A due-date calculator estimates a pregnancy due date from a starting date such as the first day of the last menstrual period (LMP) or, in some calculators, a conception date. A due date is an estimate rather than a promise of the exact day a baby will be born. The dating method and the underlying assumptions matter, so the result should be used as a planning reference rather than a prediction with certainty.
How pregnancy dating is estimated
A common LMP-based calculation begins with the first day of the last menstrual period and estimates about 280 days, or 40 weeks, to the due date. This approach assumes a typical cycle and ovulation timing, so the estimate can be less useful when cycles vary or the LMP is uncertain.
How to use Tervilo's Due Date Calculator
- Open the Tervilo Due Date Calculator.
- Select the dating method available to you.
- Enter the date carefully.
- Review the estimated due date and the calculation basis.
- Use the result as an estimated date, not a guarantee.
Why the last menstrual period matters
The first day of the last menstrual period is used as a reference point in many pregnancy-dating calculations. A mistaken date, irregular cycle or uncertainty about the LMP can change the estimate.
Alternative dating inputs
Some calculators support a conception-based starting point, but that is a different input method from an LMP date. Enter the date type that the calculator requests rather than substituting one for another.
Why the due date can change
Pregnancy dating may be revised when better clinical information becomes available. Ultrasound findings and other clinical assessments can sometimes provide more useful dating information than an uncertain LMP. ACOG guidance emphasizes the importance of establishing an estimated due date using the best available obstetric information and documenting changes when appropriate. ACOG: Methods for Estimating the Due Date.
What “due date” does and does not mean
The estimated due date is a reference point for pregnancy care. It does not mean labor will occur on that exact day. Pregnancy length varies, and clinical decisions should use the full pregnancy context rather than the calculator alone.
Cycle length and irregular periods
An LMP-based estimate assumes a typical relationship between the period and ovulation. If cycles are regularly much shorter or longer than the standard assumption, the estimated date may differ from the simple calculation. Tell a healthcare professional when cycle information is uncertain or atypical.
Example
If an LMP date is entered into an LMP-based calculator, the output is a calendar estimate based on that starting date. The important part is not memorizing the date but understanding that the result depends on the starting date and the dating method.
When the calculator is not enough
Pregnancy calculators cannot evaluate symptoms, fetal development or complications. Contact a qualified maternity or healthcare professional for pregnancy concerns, bleeding, pain, reduced fetal movement or other symptoms that need clinical assessment.
What to do when the LMP date is uncertain
If the first day of the last menstrual period is not known confidently, the calculator's estimate inherits that uncertainty. Do not invent a date simply to produce a precise-looking result. Use the best information available and discuss uncertain dating with a healthcare professional.
Why calendar arithmetic is not a clinical assessment
A calculator can add days to a date, but it cannot assess fetal development or determine whether a pregnancy is progressing normally. The numerical output is therefore useful for orientation and planning, while clinical care provides the medical context.
Quick checklist
- □ I entered the correct date type.
- □ I understand the result is an estimate.
- □ I know an uncertain LMP can affect the result.
- □ I understand clinical dating can differ.
- □ I use professional pregnancy care for health decisions.
Quick answer
A due-date calculator estimates a pregnancy due date from a date such as the last menstrual period or conception. The calculation is a planning estimate, and clinical information can lead to a different or revised estimated due date.