My cycles are not 28 days; does this still work?

Partly. Each extra day of typical cycle length shifts ovulation (and effectively the true due date) about a day later. Mention cycle length to your provider for adjustment.

What if I know my conception date instead?

Add 266 days to conception instead. IVF transfer dates allow precise calculation; your clinic provides the adjusted due date.

Why does the calculator reject future dates?

Gestational age counts from an event that has happened. A future LMP means a typo or, rarely, a very early pregnancy where waiting a few days gives a valid date.

Health

Pregnancy Due Date Calculator

What this does

Estimate your due date from the first day of your last menstrual period using Naegele’s rule, plus your current gestational week and trimester.

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Runs in your browser

How to use it

Using the pregnancy due date calculator

01
Recall your LMP
First day of your most recent period. If cycles are irregular or you conceived while breastfeeding, the estimate is shakier.
02
Enter the date
The date picker returns the estimated due date instantly along with your current week and trimester.
03
Confirm with your provider
Bring the estimate to your first appointment; early ultrasound will refine or replace it.

Good to know

Why 280 days?

Obstetric convention ages pregnancy from the LMP rather than conception, adding the roughly two pre-ovulation weeks to nature’s ~266 days from fertilization. The result is the familiar 40-week framework used in every prenatal chart.

Due dates are midpoints, not deadlines

Spontaneous births cluster between 37 and 42 weeks, with first-time mothers skewing later. An "overdue" baby at 40+5 is statistically unremarkable; induction conversations typically wait until 41+ in uncomplicated pregnancies.

Trimesters at a glance

First trimester: weeks 1–13; organogenesis, highest miscarriage risk.
Second trimester: weeks 14–27; anatomy scan, often the comfort sweet spot.
Third trimester: weeks 28–42; growth, monitoring and preparation.

How it's calculated

The math behind this calculator

Due date = LMP + 280 days
Gestational week = ⌊days since LMP ÷ 7⌋ + 1 (clamped to 42)

Naegele’s rule adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation on day 14. Gestational age counts from the LMP itself, so week 1 begins immediately; standard obstetric convention worldwide.

All date arithmetic uses UTC calendar days, avoiding timezone drift. Only 280 days are added regardless of cycle length; providers adjust for irregular cycles or late ovulation.

Assumptions & limitations

28-day menstrual cycle with mid-cycle ovulation.
Full term spans 37–42 weeks; only ~4% of babies arrive on the exact date.
Ultrasound dating in the first trimester supersedes LMP estimates.

Worked example

With an LMP of 15 June 2026, adding 280 days gives an estimated due date in late March 2027, with gestational age advancing each Monday-free calendar day thereafter.

FAQ

Frequently asked questions

My cycles are not 28 days; does this still work?: Partly. Each extra day of typical cycle length shifts ovulation (and effectively the true due date) about a day later. Mention cycle length to your provider for adjustment.
What if I know my conception date instead?: Add 266 days to conception instead. IVF transfer dates allow precise calculation; your clinic provides the adjusted due date.
Why does the calculator reject future dates?: Gestational age counts from an event that has happened. A future LMP means a typo or, rarely, a very early pregnancy where waiting a few days gives a valid date.

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