG says it should change only in rare cases.

What if my periods are irregular?

Enter your usual cycle length, and the calculator moves the date by the difference from 28 days. If your cycles vary a lot, or you do not remember the date, an ultrasound gives a more reliable estimate; enter the scan date and the weeks and days from the report in the Ultrasound tab.

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