Pregnancy Calculator

Last updated: April 2026

First day of last period

Average cycle length

days

Pregnancy summary

8 October 2026

Due date

Gestational age

24 weeks 2 days

Trimester

Second trimester

Estimated conception date

15 January 2026

Days remaining

110 days

Progress60.7%

Medical note

This calculator provides an estimate only. Pregnancy dates and health guidance should be confirmed with a qualified healthcare professional.

Related calculators:

Due Date Calculator
Pregnancy Conception Calculator
Pregnancy Weight Gain Calculator
Ovulation Calculator

What does a pregnancy calculator show?

A pregnancy calculator estimates your due date, gestational age, trimester, possible conception date, days pregnant, and days remaining.

How pregnancy weeks are counted

Pregnancy weeks are usually counted from the first day of your last menstrual period, not from the day of conception.

Why due dates are estimates

Cycle length, ovulation timing, and dating scans can shift the estimated due date. Clinical dating from a healthcare professional may differ from this calculator.

About this calculator

The Pregnancy Calculator estimates pregnancy week, trimester, conception window, and expected due date from the last menstrual period, conception date, or scan date where supported. It helps parents understand timing while recognising that clinical dating may change after ultrasound. Use this expanded guide when you need more than a quick result. It explains the assumptions behind the Pregnancy Calculator, the records to gather, and the decisions the estimate can support. It is especially useful for parents-to-be estimating pregnancy week, trimester, appointment timing, and likely due date before clinical confirmation. The strongest use of the page is scenario comparison: change one input at a time, compare the output, and keep a note of which assumption changed.

Pregnancy dating method

Pregnancy is commonly dated from the first day of the last menstrual period. A due date estimate is often based on about 40 weeks from that date, adjusted for cycle length where the calculator supports it. The calculator result depends on the quality of the inputs and on the rule set or formula selected in the calculator above. For practical use, treat the output as a structured estimate: start with the core inputs, review the main outputs, then test the decision points that matter most to your situation. Key decisions include which date method is most reliable, how pregnancy week maps to trimester, when to discuss changed dates with a midwife.

estimated due date = first day of last period + 280 days
pregnancy week = days since last period / 7
trimester is based on completed pregnancy weeks
better estimate = accurate inputs + correct rule set + realistic assumptions
scenario difference = revised result - original result

How to use the pregnancy calculator

Enter the first day of the last menstrual period if known.
Adjust cycle length if the calculator allows and your cycle is not close to 28 days.
Use conception or IVF dates only if those are more reliable for your situation.
Review estimated week, trimester, and due date.
Use scan dating from a midwife or clinician if it differs from a home estimate.
Gather the main inputs first: last menstrual period, cycle length, conception date.
Check supporting records such as period start date and cycle history before relying on a final number.
Enter one realistic scenario first, using conservative assumptions where the future is uncertain.
Review the main outputs: estimated due date, pregnancy week, trimester.
Run at least one alternative scenario so you can see which input changes the answer most.
Compare the result with NHS pregnancy dating guidance or the relevant contract, bill, statement, or professional document.
Keep the calculation date and assumptions with your notes so you can revisit the estimate when rates, rules, or circumstances change.

Worked example

LMP due date estimate

Input: First day of last period 1 January

Calculation: 1 January + 280 days

Result: Estimated due date is around early October before clinical adjustment.

Last-period scenario

Input: A user knows the first day of the last period but has a longer cycle.

Calculation: The calculator adjusts the estimate if cycle length is entered.

Result: The due date may shift compared with a standard 28-day assumption.

Scan update scenario

Input: A dating scan gives a due date different from the calculator.

Calculation: Clinical dating is compared with the earlier estimate.

Result: The scan date should usually be used for care planning if the clinician updates it.

Why due dates are estimates

A due date is not a guarantee of the birth date. NHS guidance explains that pregnancy commonly lasts from 37 to 42 weeks from the first day of the last period. A dating scan can give a more accurate estimate when dates are uncertain.

What to check before relying on the result

A useful Pregnancy Calculator result starts with the same evidence you would use if you were checking the answer manually. The calculator can organise the arithmetic, but it cannot know whether a payslip is final, a bill is estimated, a quote excludes fees, or a personal circumstance has changed since the last statement.

Before making a decision, compare the calculator result with the source document that controls the real outcome. For this topic, that usually means checking NHS pregnancy dating guidance. If there is a difference between the calculator and an official statement, contract, assessment, or professional advice, treat the official document as the stronger source.

period start date: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
cycle history: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
fertility treatment record: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.
scan report: Use this as supporting evidence for the calculation. If it is out of date, estimated, or based on a different period, the calculator output may look precise while still being wrong for the decision.

