Due Date Calculator
Enter the first day of your last period – the calculator estimates your due date with Naegele’s rule and shows the current week of pregnancy, the trimester and the days left. If you know the date of conception, you can use that instead. The result is an estimate and does not replace advice from your doctor or midwife. Example: last period on 1 March 2026 → due date Sunday, 6 December 2026.
Source: ACOG Committee Opinion No. 700 – Methods for Estimating the Due Date. Updated: .
How it is calculated
How the due date is calculated
The estimated due date (EDD) follows Naegele’s rule: first day of the last menstrual period (LMP) plus 280 days (40 weeks). The rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts – so the calculator adjusts the date by cycle length − 28 days. If you know the date of conception, it uses conception + 266 days (38 weeks) instead.
How to calculate your due date
- Choose whether to calculate from the first day of your last period or from conception.
- Enter the date and, if needed, your cycle length.
- Read the estimated due date, current pregnancy week, trimester and days to go.
Example: last period started on 10 March 2026, 28-day cycle → 10 March 2026 + 280 days = Tuesday, 15 December 2026. With a 32-day cycle, add 4 days: 19 December 2026. On 27 September 2026 the pregnancy is at 28 weeks 5 days (201 days) – third trimester, 79 days to go.
Due dates by first day of the last period
| First day of last period | Due date (28-day cycle) |
|---|---|
| 1 January 2026 | Thursday, 8 October 2026 |
| 1 February 2026 | Sunday, 8 November 2026 |
| 1 March 2026 | Sunday, 6 December 2026 |
| 1 April 2026 | Wednesday, 6 January 2027 |
| 1 May 2026 | Friday, 5 February 2027 |
| 1 June 2026 | Monday, 8 March 2027 |
| 1 July 2026 | Wednesday, 7 April 2027 |
| 1 August 2026 | Saturday, 8 May 2027 |
| 1 September 2026 | Tuesday, 8 June 2027 |
| 1 October 2026 | Thursday, 8 July 2027 |
| 1 November 2026 | Sunday, 8 August 2027 |
| 1 December 2026 | Tuesday, 7 September 2027 |
Each due date is the first day of the last period plus 280 days. Because the months have different lengths, the result moves between the 5th and the 8th of the month nine months later. With a longer cycle, add the difference to 28 days: a 32-day cycle and a last period on 1 March 2026 give 10 December 2026.
What does “28 weeks 5 days” mean?
Gestational age is counted from the first day of the last period, even though conception happens about two weeks later. “28 weeks 5 days” (often written 28+5) means 28 full weeks and 5 days have passed – you are in week 29. The first trimester runs to 13 weeks 6 days, the second to 27 weeks 6 days, and the third starts at 28 weeks 0 days (ACOG). See the pregnancy week calculator for more.
How accurate is the due date?
Only about 1 in 25 babies arrives on the estimated day. A birth between 37 weeks 0 days and 41 weeks 6 days is considered full term, and the calculator shows that window. An ultrasound in the first trimester is the most accurate method; if it differs clearly from the LMP date, your clinician will set the due date by the scan (ACOG Committee Opinion 700).
Important: this calculator gives an arithmetic estimate, not a diagnosis. It does not replace medical advice or antenatal care. If you have questions, symptoms or an irregular cycle, talk to your doctor or midwife.
Ovulation and your cycle
When ovulation happened can only be estimated from a calendar. For cycle planning, use the ovulation calculator – which is not a method of contraception either.
More calculators: the pregnancy week calculator, the ovulation calculator and a countdown to the big day.
Frequently asked questions
How do I calculate my due date?
Take the first day of your last period and add 280 days (Naegele’s rule). If your cycle is not 28 days, add or subtract the difference. Example: 10 March 2026 + 280 days = 15 December 2026.
My last period was on 1 March 2026 – when is my due date?
Sunday, 6 December 2026, assuming a 28-day cycle: 1 March plus 280 days. With a 32-day cycle it moves 4 days later, to 10 December 2026.
Can I calculate the due date from conception?
Yes: date of conception plus 266 days (38 weeks). Because conception is rarely known exactly, clinicians usually work from the last period and the ultrasound scan.
Why is pregnancy counted from the last period?
The first day of the last period is a date you know; ovulation usually is not. So counting starts there – the first two “weeks of pregnancy” are before conception.
How many babies are born on their due date?
Only a small share, roughly 1 in 25. Most babies arrive in the weeks around it; birth between 37 weeks 0 days and 41 weeks 6 days counts as full term.
Does this replace my doctor’s due date?
No. The calculator is an estimate, not a diagnosis, and does not replace medical advice. The due date your doctor or midwife sets is the one that counts.
Sources and legal basis
- ACOG Committee Opinion No. 700 – Methods for Estimating the Due Date
- ACOG Committee Opinion No. 579 – Definition of Term Pregnancy
- NHS – Pregnancy due date calculator
- NICE guideline NG201 – Antenatal care
As of: · Editorial policy
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