Pregnancy Week Calculator
Turns the first day of the last menstrual period into the pregnancy day count, completed weeks, trimester and the expected due date.
Recent
Formula
How the calculation works
The calculator counts the days from the first day of your last menstrual period up to the reference date you supply.
It divides those days by seven and keeps the whole number to give the completed weeks of pregnancy.
It also adds two hundred and eighty days to the last period date, which is the standard estimate for the due date.
Common mistakes
- Counting from the date of conception instead of the last menstrual period, which shifts the weeks by about two.
- Treating the due date as exact, when only a small share of babies arrive on the estimated day itself.
When to use it
- Use it after a positive test to see how many weeks pregnant you are and which trimester you are in.
- Use it to plan antenatal appointments and maternity leave dates around the estimated due date.
Worked example
LMP on 2025-01-01 with 100 days elapsed: day 100, week 14, second trimester and due date 2025-10-08.
Common questions
How is the due date estimated?
The due date is the first day of the last menstrual period plus 280 days, the standard forty-week estimate used by clinicians.
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