Due Date Calculator
Estimate your pregnancy due date, see how many weeks pregnant you are, and which trimester you're in. Choose whether you're counting from your last period or your conception/ovulation date, and adjust your cycle length if it isn't 28 days. Updates instantly.
This is an estimate for general information only — not medical advice. Always confirm your due date with your midwife or doctor, usually via an early ultrasound.
How it works
due date = last period + 280 days + (cycle length − 28)
- From last period (LMP): adds 280 days (40 weeks), the standard Naegele's rule, then adjusts for any difference between your cycle length and 28 days.
- From conception: adds 266 days (38 weeks), since conception is about two weeks after the last period.
- Weeks pregnant is the time elapsed since your last period (or conception date + 14 days), shown as weeks + days.
- Trimester: 1st = weeks 1–12, 2nd = weeks 13–26, 3rd = week 27 onward.
FAQ
How is a due date calculated?
The most common method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a regular 28-day cycle with ovulation around day 14. If you know your conception or ovulation date instead, the due date is about 266 days (38 weeks) after it. This calculator does either and adjusts for cycle length.
How accurate is a due date calculator?
A calculated due date is an estimate, not a guarantee — only about 1 in 20 babies arrive on the exact date. Most healthy births happen within two weeks either side. The LMP method is less accurate if your cycles are irregular, which is why an early ultrasound is used by clinicians to confirm or adjust the date.
How many weeks is a full-term pregnancy?
Pregnancy is counted as 40 weeks from the first day of your last period, even though conception happens about two weeks into that. "Full term" is 39–40 weeks; 37–38 weeks is "early term" and 41–42 weeks is "late term".
What if my cycle is not 28 days?
If your cycle is longer or shorter than 28 days, ovulation shifts, so the estimate shifts too. Enter your average cycle length and the calculator adjusts the due date accordingly. For irregular cycles, treat the result as a rough guide and confirm with an early scan.
When does each trimester start?
The first trimester is weeks 1–12, the second is weeks 13–26, and the third is weeks 27 to birth. The calculator shows which trimester you're currently in based on weeks elapsed.
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More tools: ovulation calculator and the full tools list.