Pregnancy Due Date Calculator

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Estimate the due date, current week of pregnancy and each trimester from your last period.

Inputs

First day of last period

Average cycle length (days)

How to use this calculator: Pregnancy Due Date Calculator

Estimate the due date, current week of pregnancy and each trimester from your last period. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Pregnancy Due Date Calculator matches the quantity or design check you need.
2Enter Average cycle length using the units printed beside each field.
3Check every value before calculating, especially decimal points and measurement units.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
First day of last period	1969-11-02	Measured or known first day of last period used by the calculation engine.
Average cycle length	28 days	Measured or known average cycle length used by the calculation engine.

Formula inputs & variables for Pregnancy Due Date Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
First day of last period	—	Measured or known first day of last period used by the calculation engine.
Average cycle length	days	Measured or known average cycle length used by the calculation engine.

How the Pregnancy Due Date Calculator works

The Pregnancy Due Date Calculator uses First day of last period and Average cycle length to calculate Estimated due date, Current stage, Conception (approx.), and Days remaining. Its engine applies Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days; the worked values below come from that same live calculation rather than a separately typed example.

With First day of last period 1969-11-02 and Average cycle length 28 days, the main worked-example result is Estimated due date = Aug 9, 1970.

How each Pregnancy Due Date Calculator input is used

First day of last period

The Pregnancy Due Date Calculator worked example uses First day of last period = 1969-11-02. This value is passed directly into the calculation.

Average cycle length

The Pregnancy Due Date Calculator worked example uses Average cycle length = 28 days. This value is passed directly into the calculation, with an allowed minimum 20 and maximum 45.

Pregnancy Due Date Calculator formulas and result interpretation

Formula 1: relationship used

In the Pregnancy Due Date Calculator, Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Estimated due date

For the displayed Pregnancy Due Date Calculator worked example, Estimated due date is Aug 9, 1970. Verify First day of last period and Average cycle length and their units before relying on this output.

Current stage

For the displayed Pregnancy Due Date Calculator worked example, Current stage is 2967 weeks 6 days. Third trimester Verify First day of last period and Average cycle length and their units before relying on this output.

Conception (approx.)

For the displayed Pregnancy Due Date Calculator worked example, Conception (approx.) is Nov 16, 1969. Verify First day of last period and Average cycle length and their units before relying on this output.

Days remaining

For the displayed Pregnancy Due Date Calculator worked example, Days remaining is 0 days. Verify First day of last period and Average cycle length and their units before relying on this output.

Pregnancy Due Date Calculator accuracy, checks and limitations

Pregnancy Due Date Calculator units check: confirm First day of last period and Average cycle length (days) before calculating.
Pregnancy Due Date Calculator result check: compare Estimated due date, Current stage, Conception (approx.), and Days remaining with the substituted formula steps and the displayed rounding precision.
Pregnancy Due Date Calculator: This is a general estimate, not a diagnosis or personal medical advice. Discuss health decisions with a qualified clinician.

Formula, derivation and worked example

Naegele's rule counts 40 weeks from the first day of the last period. If your cycle is not 28 days, ovulation shifts and so does the due date. Only about 4% of babies arrive on the exact date — anywhere from 37 to 42 weeks is full term.

Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days

Substitution steps

1. Naegele's rule
LMP + 280 days
= Aug 9, 1970

Computed example results

Estimated due date: Aug 9, 1970
Current stage: 2967 weeks 6 days; Third trimester
Conception (approx.): Nov 16, 1969
Days remaining: 0 days

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Assumptions and calculation scope

Average cycle length is assumed to be entered in days; converting from another unit before entry avoids changing the numerical meaning of the calculation.
Pregnancy Due Date Calculator applies the displayed Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days relationship to the entered health inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Common mistakes when using Pregnancy Due Date Calculator

Do not mix units for Average cycle length (days). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not replace the displayed Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Estimated due date = Aug 9, 1970 from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Pregnancy Due Date Calculator is useful

Pregnancy Due Date Calculator is designed for cases where First day of last period, Average cycle length are known and you need Estimated due date, Current stage, Conception (approx.). The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

First day of last period and Average cycle length: what changes the answer

The worked example uses First day of last period = 1969-11-02, Average cycle length = 28 days. With those values, Estimated due date is Aug 9, 1970. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: First day of last period: Measured or known first day of last period used by the calculation engine. Average cycle length (days): Measured or known average cycle length used by the calculation engine.

How to sanity-check a Pregnancy Due Date Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Estimated due date, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Ovulation Calculator

Useful next check because both tools use First day of last period, while Ovulation Calculator answers a different part of the same workflow.

Open Ovulation Calculator

Formula

Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days

Warnings

This is an estimate for planning only. An ultrasound dating scan is far more accurate.

Frequently asked questions

What inputs does the Pregnancy Due Date Calculator use?

It uses First day of last period and Average cycle length. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Pregnancy Due Date Calculator calculate?

It calculates Estimated due date, Current stage, Conception (approx.), and Days remaining. With the displayed default inputs, Estimated due date is Aug 9, 1970.

Which formula does the Pregnancy Due Date Calculator use?

The primary relationship is Due date = LMP + 280 days, adjusted for cycles longer or shorter than 28 days. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Pregnancy Due Date Calculator result?

First verify the units for First day of last period and Average cycle length. Then compare the substituted formula steps with Estimated due date and its displayed precision.

What are the limitations of the Pregnancy Due Date Calculator?

Pregnancy Due Date Calculator: This is a general estimate, not a diagnosis or personal medical advice. Discuss health decisions with a qualified clinician.

Where can I find related Health tools?

Use the related-tools section on this page to compare another method, change units or continue the same health calculation.

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