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Pregnancy Due Date Calculator

Estimate your due date from the first day of your last period or from conception, with gestational age and trimester.

Enter the values to see the result here.

For general information, not medical advice. Talk to a doctor before making health decisions.

Formula

From the last period, with cycle length cc days:

due date=LMP+280+(c−28) days\text{due date} = \text{LMP} + 280 + (c - 28)\ \text{days}

From the conception date:

due date=conception+266 days\text{due date} = \text{conception} + 266\ \text{days}

Gestational age is the days since the (adjusted) LMP, split into weeks and a remainder: weeks=⌊n/7⌋\text{weeks} = \lfloor n / 7 \rfloor, days=n mod 7\text{days} = n \bmod 7. Conception is estimated at LMP + 14 + (c − 28).

Worked example

First day of last period: 1 January 2025, cycle 28 days, as of 5 March 2025.

  • Due date: 1 January + 280 days = 8 October 2025.
  • Days since LMP: 31 + 28 + 4 = 63, so 9 weeks 0 days, still the first trimester.
  • Days left: 217; progress 63/280=22.5%63 / 280 = 22.5\%.
  • Estimated conception: 15 January 2025.

With a 32-day cycle the due date moves four days later, to 12 October 2025.

Common mistakes

  • Counting 9 months from the LMP. Nine calendar months is about 274 days, so it differs from 280 days by roughly a week.
  • Using the date of a positive test as the LMP.
  • Ignoring cycle length when it is clearly far from 28 days.
  • Treating the due date as a deadline. A pregnancy of 37 to 42 weeks is full term.

Frequently asked questions

What is Naegele's rule?

It dates a pregnancy from the first day of the last menstrual period (LMP) and adds 280 days, or 40 weeks. It assumes a 28-day cycle with ovulation on day 14, which is why the tool adjusts for longer or shorter cycles.

Why does pregnancy count from the last period and not conception?

Conception is rarely known exactly, but the first day of a period usually is. That makes the LMP the standard starting point, even though about two weeks of the 40 pass before conception.

How reliable is the date?

Only about 4 to 5 in 100 babies are born on the due date. Early scans (before about 13 weeks) measure the baby and can move the date by a few days if it differs from the LMP date by more than a week.

What do the trimesters cover?

The first trimester is weeks 1 to 12, the second weeks 13 to 26, and the third from week 27 until birth.

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