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Pregnancy Due Date Calculator

Find your estimated due date from your last period, conception date or IVF transfer, and see how many weeks pregnant you are, your trimester and the key dates ahead.

Defaults to your region. Switch to compare any other scale.

Trimesters: 0–13+6, 14–27+6 and 28+ weeks. Maternity leave dates are not shown because the rules differ by country — pick your country to see them.

Calculate from
days

28 days if you are not sure. Each extra day moves the due date one day later.

Result

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Estimated due date

Enter a date to see your due date.

    About this tool

    How it works: doctors count a pregnancy from the first day of the last period, about two weeks before conception, so a typical pregnancy lasts 280 days (40 weeks). That is Naegele's rule. If your cycle is longer or shorter than 28 days, ovulation shifts by the same number of days, and so does the due date. From a known conception date the calculator adds 266 days; after IVF it adds 261 days for a day-5 embryo transfer and 263 days for a day-3 transfer, as recommended by ACOG.

    Local conventions: the Standard menu changes how the result is shown. The UK (NHS) ends the first trimester with week 12, while ACOG, WHO-based guidance and Japan use 13 weeks 6 days. Germany writes 12+3 and calls it the 13th SSW. Japan also counts 4-week pregnancy months. Pick your country to see the date your maternity leave starts, or the earliest date it can start, under local law.

    Useful next steps: track dates with the date difference calculator, check your energy needs with the calorie calculator, or see your pre-pregnancy BMI.

    Formula

    • Due date (period) = first day of last period + 280 days + (cycle length − 28)
    • Due date (conception) = conception date + 266 days
    • Due date (IVF) = transfer date + 261 days (day-5 embryo) or + 263 days (day-3 embryo)
    • Weeks pregnant = today − (due date − 280 days)
    • Trimesters (ACOG/WHO): 0–13+6 · 14–27+6 · 28+ weeks

    How it is calculated

    1. Choose what you know: your last period, the conception date or the IVF transfer date.
    2. Enter the date. For the period method, adjust the cycle length if yours is not 28 days.
    3. Read the due date, how far along you are today, the trimester and the countdown.
    4. Check the milestones table for key dates such as the anatomy scan window and full term.
    5. Pick your country in the Standard menu to see local week notation and maternity leave dates.

    Questions

    How is the due date calculated?

    With Naegele's rule: take the first day of your last period and add 280 days (40 weeks). If your cycle is not 28 days, add or subtract the difference, because ovulation happens later in longer cycles. From a known conception date add 266 days, and after IVF add 261 days for a day-5 transfer or 263 days for a day-3 transfer.

    Why is pregnancy counted from the last period, not from conception?

    Most people know when their last period started, but not the exact day they ovulated. Counting from the period gives everyone the same reference point. That is why at conception you are already counted as about 2 weeks pregnant, and a 40-week pregnancy is really about 38 weeks after conception.

    How accurate is the due date, and can it change after an ultrasound?

    It is an estimate: most babies arrive between 37 and 42 weeks. An early ultrasound measuring the baby's length is the most accurate way to date a pregnancy. ACOG advises changing the due date when the scan differs from the period-based date by more than 5 days before 9 weeks, or by more than 7 days between 9+0 and 13+6 weeks. Once your doctor or midwife sets the date, use that one.

    When does each trimester start, and why do sources disagree?

    ACOG and most WHO-based guidance use: 1st trimester up to 13 weeks 6 days, 2nd from 14+0 to 27+6, 3rd from 28+0. The NHS in the UK ends the 1st trimester with week 12 and starts the 2nd at week 13. Germany usually writes 12+3, meaning 12 completed weeks and 3 days, which is the 13th SSW. The Standard menu switches between these conventions; the due date stays the same.

    How are the maternity leave dates worked out?

    They are counted back from the due date using the rule of the country chosen in the Standard menu, for example 6 weeks (42 days) in Germany, 56 or 70 days in Latvia, 2 months in Italy and from the 11th week before the expected week of childbirth in the UK. Some countries set a fixed start; others only set the earliest possible start. The legal start is always based on the due date on your medical certificate, and employer or insurer rules can add conditions, so treat the result as a guide.

    What if my cycle is irregular or I don't remember my last period?

    Enter your average cycle length (20–45 days) and the calculator shifts the date accordingly. If your cycles vary a lot, you recently stopped hormonal contraception or you don't know the date, the period-based estimate can be off by a week or more. In that case use the conception or IVF method if you know that date, and rely on the early ultrasound for the final due date.

    Sources

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