Pregnancy due date calculator
This calculator estimates a pregnancy due date from the first day of the last menstrual period, or from a conception date, following the Naegele rule of adding 280 days from the last period. It also shows how many weeks pregnant someone is today and which trimester that falls in. The result is an estimate, not a prediction.
How is a due date calculated?
A due date is calculated by adding 280 days to the first day of the last menstrual period, under the Naegele rule.
Pregnancy is counted from that date rather than from conception, because the start of a period is easier to know than the date of fertilisation.
If a conception date is used instead, the calculator adds 266 days, since fertilisation happens roughly two weeks after a period starts.
| Trimester | Weeks |
|---|---|
| First | Up to week 12 |
| Second | Week 13 to week 26 |
| Third | Week 27 to birth |
The calculator compares today's date with that timeline to show how many weeks and days have passed, and how many days remain.
Only about one in twenty babies arrives on the estimated due date itself, so the result is an estimate rather than a prediction.
A dating scan between 8 and 14 weeks measures the baby directly and is the figure a maternity team will use, so it may move this estimate.
What would this look like for an example last period date?
Take a last period date of 1 June 2026.
Adding 280 days under the Naegele rule gives an estimated due date of Monday, 8 March 2027.
Measured against today's date, that is about 13 weeks along, in the second trimester, with roughly 183 days to go.
Using the conception-date method instead would move the starting point back roughly two weeks and shift the due date accordingly.
This is an estimate rather than a prediction, and a midwife or dating scan gives the figure a maternity team will use.
Common questions
How is a due date calculated?
The standard method adds 280 days, about 40 weeks, to the first day of your last menstrual period. If you know the conception date instead, the tool adds 266 days. Both assume a roughly average cycle, so the result is an estimate rather than a fixed date.
How accurate is a due date?
Only about one baby in twenty arrives on the estimated date. Most births happen in the two weeks either side of it, so treat it as the middle of a window. A dating scan, usually between 8 and 14 weeks, gives the most reliable estimate and may adjust the date.
What if my cycle is not 28 days?
The last-period method assumes ovulation around day 14. If your cycles are much longer or shorter, ovulation shifts, so the date from your period can be out by several days. The conception-date option, or a dating scan, is more accurate in that case.
When does each trimester start?
Counting from the first day of your last period, the first trimester runs to the end of week 12, the second from week 13 to week 26, and the third from week 27 to birth. The tool shows which trimester today falls in.
Does this replace a midwife or scan?
No. This is a quick estimate to help you plan. Your midwife and dating scan confirm the date and check that the pregnancy is progressing well, so book in with your GP or midwife as early as you can.
What would this look like for an example last period date?
Starting from a last period date of 1 June 2026, adding 280 days under the Naegele rule gives an estimated due date of Monday, 8 March 2027. Measured against today's date, that is about 13 weeks along, in the second trimester, with roughly 183 days to go. Using the conception-date method instead would move the starting point back roughly two weeks and shift the due date. This is an estimate rather than a prediction.
Sources & Rate Verification
Rates verified against official statutory guidance on 6 September 2026.