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Pregnancy Due Date Calculator

Find out when your baby is due based on the first day of your last menstrual period. This calculator uses Naegele's Rule, adjusted for your cycle length, to estimate your due date, current week of pregnancy, trimester, and likely conception date.

The year your last menstrual period started.

The month your last menstrual period started (1-12).

The day of the month your last menstrual period started.

Your average menstrual cycle length in days. The standard is 28 days.

This calculator applies Naegele's Rule to estimate your expected delivery date. It adds 280 days (40 weeks) to the first day of your last menstrual period and adjusts for cycle lengths that differ from the standard 28 days. You will also see your current gestational week, trimester, days remaining, and estimated conception date.

How It Works

Naegele's Rule

Due Date = LMP + 280 days + (Cycle Length - 28)

The due date is calculated by adding 280 days to the first day of your last menstrual period, then adjusting for cycle lengths that differ from the standard 28 days.

Start with the first day of your last menstrual period (LMP)

Add 280 days (40 weeks) to get the standard due date

If your cycle is longer or shorter than 28 days, add or subtract the difference

Conception is estimated at LMP + 14 days (adjusted for cycle length)

Current gestational age is calculated from LMP to today

Important Notes:

  • •Naegele's Rule is the standard method used by healthcare providers worldwide
  • •Only about 5% of babies are born on their exact due date
  • •A normal full-term pregnancy ranges from 37 to 42 weeks
  • •Cycle length adjustment improves accuracy for women with irregular cycles

Worked Example

A woman whose last menstrual period started on January 1, 2026, with a standard 28-day cycle.

Inputs:

  • last Period Year:2,026
  • last Period Month:1
  • last Period Day:1
  • cycle Length:28

Result:

Her estimated due date is October 8, 2026. The estimated conception date is around January 15, 2026. At the time of calculation, her current gestational week and trimester will depend on today's date.

Who Is This Calculator For?

  • expectant parents
  • women planning pregnancy
  • healthcare providers seeking a quick estimate

Frequently Asked Questions

Due date calculators based on Naegele's Rule are a reliable starting estimate, but only about 5% of babies are born on the exact predicted date. Most births occur within a two-week window around the due date. An early ultrasound between 8 and 12 weeks can refine the estimate further.
The first trimester spans weeks 1 through 12, the second trimester covers weeks 13 through 26, and the third trimester runs from week 27 until delivery (typically around week 40). Each trimester is associated with different developmental milestones for the baby and physical changes for the mother.
Yes. Naegele's Rule assumes a standard 28-day cycle. If your cycle is longer, ovulation likely occurred later, which pushes the due date forward. If your cycle is shorter, ovulation may have happened earlier, moving the due date back. This calculator adjusts automatically based on the cycle length you enter.
If you are unsure of your last menstrual period date, your healthcare provider can estimate your due date with an early ultrasound. Ultrasound measurements of the embryo between 8 and 12 weeks are considered the most accurate way to determine gestational age when the LMP is unknown.
Yes. Your healthcare provider may adjust your due date after an ultrasound, especially if the ultrasound-based estimate differs from the LMP-based date by more than a week in the first trimester or more than two weeks in the second trimester. The adjusted date becomes your new official due date.
Gestational age is counted from the first day of your last menstrual period, which is about two weeks before conception actually occurs. Fetal age (also called embryonic age) is counted from the estimated date of conception. So the fetal age is typically about two weeks less than the gestational age.

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Last updated: April 20, 2026