Pregnancy Due Date Calculator

How Pregnancy Due Date Calculator Works

This calculator estimates your baby's due date from either the first day of your last menstrual period (LMP) or a known conception date, and also shows your current gestational age and trimester based on today's date.

Formula & Method

From your last menstrual period, the due date is estimated using Naegele's rule: LMP + 280 days, adjusted by the difference between your average cycle length and the standard 28-day cycle (so a 30-day cycle pushes the due date 2 days later). From a known conception date, the due date is simply conception date + 266 days (38 weeks), since conception is assumed to occur roughly 14 days into a standard cycle. Gestational age is measured in completed weeks and days from the effective LMP (or from conception minus 14 days) to today, with trimesters set at under 14 weeks (1st), 14-27 weeks (2nd), and 28 weeks or more (3rd).

Worked Example

For a last menstrual period of January 1, 2026 and a standard 28-day cycle, the estimated due date is October 8, 2026 (280 days after the LMP). If today were August 3, 2026, that pregnancy would show a gestational age of 30 weeks 4 days, placing it in the 3rd trimester, with about 66 days remaining until the due date.

Frequently Asked Questions

Why does cycle length change the due date?

Naegele's rule assumes ovulation and conception happen around day 14 of a standard 28-day cycle. If your average cycle is longer or shorter than 28 days, ovulation likely happened correspondingly later or earlier, so this calculator shifts the due date by the same number of days to better match your actual cycle.

Why is the conception-date method different from the LMP method?

When you enter a conception date directly, the calculator already knows when conception happened, so it simply adds the standard 266-day (38-week) gestation length from conception. For gestational-age purposes it also estimates an "effective LMP" as 14 days before conception, matching standard obstetric dating conventions.

How accurate is this due date estimate?

It's a standard statistical estimate -- most babies are born within about two weeks of it, but very few arrive on the exact calculated date. An ultrasound-based estimate from your healthcare provider, especially from early pregnancy, is generally considered more accurate than a calculation from dates alone.

What are the trimester cutoffs used here?

This calculator marks under 14 weeks as the 1st trimester, 14 up to 28 weeks as the 2nd trimester, and 28 weeks onward as the 3rd trimester -- a common convention, though some healthcare providers use slightly different week ranges.