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Open toolPregnancy Due Date Calculator
Estimate a due date from the first day of the last menstrual period and a regular-cycle length. The date is provisional and can be revised by a clinician.
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How Pregnancy Due Date Calculator works
A calendar-day offset is adjusted by the entered regular-cycle length.
The tool adds 280 days to the last-period start date, then adjusts that offset by the entered cycle length minus 28 days. Calendar arithmetic uses UTC dates; the estimated date is not a clinical finding.
This is a rough estimate based on a regular cycle. Ultrasound and a clinician's assessment may change the due date. It should not guide medical decisions.
Reference: ACOG: Methods for Estimating the Due Date.
How to use Pregnancy Due Date Calculator
Enter the first day of the last period and the assumed average cycle length.
Enter the period date and cycle length
Choose the first day of the last menstrual period and enter a whole-number average cycle length from 21 to 45 days. The date cannot be in the future. This model assumes a regular cycle and does not collect clinical measurements.
First day of last period
Calculate the provisional date
Select Calculate to read the estimated due date. The date is derived from the entered period start and cycle-length adjustment. Check those inputs if the output is unexpected; the page does not measure conception or confirm fetal development.
October 10, 2026
Read the date labels and scope
Compare the estimate with the separate 40-week date for a 28-day cycle. Where displayed, weeks since the last period reflect the calendar-date basis. Keep the labels with the dates; a clinician may revise a provisional estimate using clinical information.
Estimated due date
Check a pregnancy due date estimate from the last period
These uses concern a calendar estimate and do not replace pregnancy care.
Review the date entered
Use the result to check whether the calendar input matches the intended first day of the last period. An accidental month or year change can still produce a valid calendar date, so the form's validity check cannot catch every entry mistake.
Compare cycle lengths in the due date estimate
Compare the selected cycle length with the twenty eight day basis shown in the result. The difference between those dates reflects the entered cycle assumption. It does not establish conception timing or explain an individual pregnancy's development.
Prepare questions for a clinician
Keep the entered date and cycle assumption with the rough estimate if discussing it with a clinician. Clinical information and ultrasound can change the assessment. The page does not collect examination findings or provide a plan for medical care.
Due date, 40-week estimate, and weeks since the last period
Read the provisional dates with their period-start and cycle assumptions.
The main date includes the entered cycle adjustment. The separate forty week date keeps the twenty eight day basis, making that assumption visible. These are calculated estimates and should not guide medical decisions.
When shown, weeks since last period count elapsed calendar time from the entered date to today. That line depends on the current date and does not establish a clinical assessment of gestational age.
Check dates and recognise the estimate limits
Check the calendar inputs and keep the model’s clinical limits visible.
Use a valid calendar date on or before today. A blank date, a future first day or a cycle length outside the field range prevents calculation. Recheck the date picker rather than changing inputs merely to obtain a result.
The method assumes a regular cycle. It does not assess irregular cycles or incorporate ultrasound information. A clinician may revise the estimate; use their assessment for care decisions instead of relying on this page alone.
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Open toolUse Pregnancy Due Date Calculator
Estimate a due date from the first day of the last menstrual period and a regular-cycle length.
Pregnancy Due Date Calculator: common questions
Answers about using Pregnancy Due Date Calculator and understanding its results.
Why add 280 days?
A 40-week estimate from the first day of the last menstrual period is the standard convention for a 28-day cycle.
Can the date change?
Yes. A clinician may revise it using an early ultrasound or other clinical information.
What cycle lengths does the form accept?
The cycle-length field accepts whole numbers from 21 to 45 days. The model assumes a regular cycle.
Does this tool estimate the exact conception date?
No. It uses the last-period date and cycle-length convention to estimate a due date. It does not measure conception or fetal development.