This pregnancy calculator sketches timing from last menstrual period and cycle length. With sample LMP 2026-01-01 and cycle 28, Naegele style dating gives due date 2026-10-08. Educational estimate only, not medical advice.
For a focused due date workflow use the due date calculator. Cycle logging can start with the period calculator. Mid cycle fertility sketches belong on the ovulation calculator.
How the formula works
due = LMP + 280 + (cycle – 28). The 280 day term is the familiar 40 week baseline from LMP for a 28 day cycle. Longer cycles add days. Shorter cycles subtract days.
Worked example
LMP 2026-01-01, cycle 28. Adjustment (28 – 28) = 0. Adding 280 days lands on 2026-10-08. That is the sample due estimate shown throughout this page.
| Input | Value |
|---|---|
| LMP | 2026-01-01 |
| Cycle length | 28 days |
| Baseline add | 280 days |
| Cycle adjustment | 0 days |
| Estimated due date | 2026-10-08 |
How to use the fields
- Enter LMP as the first day of the last period.
- Enter your typical cycle length in days.
- Read the estimated due date as a planning sketch, not a clinical guarantee.
Why LMP dating starts before conception
Clinicians often date pregnancy from LMP even though conception occurs later. The convention keeps charts consistent. Ultrasound dating may revise the estimate when measurements disagree.
Common mistakes
- Entering the last spotting day instead of the period start as LMP
- Using a single unusual cycle as if it were the long term average
- Treating the calculator output as a diagnosis of pregnancy
- Ignoring clinician guidance when ultrasound dating differs
Cycle length adjustments
If cycle is 30, add 2 days beyond the 280 day baseline. If cycle is 26, subtract 2 days. The arithmetic is simple, but irregular cycles still widen uncertainty.
Educational boundary
This page does not replace prenatal care, labs, or ultrasound. Bleeding, pain, or uncertain dates need clinician evaluation. Keep the estimate labeled as educational.
Planning around a date
People use due estimates for work leave sketches and appointment planning. Build buffers because delivery timing varies. A single calendar day is a center point, not a promise.
Write LMP, cycle length, and the calculated due date together so later revisions are auditable.
Irregular cycles
When lengths swing widely, Naegele style math is less reliable. Mention irregularity to a clinician and rely on medical dating tools they recommend.
You can still run the calculator with a best average for a rough window, then avoid treating the result as exact.
How related Multicalify tools differ
Period tools project the next bleed start. Ovulation tools sketch mid cycle timing. Due date and pregnancy pages push further along the calendar toward an estimated birth date. Same LMP can feed more than one view without changing the need for clinical care.
What to bring to a visit
Bring LMP, typical cycle length, any home test dates, and the calculator estimate 2026-10-08 if that was your sample path. Clear notes help a clinician reconcile calendar math with exam findings.
Do not delay care because an online estimate looked reassuring or alarming on its own.
Limitations
Fixed day count model from LMP and cycle length. No ultrasound biometry, no lab interpretation, and no personalized medical risk assessment. Educational estimate only, not medical advice.
Revisiting the sample path
If LMP stays 2026-01-01 and cycle stays 28, the due estimate remains 2026-10-08 under this rule. If either input changes, recompute instead of editing the old date by guesswork. Small LMP shifts move the due date by the same number of days.
Keep clinical appointments even when the online date looks tidy. The calculator supports education and planning language, not diagnosis or delivery prediction with clinical authority.
Multiple gestation and special cases
Twins and other special pregnancies often follow different clinical schedules. Do not stretch a simple LMP plus 280 day sketch to cover those cases. Ask a clinician for the dating method they want you to use.