Pregnancy Due Date Calculator

Calculate Due Date

Estimated Due Date

May 18, 2027

Current Progress

6 weeks, 0 days

Trimester

First

See how this is calculated

How This Calculation Works

This calculator uses Naegele's rule, the standard method most healthcare providers use for an initial due date estimate: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a typical 28-day cycle with ovulation around day 14, which is why it's an estimate rather than a guarantee.

Pregnancy is conventionally measured in weeks starting from the first day of the LMP — not from the estimated date of conception, which is usually about two weeks later. This is why a pregnancy is described as "40 weeks" even though actual fetal development time from conception is closer to 38 weeks.

The calculator also shows which trimester you're in: the first trimester spans weeks 1-13, the second spans weeks 14-27, and the third spans weeks 28-40. Only about 1 in 20 babies are actually born exactly on their calculated due date — most arrive within a couple of weeks before or after.

Common Mistakes to Avoid

  • Assuming a 28-day cycle when yours is different. Naegele's rule assumes a standard 28-day cycle. If your cycles are consistently longer or shorter, your actual due date may differ from this estimate — an early ultrasound often provides a more precise date.
  • Treating the due date as fixed. A due date is a statistical estimate, not a deadline. Full-term birth is considered anywhere from 37 to 42 weeks, and only a small percentage of babies are born exactly on their calculated date.
  • Confusing gestational age with fetal age. Gestational age (used by this calculator and doctors) is counted from the LMP. Fetal age, counted from conception, is roughly two weeks less — mixing these up can cause confusion when comparing development milestones.
  • Relying solely on an LMP-based estimate later in pregnancy. Early ultrasound dating (typically in the first trimester) is generally considered more accurate than LMP alone, especially for people with irregular cycles — your healthcare provider may adjust your due date based on ultrasound measurements.

Worked Example

Scenario: The first day of the last menstrual period was March 1st.

Step 1 — Apply Naegele's rule: Add 280 days (40 weeks) to March 1st.

Step 2 — Result: The estimated due date is December 5th of the same year.

Step 3 — Track progress: As each week passes from the LMP date, the calculator updates the current gestational week and identifies the trimester automatically.

Frequently Asked Questions

How accurate is a due date calculated from the last period?
It's a reasonable initial estimate, especially for people with regular 28-day cycles, but it has a margin of error of roughly one to two weeks. An early first-trimester ultrasound is generally considered the most accurate way to date a pregnancy, particularly for those with irregular cycles.
Why is pregnancy counted from the last period instead of conception?
The exact date of conception is usually unknown, since it depends on ovulation timing, which can vary. The last menstrual period is a known, reportable date, making it a practical and consistent starting point that healthcare providers have used for generations.
What if my due date changes after an ultrasound?
This is common and normal, especially in early pregnancy. Ultrasound measurements of fetal size can provide a more precise estimate than LMP-based dating alone, and your healthcare provider may adjust your due date based on those measurements — the new estimate is generally considered more reliable.
How many babies are actually born on their due date?
Only about 5% of babies are born on their exact calculated due date. Most births occur within about two weeks before or after, and "full term" is medically defined as anywhere from 37 to 42 weeks of gestation.

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