Inputs that usually change the answer

The most important input is not always the largest number on the form. Sometimes a date, threshold, percentage, eligibility flag, or timing assumption changes the result more than the headline amount. This is why scenario testing is more useful than a single calculation.

Input	Why it matters	What to double-check
last menstrual period	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
cycle length	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
conception date	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
IVF transfer date	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.
scan date where supported	It feeds directly into the estimate or changes which rule is applied.	Check the period, units, eligibility, and whether the figure is final or estimated.

How to interpret the output

The output should be read as a decision aid, not just a number. For Pregnancy Calculator, the useful question is often what the result means for timing, affordability, eligibility, comparison, or next steps.

estimated due date: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
pregnancy week: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
trimester: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.
key date ranges: Use this output alongside the other results rather than in isolation. A monthly amount, percentage, date, or payback figure can look acceptable until fees, timing, evidence, or eligibility conditions are added.

Scenarios worth comparing

A single estimate is a snapshot. A better approach is to save a base case, then adjust one assumption at a time. This shows whether the result is stable or whether a small change in timing, rate, usage, income, or cost creates a very different answer.

Scenario	Change one assumption	What the comparison shows
Base case	Use the best current evidence.	Shows the result you would expect if nothing important changes.
Conservative case	Use lower income, higher cost, slower growth, or less favourable timing.	Shows whether the decision still works with less optimistic assumptions.
Improved case	Use the realistic upside, such as lower cost, better rate, higher usage, or stronger evidence.	Shows the potential benefit without treating it as guaranteed.

Common mistakes and edge cases

Most errors come from using the right formula with the wrong assumption. Dates can be counted differently, rates can change, official thresholds can move, and real bills or contracts often include conditions that a simple calculator cannot infer automatically.

Due dates are estimates, not guarantees.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Dating scans can change the estimated due date.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Irregular cycles make period-based dating less reliable.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.
Pain, bleeding, severe sickness, or reduced movements need clinical help, not a calculator.: Check this point before using the estimate for a payment, claim, purchase, application, employment decision, or health-related decision.

Next steps after calculating

Once you have a result, write down the key assumptions and compare them with NHS pregnancy dating guidance. If the number affects a deadline, tax return, benefit claim, employment issue, medical question, finance agreement, or major purchase, use the calculator as preparation for a more formal check.

For lower-stakes use, the next step may simply be comparing two or three scenarios. For higher-stakes use, the next step should be checking the official guidance, speaking to the relevant organisation, or getting qualified advice before acting.

Important edge cases

Due dates are estimates, not guarantees.
Dating scans can change the estimated due date.
Irregular cycles make period-based dating less reliable.
Pain, bleeding, severe sickness, or reduced movements need clinical help, not a calculator.

Limitations

This guide is general information only and is not medical advice. This is general health information and not medical advice. The calculator is designed to support understanding and planning, but it cannot verify documents, predict future rule changes, or account for every exception. Use it as an estimate and check the official source before acting where the result matters.

It cannot diagnose pregnancy or complications.
Irregular cycles, recent contraception, breastfeeding, or uncertain dates can reduce accuracy.
Speak to a midwife, GP, or clinician for personal advice.
Check NHS pregnancy dating guidance for current rules, rates, definitions, and eligibility where relevant.
Do not rely on a single scenario where income, costs, dates, rates, usage, or health circumstances may change.
Keep records of the inputs used so that the estimate can be reviewed later.

Frequently asked questions

Is the due date exact?

No. It is an estimated date and many babies arrive before or after it.

Why does a scan change my due date?

Early ultrasound measurements can be more reliable than period dates when cycle timing is uncertain.

Can I use conception date instead?

Yes, if known, but clinical dating may still use scan findings.

Why is pregnancy counted from the last period?

It is a standard dating convention because the exact conception date is often unknown.

Can the calculator tell if pregnancy is healthy?

No. It estimates dates only and cannot assess symptoms, fetal growth, or complications.

When should I contact a clinician?

Contact a midwife, GP, NHS service, or emergency care for concerning symptoms or urgent worries.

Related calculators

Due Date Calculator
Pregnancy Conception Calculator
Ovulation Calculator
Period Calculator

What does this mean?

This calculator is designed to help you understand the likely number before you make a decision or start an application.

Your result should be checked against official UK guidance, especially if your circumstances include dependants, exemptions, prior leave, or a complex immigration history.

Treat the figure as a planning tool rather than legal advice. Where the answer affects an application deadline or major payment, speak to an authorised adviser.