This pregnancy calculator estimates your due date from the first day of your last menstrual period (LMP) or from the date of conception. It then shows how many weeks and days pregnant you are today, which trimester you are in, how many days are left, and the dates of key milestones such as the end of the first trimester and the start of full term.
The LMP method uses Naegele’s rule, adjusted for your average cycle length, which is the same method most doctors and midwives use at the first appointment. Your care provider may later update the date after an ultrasound scan.
How to use the Due Date Calculator tool
- Choose whether you know the first day of your last period or the conception date.
- Enter the date.
- For the period method, enter your average cycle length (28 days is typical).
- Read your estimated due date and how many weeks pregnant you are today.
- Check your trimester, the estimated conception date and the milestone table.
Why use ToolOrca Due Date Calculator
- Due date by last period or conception date
- Adjusts for cycle lengths from 20 to 45 days
- Current week, day and trimester
- Days remaining with a progress bar
- Milestone dates (trimesters, anatomy scan, full term)
- Private: nothing is saved or sent
How the due date is calculated (Naegele’s rule)
Naegele’s rule adds 280 days (40 weeks) to the first day of your last period. This assumes a 28-day cycle with ovulation on day 14; for longer or shorter cycles the date is moved by the difference, so a 30-day cycle adds 2 days. If you know the conception date, the due date is conception + 266 days (38 weeks). Pregnancy weeks are always counted from the LMP, so at conception you are already about 2 weeks pregnant.
Worked example: if your last period started on 1 January 2026 and your cycle is 28 days, the due date is 1 January + 280 days = Thursday, 8 October 2026. With a 30-day cycle it is Saturday, 10 October 2026. A conception date of 10 March 2026 gives a due date of Tuesday, 1 December 2026.
Trimesters and accuracy
The first trimester runs to the end of week 13, the second from week 14 to week 27 and the third from week 28 until birth. Only about 4% of babies are born on their due date; full term is 39 to 40 weeks and most births happen between 37 and 42 weeks. A dating ultrasound in the first trimester is the most accurate way to confirm the date. This calculator is for information only and is not medical advice.
Frequently asked questions
How is my due date calculated?
By adding 280 days to the first day of your last period (Naegele’s rule), adjusted for your cycle length, or 266 days to the conception date. A last period on 1 January 2026 gives a due date of 8 October 2026.
How many weeks pregnant am I?
Count the weeks from the first day of your last period to today. For example, if your period started 70 days ago you are 10 weeks and 0 days pregnant, in the first trimester.
How long is a pregnancy?
About 40 weeks (280 days) from the first day of the last period, or 38 weeks (266 days) from conception. That is roughly 9 months and 1 week.
Can the due date change?
Yes. If an early ultrasound measures the baby as more than about 5–7 days different from the period-based date, your provider will usually change the due date to the scan date.
When does the second trimester start?
At 14 weeks (day 98 from the last period). With a last period on 1 January 2026, that is 9 April 2026.