Estimating a due date from the last menstrual period
Due-date estimation gives a planning date based on menstrual dating. This page also counts weeks since the first day of the last period, which differs from time since conception. If the due date is shifted for cycle length, the separate week count is still LMP-based; it is not a clinician-established gestational age.
The separate week count is elapsed whole weeks and days since LMP. In a typical 28-day cycle, that count is about two weeks longer than time since conception. A cycle-adjusted due date and an unchanged LMP-based week count can imply different dating; neither establishes a clinical gestational age on its own.
The due-date estimate, cycle adjustment and separate LMP-based week count are shown together. The formula states the 280-day convention, while the result labels the date as an estimate rather than a prediction of delivery.
How the period date, cycle length and selected date affect this estimate
This is a menstrual-date estimate. A clinician may revise the due date using ultrasound and other clinical information.
- First day of the last menstrual period
- The starting day for the 280-day convention. Enter the first day of bleeding rather than the day the period ended; moving this date changes both the due-date estimate and the LMP-based week count.
- Usual cycle length
- A 28-day cycle leaves the date unchanged. The calculator shifts the estimate by the difference between your entered 21–35-day cycle and 28 days; irregular cycles make this adjustment less dependable.
- Weeks since last period on
- Choose the date for the LMP-based weeks-and-days reading. It does not alter the cycle-adjusted due date; a clinician may establish different dating.
Steps
- Choose the first day of the last menstrual period.
- Enter the usual cycle length in days; 28 days produces no adjustment.
- Select a date and calculate to see the estimated due date and the separate LMP-based week count.
Reading tip: The trimester cards illustrate stages using the LMP-based week count. The due-date card uses a separate, cycle-adjusted planning estimate.
Planning questions a menstrual-date estimate can help frame
Plan appointments
Use an estimated due date as a calendar reference for conversations with a clinician.
Count weeks since the last period
See elapsed whole weeks and days from the last menstrual period; clinical dating may differ.
Understand trimesters
Find the stage used by this tool on a selected date.
Compare cycle lengths
See how the simple cycle adjustment moves the estimate.
How the calculation works
- LMP
- First day of the last menstrual period, rather than an estimated conception date.
- 280 days
- The conventional 40-week menstrual dating interval for a 28-day cycle.
- cycle adjustment
- Difference in days between the entered regular cycle length and 28 days.
- elapsed days
- Calendar-day difference between LMP and the selected as-of date.
The cycle shift is a site assumption; the week count does not use it. A clinician may establish different dating after reviewing history or ultrasound.
What every part means
- LMP
- First day of the last menstrual period, rather than an estimated conception date.
- 280 days
- The conventional 40-week menstrual dating interval for a 28-day cycle.
- cycle adjustment
- Difference in days between the entered regular cycle length and 28 days.
- elapsed days
- Calendar-day difference between LMP and the selected as-of date.
Calculate it step by step
A 28-day cycle adds zero days; this is a simplified shift for regular cycles.
The result is a planning estimate, not a predicted delivery day.
Elapsed days are measured from LMP to the selected date without the cycle shift.
ACOG describes the 280-day LMP convention for a 28-day cycle and the limits of menstrual dating. The extra cycle-length shift is this site's simplifying assumption, not an ACOG dating instruction. The engine counts whole seven-day weeks from LMP without that shift. Its trimester illustration uses boundaries of 14 and 28 weeks.
See how Calculation Guide tests examples and handles calculation limits.
See it in practice
- First day of last menstrual period
- 2026-01-01
- Usual cycle length
- 28 days
- Count weeks since last period on
- 2026-04-01
LMP January 1, 2026; cycle 28 days → adjustment = 0. April 1 is 90 days later → 12 weeks + 6 days; due date is LMP + 280 days.
- Weeks since last period on selected date12 weeks, 6 days
- LMP-based trimester illustrationFirst
Interpretation & limitations
How to read your result
The highlighted trimester follows the LMP-only count, not the adjusted due date. It is an illustration, not a clinical assessment. A selected date after the estimated due date does not itself indicate a problem or determine when delivery will occur.
Where this estimate stops
Irregular cycles, uncertain LMP, recent hormonal contraception, assisted reproduction and individual variation can make this simple estimate inaccurate. Discuss dating and care with a qualified clinician.
Frequently asked questions
01Why is pregnancy dated from the last period?
Clinical gestational age convention begins on the first day of the last menstrual period, around two weeks before typical conception in a 28-day cycle.
02Can an ultrasound change the due date?
Yes. Clinical dating uses the menstrual history and accurate ultrasound findings together.
03Does a longer cycle move the estimate later?
Yes. For example, a regular 30-day cycle adds two days to the 28-day-cycle estimate.
04Is the estimated due date the day birth will happen?
No. It is a reference date used for planning and clinical dating; actual delivery timing varies.
05What if my cycles are irregular?
The single cycle-length adjustment may be unreliable. A clinician can use history and ultrasound to assess dating.
06What does a negative days-until-due figure mean?
The selected as-of date falls after this calculator's estimated due date. The figure does not diagnose any condition.