Pregnancy Due Date Calculator
Estimate your due date, how many weeks pregnant you are and key trimester dates from your last period.
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Quick Tip
Enter the first day of your last menstrual period, not the day it ended.
About This Tool
Why use Pregnancy Due Date Calculator?
This due date calculator uses Naegele's rule — the method clinicians use — adjusted for your cycle length. It adds 280 days to the first day of your last menstrual period, then shifts the result by the difference between your cycle and a standard 28 days.
Example: a last period starting 1 March 2026 with a 28-day cycle gives a due date of 6 December 2026, with conception estimated around 15 March. The first trimester ends on 31 May and the second on 6 September. With a 32-day cycle, everything shifts four days later, putting the due date at 10 December.
Key Benefits
Adjusted for your cycle
Standard due date calculations assume a 28-day cycle. Longer cycles mean later ovulation and a later due date, which this accounts for.
Gestational age in weeks and days
Shows how far along you are in the format maternity care actually uses, such as 14 weeks and 3 days.
Trimester milestones
Marks the end of the first and second trimesters, which is when many scans and tests are scheduled.
How to use Pregnancy Due Date Calculator
- 1
Enter the first day of your last menstrual period, not the day it ended.
- 2
Enter your average cycle length so the estimate is adjusted for longer or shorter cycles.
- 3
Review your due date, current gestational age in weeks and days, and trimester dates.
- 4
Take the date to your first scan, which will confirm or revise it.
Best use cases
Estimate your due date before your first midwife or doctor appointment.
Work out how many weeks pregnant you are today.
Plan maternity leave around trimester dates.
Understand why a dating scan moved your due date.
How this tool compares
Pregnancy Due Date Calculator FAQs
How is a due date calculated?
Naegele's rule adds 280 days — 40 weeks — to the first day of your last menstrual period. This counts pregnancy from your last period rather than from conception, which is why you are considered about two weeks pregnant at the point of conception.
How accurate is a due date?
It is an estimate, not a deadline. Only around 4% of babies arrive on their due date, and most births happen within two weeks either side. A dating scan in the first trimester is the most accurate method and may shift the date by several days.
Why does my cycle length change the due date?
Because the 280-day rule assumes you ovulated on day 14. If your cycle is 32 days, you probably ovulated around day 18, so conception happened four days later and your due date moves four days later too.
When do the trimesters start and end?
The first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until birth. Boundaries vary slightly between sources, and some count the second trimester from week 13.
Can this replace a dating scan?
No. This gives a reliable working estimate from your dates, but an early ultrasound measures the baby directly and is more accurate, particularly if your cycles are irregular or you are unsure of your last period. Always follow the date your care provider gives you.
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