Pregnancy due date calculator
Enter the first day of your last period, or a conception date, to see your estimated due date, how far along you are today and which trimester you are in.
How it works
A pregnancy is counted from the first day of your last menstrual period, not from conception, because that date is easier to know. The tool follows the standard rule and adds 280 days, which is 40 weeks, to give the estimated due date. If you would rather work from a known conception date, it adds 266 days instead, since fertilisation happens roughly two weeks after a period starts. It then compares today with that timeline to tell you how many weeks and days along you are, and how many days remain.
The word estimate matters. Only about one in twenty babies arrives on the due date itself, and a normal birth can happen at any point in the weeks around it. The last-period method also assumes an average cycle with ovulation near day 14, so if your cycles run long or short the date can be out by several days. A dating scan between 8 and 14 weeks measures the baby directly and is the figure your maternity team will use, so it may move this estimate.
Worked example
Suppose the first day of your last period was 1 June 2026. Adding 280 days lands on Monday, 8 March 2027. Measured against early September, that puts you around 13 weeks along, in the second trimester, with roughly 183 days to go. Switching to the conception-date method would shift the start point about two weeks later and move the due date accordingly.
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.