How to use the due date calculator
- Choose the date you know: the first day of your last period, a conception date, or an IVF embryo transfer date.
- For a last period, enter your average cycle length. Leave it at 28 days if you are not sure.
- For IVF, choose whether the embryo was transferred on day 5 (a blastocyst) or day 3.
- Read your due date, the 37 to 42 week window and how far along you are today.
How the due date is calculated
Naegele's rule assumes a 28-day cycle with ovulation on about day 14. The pregnancy then lasts 280 days (40 weeks) from the first day of your last menstrual period (LMP). If your cycles are longer, you probably ovulated later, so the date moves later by the same number of days. A 32-day cycle adds 4 days.
From conception, the time is 280 − 14 = 266 days. For IVF, the embryo was already 5 or 3 days old at transfer, so those days come off too. The 261 and 263 day figures are the ones the American College of Obstetricians and Gynecologists (ACOG) gives in its guidance on estimating the due date (Committee Opinion 700).
Worked example
The calculator opens with a last period 70 days before today and a 28-day cycle. On October 29, 2026, that last period started on August 20, 2026.
- Cycle adjustment: 28 − 28 = 0 days
- Due date: August 20, 2026 + 280 days = May 27, 2027, a Thursday
- Full term begins at 39 weeks: May 20, 2027
- The 37 to 42 week window: May 6, 2027 to June 10, 2027
Change the cycle to 32 days and the due date moves 4 days later, to May 31, 2027.
The original calendar version of Naegele's rule
Before calculators, the rule was done on a calendar: take the first day of the last period, add 7 days, subtract 3 months and add 1 year. Because months have different lengths, this can land a day or two away from the 280-day count. For a last period on March 1, 2026, the calendar rule gives December 8, 2026, while 280 days gives December 6. The calculator shows both when you use a last period. Most clinics and apps use the 280-day count.
| Last period | LMP + 280 days | Calendar rule |
|---|---|---|
| Jan 1, 2026 | Oct 8, 2026 | Oct 8, 2026 |
| Mar 1, 2026 | Dec 6, 2026 | Dec 8, 2026 |
| Aug 20, 2026 | May 27, 2027 | May 27, 2027 |
Limits of a calculated due date
A date from your last period is only as good as the guess about when you ovulated. Cycles vary, and many people are not sure of the exact day their last period started. That is why a first-trimester ultrasound is the preferred way to set the due date: ACOG puts its accuracy at about 5 to 7 days. If your scan gives a different date, go by the scan. Only a small share of babies arrive on the due date itself; most come within the 37 to 42 week window.
Frequently asked questions
Is the due date the same as 40 weeks?
Yes. The due date is the day you reach 40 weeks, 0 days, counted from the first day of your last period (or the equivalent date for conception and IVF). The pregnancy has lasted about 38 weeks from conception at that point.
My cycles are irregular. Will this work?
Only roughly. With irregular cycles it is hard to know when you ovulated, so a date from your last period can be off by a week or more. An early ultrasound is much more reliable; ask your doctor or midwife about a dating scan.
Why does IVF give a more exact date?
Because the embryo's age and the transfer date are known to the day. There is no guessing about ovulation or fertilization, so ACOG recommends dating IVF pregnancies from the transfer date and embryo age.
When should I call my doctor or midwife?
Contact them as soon as you have a positive test to arrange care, and any time you have bleeding, severe pain or other symptoms that worry you. This calculator gives estimates only and cannot tell you anything about the health of a pregnancy.
Related calculators
- Pregnancy Calculator — How many weeks pregnant you are, your due date, trimester and key milestones, from your last period, conception, ultrasound or IVF.
- Pregnancy Conception Calculator — Estimate your conception date and fertile window from your last period or due date, with the early pregnancy timeline.
- Ovulation Calculator — Your estimated ovulation date and fertile window for the next 6 cycles, from your last period and cycle length.
Results are estimates for general information and are not medical advice. Talk to a doctor or other qualified professional about your health. See our disclaimer.