Fitness & Health

Pregnancy Due Date Calculator

Estimate your due date, week and trimester.

Last reviewed July 16, 2026 · Written & reviewed by the Calculator Kong team — methodology & sources

First day of last period

Cycle length

days

Last period:

Estimated due date

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You are

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Trimester

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Naegele's rule

Due date = first day of last period + 280 days (40 weeks), ± (cycle length − 28)

"When is my baby due?" This pregnancy due date calculator estimates your due date, how many weeks pregnant you are, and your current trimester — from just the first day of your last menstrual period (LMP) and your typical cycle length.

How your due date is estimated

The standard method, Naegele's rule, counts 280 days (40 weeks) forward from the first day of your last period. It assumes a 28-day cycle with ovulation around day 14 — so if your cycles run longer or shorter, the calculator shifts the date by the difference, since ovulation (and therefore conception) moves with cycle length.

Worked example

If your last period started on March 1 with a 28-day cycle, the estimated due date is December 6 (March 1 + 280 days). With a 33-day cycle, ovulation runs about 5 days later, nudging the estimate to December 11.

How many weeks pregnant am I?

Pregnancy is dated from the LMP, not conception — so at "6 weeks pregnant" the embryo is about 4 weeks old. The calculator shows your exact week and day, plus the trimester: first (weeks 1–13), second (14–27) and third (28–40).

How accurate are due dates?

Treat the date as the middle of a window, not an appointment: only about 4% of babies arrive on their exact due date, while most come within a week either side, and full term spans weeks 37–42. A first-trimester dating ultrasound is the most precise method and may adjust the LMP-based estimate — your provider will use whichever is more reliable.

This is an estimate for general information only and is not medical advice. Please confirm dates and care decisions with your healthcare provider.

Frequently Asked Questions

How is a due date calculated? +

By Naegele's rule: the due date is 280 days (40 weeks) from the first day of your last menstrual period (LMP), adjusted for your cycle length since ovulation shifts with longer or shorter cycles.

How many weeks pregnant am I? +

Pregnancy is counted from the first day of your last period, not from conception. Enter your LMP and the calculator shows your exact week and day, plus your current trimester.

How accurate is a due date? +

It is an estimate — only about 4% of babies arrive on the exact date, and most come within a week either side. Full term spans weeks 37–42, and a dating ultrasound is the most precise method.

What if my cycle is not 28 days? +

Adjust the cycle length and the calculator shifts the due date by the difference from 28 days. A 33-day cycle typically means ovulation about 5 days later, so the due date moves 5 days later too.

What are the trimesters? +

The first trimester is weeks 1–13, the second weeks 14–27, and the third weeks 28 to birth. The calculator shows which one you are in today.

Due date from conception vs last period — what's the difference? +

Clinicians date pregnancy from the LMP because it is a known date; conception typically happens about two weeks later. From conception, a pregnancy is about 38 weeks; from the LMP, 40.