Due Date Calculator
Calculate your estimated due date (EDD) and pregnancy milestones from your last menstrual period (LMP).
Did this tool work for you?
How to use this calculator
Add 280 days to the first day of your last menstrual period. This assumes a 28-day cycle.
- 1
Enter the first day of your last menstrual period (LMP).
- 2
Select your average menstrual cycle length.
- 3
The calculator shows your estimated due date, current pregnancy week, and key milestones.
Frequently asked questions
Exactly how is the due date calculated?
The code adds 280 days to the first day of your last period, then adds the difference between your chosen cycle length and 28 days. A 35-day cycle adds 7 days to the base; a 21-day cycle subtracts 7. For an LMP of 10 January 2026 that gives 17 October 2026 with a 28-day cycle.
How is the estimated conception date derived?
It is the LMP plus 14 days, adjusted by the same cycle-length difference. For a 28-day cycle that is day 14, and for a 35-day cycle it is day 21. It is an arithmetic estimate based on an assumed ovulation day, not a detection of when conception actually happened.
Why do “Currently” and “Days Until Due Date” change when I reopen the page?
They are measured against your device’s clock at the moment you calculate. The due date, conception estimate and trimester end dates depend only on the date and cycle length you pick, so they stay the same. “Days Until” stops at zero once the due date has passed.
How are the trimester dates and labels set?
The first trimester ends 13 weeks (91 days) after the LMP and the second ends at 27 weeks (189 days). These two dates use the LMP only and do not move with cycle length. The label compares your completed weeks with those cut-offs: under 13 is first, under 27 is second, otherwise third.
What if I enter a date in the future or leave it blank?
A blank or invalid date returns an error. A future LMP still produces dates, but the Currently line says the LMP is in the future and the trimester shows a dash. The default is today’s date, which is only a placeholder you should replace.
Can it use an ultrasound date or a known conception date?
No. The only inputs are the LMP and one of seven cycle lengths, from 21 to 35 days. Dating changed by scans, IVF transfers or irregular cycles needs a clinician’s calculation, and their date takes priority over this one.
Is the due date when my baby will be born?
It is a calculated reference point, not a prediction. Births happen before and after it, and the result should be treated as general information rather than medical advice. Your midwife or doctor sets the official date.
Due date from your last period — the date arithmetic explained
From LMP to estimated due date
The calculator treats the first day of your last menstrual period as day zero and counts forward 280 days, which is 40 weeks. If your cycle is shorter or longer than 28 days it shifts the whole timeline by the difference, on the assumption that ovulation moves with the cycle. It then reports the due date, an estimated conception date, the two trimester boundaries, and, using your device’s clock, the weeks and days you are into the pregnancy.
How cycle length changes the dates
Dates below were produced by the calculator for an LMP of 10 January 2026.
| Cycle length | Estimated due date | Estimated conception |
|---|---|---|
| 21 days | October 10, 2026 | January 17, 2026 |
| 28 days | October 17, 2026 | January 24, 2026 |
| 35 days | October 24, 2026 | January 31, 2026 |
A second worked example with milestones
For an LMP of 1 March 2027 and a 28-day cycle, the calculator gives a due date of December 6, 2027 and a conception estimate of March 15, 2027. The first trimester ends May 31, 2027 and the second ends September 6, 2027. Notice that the due date is 280 days after the LMP, so a pregnancy dated this way counts about two weeks before conception, when the egg has not yet been fertilised.
What this page assumes
The method assumes regular cycles and that the date you remember is the true first day of bleeding. If your cycles vary widely, or you are unsure, the output is only as good as that memory. The trimester boundaries are fixed week counts rather than anything personal, so they will not match a calendar your clinic builds from a scan. Nothing here accounts for twins, previous pregnancies or medical history, and the cycle-length choice only offers seven fixed values between 21 and 35 days.
Treat the date as a starting point
Clinics commonly confirm or revise dates with an early ultrasound, and when that happens their date is the one to use. Use this calculator to get oriented, plan appointments and see roughly where each trimester begins and ends. It is general information and not medical advice, and decisions about care should be made with your doctor or midwife.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →