Due Date Calculator
Pick the method that fits what you know: last period, conception, IVF transfer or ultrasound. You get an estimated due date, its weekday and the term stages.
Last updated November 4, 2026
First day of one period to the first day of the next
Result
Estimated due date: May 27, 2027 (Thursday)
| Measure | Value |
|---|---|
| Estimated due date | Thursday, May 27, 2027 |
| Method used | last menstrual period |
| Estimated conception | September 3, 2026 |
| Second trimester starts (week 14) | November 26, 2026 |
| Third trimester starts (week 28) | March 4, 2027 |
| Term window (37w0d to 41w6d) | May 6, 2027 to June 9, 2027 |
| Stage | Weeks | Dates |
|---|---|---|
| Early term | 37w0d to 38w6d | May 6, 2027 to May 19, 2027 |
| Full term | 39w0d to 40w6d | May 20, 2027 to June 2, 2027 |
| Late term | 41w0d to 41w6d | June 3, 2027 to June 9, 2027 |
| Post-term | 42w0d and later | From June 10, 2027 |
Few babies arrive on the exact date. These dates are estimates, not a diagnosis or medical advice. Your doctor or midwife sets the due date that counts, often with an ultrasound.
Show the work
Formula: Due date = date entered + days for the method: last period 280 (adjusted by cycle length - 28), conception 266, IVF 266 - embryo age, ultrasound 280 - gestational age in days.
- Last period starts 2026-08-20; average cycle 28 days, so the adjustment is 0 days.
- 2026-08-20 + 280 + (28 - 28) = 2026-08-20 + 280 days = 2027-05-27.
- Start of the 280-day count: 2027-05-27 - 280 days = 2026-08-20.
- Trimesters from that start: week 14 = 2026-11-26, week 28 = 2027-03-04.
- Term stages from that start: 37 weeks = 2027-05-06, 39 weeks = 2027-05-20, 41 weeks = 2027-06-03, 42 weeks = 2027-06-10.
Result: Estimated due date: May 27, 2027 (Thursday)
How each method reaches a due date
due date = date entered + days for the method (period 280, conception 266, IVF 266 − embryo age, ultrasound 280 − gestational age)
Every method lands on the same idea: by convention a pregnancy is counted as 280 days (40 weeks) from the first day of the last period, assuming ovulation on day 14, about 266 days before the due date. The last period method adds 280 days and shifts by the difference between your cycle length and 28 days. The conception method adds 266 days.
An IVF transfer date is dated from the embryo’s age: a day-3 embryo adds 263 days (266 − 3) and a day-5 blastocyst adds 261 days (266 − 5). An ultrasound report gives the gestational age on the scan date in weeks and days, and the due date is the scan date plus the days left to 280.
See how we calculate for the formulas, sources and review schedule behind every calculator.
This is an estimate, not medical advice
The dates on this page are estimates. Cycle length and pregnancy length vary from person to person and from month to month, so real dates can differ by days or weeks. The tool cannot confirm a pregnancy, ovulation or fertility, is not a method of birth control, and does not replace care from a doctor or midwife. This page has not been reviewed by a medical professional.
Comparing two methods
If you have both a last period and an ultrasound report, turn on the comparison. The page shows both due dates and how many days apart they are. A gap of a few days is common, because ovulation does not always fall on day 14.
Sources
Official and primary sources for the statements on this page. Their authors have not reviewed or endorsed this page.
- NICHD (National Institutes of Health), About Pregnancy (web page, checked November 2, 2026). States that pregnancy usually lasts about 40 weeks, measured from the last menstrual period to delivery
- American College of Obstetricians and Gynecologists, Committee Opinion No. 700: Methods for Estimating the Due Date, Obstetrics & Gynecology 2017;129(5):e150-e154. Says the due date should be set once the last menstrual period, an early ultrasound, or both are available, and that for research and surveillance the best obstetric estimate should be used rather than the last menstrual period alone. By convention the due date is 280 days after the first day of the last period, and an IVF due date uses the embryo age and transfer date (261 days after a day-5 embryo, 263 after a day-3 embryo). It also gives the second trimester as 14 0/7 to 27 6/7 weeks and the third 28 0/7 weeks and beyond. These calculators do not apply its ultrasound redating rules, so their dates are estimates and not clinical dating
- American College of Obstetricians and Gynecologists, Committee Opinion No. 579: Definition of Term Pregnancy, Obstetrics & Gynecology 2013;122(5):1139-1140. Defines early term as 37 0/7 through 38 6/7 weeks, full term as 39 0/7 through 40 6/7 weeks, late term as 41 0/7 through 41 6/7 weeks and postterm as 42 0/7 weeks and beyond
Checked against the full text of ACOG Committee Opinion 700 on November 4, 2026: by convention the due date is 280 days after the first day of the last period; for an IVF pregnancy it is assigned from the age of the embryo and the transfer date, 261 days after a day-5 embryo transfer and 263 days after a day-3 embryo transfer; and it says that the 280-day convention assumes a 28-day cycle with ovulation on day 14.
The term stages were checked against the abstract of ACOG Committee Opinion 579 on November 4, 2026, and match its definitions exactly.
Not in these sources: the cycle-length adjustment (adding or subtracting days from a 28-day cycle) and the 266-day count from conception. The 266 days is 280 minus 14 and follows from ACOG’s stated assumption of ovulation on day 14; the cycle-length adjustment is a common extension of that rule. Treat both as conventions.
ACOG Committee Opinion 700 also sets rules for when an ultrasound should replace a last-period date, based on how early the scan was done and how many days the two differ. This page does not apply those rules; it reports the gap between two dates, and your doctor or midwife decides which date is used.
How to use the Due Date Calculator
- Choose the dating method that matches the date you have.
- Enter that date, plus the cycle length, embryo age or scan gestational age if asked.
- Optionally compare with an ultrasound due date.
- Read the due date, weekday and term stages, then use Copy, Print or Share.
Worked example
These five starting points describe the same pregnancy, so every method gives Thursday, October 8, 2026.
| Method | Date entered | Calculation | Due date |
|---|---|---|---|
| Last period (28-day cycle) | January 1, 2026 | + 280 days | October 8, 2026 |
| Conception | January 15, 2026 | + 266 days | October 8, 2026 |
| IVF, day-3 embryo | January 18, 2026 | + 263 days | October 8, 2026 |
| IVF, day-5 blastocyst | January 20, 2026 | + 261 days | October 8, 2026 |
| Ultrasound at 9 weeks 0 days | March 5, 2026 | + 217 days (280 − 63) | October 8, 2026 |
Frequently asked questions
Why does IVF use a different number of days?
The embryo is already a few days old at transfer, so the count from transfer is 266 days minus the embryo’s age: 263 for day 3, 261 for day 5. ACOG Opinion 700 gives both.
How do I enter an ultrasound result?
Use the scan date and the gestational age on the report, for example 9 weeks 0 days. The due date is the scan date plus the days left to 280.
What does the term window mean?
It shows the stages around the due date, from early term at 37 weeks to post-term at 42 weeks.
Why do two methods give different dates?
Ovulation and cycle length vary, and ultrasound measurements have their own margin. Ask your doctor or midwife which date to use.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser and nothing you enter is saved or uploaded.