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A last period on 15 March 2026
Enter year 2026, March, day 15, and a 28-day cycle. ACOG’s 280-day count lands on 20 December 2026. No extra days are added because the cycle already matches the 28-day assumption.
Day of last period: 25% lower
Change day of last period to 11 while keeping the other example values the same.
Day of last period: 25% higher
Change day of last period to 19 while keeping the other example values the same.
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Quick answer
What does the Pregnancy due date calculate?
What due date is 280 days after the first day of your last period? This calculator uses year of last period, month of last period, day of last period, and usual cycle length to estimate estimated due date immediately in your browser.
With the values currently entered, the result is Dec 20, 2026 — ACOG 280-day estimated due date. It also shows last period used, days after lmp, cycle adjustment from 28 days, and next period on this cycle.
How to use the Pregnancy due date
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Year of last period
- Month of last period
- Day of last period
- Usual cycle length — entered in days
Pregnancy due date formula
EDD = first day of LMP + 280 days + (cycle length − 28). ACOG’s 280-day convention assumes a 28-day cycle.
Assumptions
- ACOG Committee Opinion 700 dates the estimated due date as 280 days after the first day of the last menstrual period. That convention assumes a regular 28-day cycle with ovulation on day 14.
- If the usual cycle is not 28 days, the calculator adds cycle length minus 28 so the luteal phase stays 14 days. A first-trimester ultrasound can change the date. This is not prenatal care.
Verify the inputs
Authoritative sources
These sources explain the definitions, factors, or rules behind this tool. Their geographic scope is shown because an official source for one country is not automatically valid somewhere else.
Sources do not endorse Calculum. Check the source date, scope, and your own documents before making a financial, tax, insurance, or reporting decision.
Practical guide
Pregnancy due date example and edge cases
ACOG dates an estimated due date as 280 days after the first day of the last period. That convention assumes a regular 28-day cycle.
Example: A last period on 15 March 2026
Enter year 2026, March, day 15, and a 28-day cycle. ACOG’s 280-day count lands on 20 December 2026. No extra days are added because the cycle already matches the 28-day assumption.
- Year of last period
- 2,026
- Month of last period
- March
- Day of last period
- 15
- Usual cycle length
- 28 days
Calculated resultDec 20, 2026ACOG 280-day estimated due date
If the usual cycle is 32 days, add 4 days to that date. The ovulation calculator uses the same last-period fields when you want the fertile window instead of the due date.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- The main result is a calendar date, not a promise. Most births do not land on the estimated day.
- ACOG says first-trimester ultrasound is more accurate than last period alone. Use this number to start planning, then confirm it in care.
Edge cases worth checking
Cycles are irregular
Keep the real average if you have one. A wildly changing cycle makes the 280-day count a weak starting point. That is when ultrasound dating matters more.
The day of the month does not exist
31 February is not a date. The calculator asks for a valid last-period day before it adds 280.
What changes the result most
Year of last period
Keep year of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Month of last period
Keep month of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Day of last period
Keep day of last period on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Day of last period: 25% lower
11Dec 16, 2026ACOG 280-day estimated due date
Day of last period: 25% higher
19Dec 24, 2026ACOG 280-day estimated due date
Year of last period: 25% higher
2,100Dec 20, 2100ACOG 280-day estimated due date
Common mistakes
Check year of last period
ACOG Committee Opinion 700 dates the estimated due date as 280 days after the first day of the last menstrual period. That convention assumes a regular 28-day cycle with ovulation on day 14. Make sure this matches the number you enter.
Keep usual cycle length consistent
If the usual cycle is not 28 days, the calculator adds cycle length minus 28 so the luteal phase stays 14 days. A first-trimester ultrasound can change the date. This is not prenatal care. Use the same units and time period throughout the calculation.
Do not rely on one pregnancy due date scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
What due date is 280 days after the first day of your last period?
Use it to set a starting budget, then confirm real quotes and each person’s needs.