How accurate is the estimated due date?

Naegele's rule gives a rough target — only about 5% of babies are born on the exact predicted day. About 80% of births fall within two weeks either side. An early ultrasound (especially in the first trimester) gives a more accurate estimate than LMP-based dating.

How does the cycle-length adjustment work?

The 280-day rule assumes a 28-day cycle with ovulation around day 14. If your cycle is longer, ovulation happens later and the due date shifts later by the same number of days; a shorter cycle shifts it earlier. The calculator adds (cycle length − 28) days to the standard 280-day estimate.

What is Naegele's rule?

Naegele's rule estimates the expected delivery date as the first day of the last menstrual period plus 280 days (40 weeks). It assumes a regular 28-day cycle and ovulation on day 14, which is why a cycle adjustment is offered for non-standard cycles.

How are trimesters defined here?

Using the common clinical convention measured from LMP: first trimester covers weeks 1–13, second trimester covers weeks 14–27, and third trimester runs from week 28 to week 40. After week 40 the calculator reports "post-term".

Why does it ask for the first day of the last period and not the conception date?

The exact day of conception is rarely known, but the first day of the last menstrual period is usually easy to recall and is the standard clinical anchor for pregnancy dating.

Can I use this if my cycles are irregular?

The estimate is least reliable when cycles are irregular, since ovulation timing varies. If you have irregular cycles, an early ultrasound is the most accurate way to date the pregnancy.

HEAHealth & Fitness

Pregnancy Due Date Calculator

Estimate your pregnancy due date from the first day of your last menstrual period using Naegele's rule. Adjust for a non-28-day cycle to see your expected due date, current trimester, weeks pregnant, and days remaining.

First day of last period

Average cycle length(optional)

days

Try an example

Enter values above to see results

How It Works

Formula

\text{dueDate} = \text{LMP} + 280\,\text{d} + (\text{cycleLength} - 28)\,\text{d}

\text{conceptionDate} \approx \text{LMP} + 14\,\text{d} + (\text{cycleLength} - 28)\,\text{d}

\text{daysPregnant} = \text{today} - \text{LMP}

\text{weeksPregnant} = \left\lfloor \frac{\text{daysPregnant}}{7} \right\rfloor

Where

$\text{LMP}$: First day of the last menstrual period
$\text{cycleLength}$: Average length of the menstrual cycle(days)
$\text{dueDate}$: Estimated date of delivery
$\text{conceptionDate}$: Approximate date of conception (LMP + ~14 days, cycle-adjusted)
$\text{daysPregnant}$: Whole days elapsed since the LMP(days)
$\text{weeksPregnant}$: Completed weeks of pregnancy(weeks)

Enter the first day of your last menstrual period (LMP). The calculator adds 280 days (40 weeks) to estimate the due date — Naegele's rule. If your cycle is shorter or longer than 28 days, the due date shifts by the same number of days, because ovulation usually happens about 14 days before the next expected period. Today's date is compared against the LMP and the due date to show your current trimester, weeks pregnant, and days remaining.

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