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Pregnancy Due Date Calculator

Details

How to use Pregnancy Due Date Calculator

What the tool does, how to run it, and what to expect from the result.

How to estimate a due date from your last period

The standard estimate, Naegele's rule, adds 280 days to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, which is why entering your real cycle length matters: a longer cycle means later ovulation and a later due date.

This calculator applies that rule, adjusts for your cycle, and shows your current gestational age, trimester, and the milestone dates ahead. It is arithmetic, done in your browser, and it is not medical advice.

  • Enter the first day of your last menstrual period.
  • Enter your typical cycle length, between 20 and 45 days. The default of 28 will be wrong for many people.
  • Read the estimated due date, your gestational age in weeks and days, and your current trimester.
  • Check the milestone dates: end of first trimester (week 13), viability (week 24), end of second trimester (week 27), full term (week 37).
  • If you have had a dating scan, use the date your clinic gave you instead of this one.
Tips

Getting a better result out of Pregnancy Due Date Calculator

Specific settings and thresholds, not general advice.

  • This is a date calculator, not a clinician. Naegele's rule gives an estimated date of delivery, and about 4% of babies arrive on it. It cannot see your scan, your history, or anything else, and it does not replace a midwife or an obstetrician.
  • The tool uses Naegele's rule with a cycle adjustment: 280 days from the first day of your last period, shifted by (cycle length minus 28) days. So a 32-day cycle pushes the estimate 4 days later, which is why the cycle field matters and why the default of 28 will mislead anyone whose cycle is not 28.
  • Gestational age is counted from the first day of your last period, not from conception, which means you are notionally two weeks pregnant on the day you conceive. That convention exists because the LMP date is knowable and the conception date usually is not.
  • A dating scan in the first trimester is more accurate than any LMP-based estimate, typically within about five days, because it measures the fetus rather than assuming an ovulation date. If your scan and this calculator disagree, the scan wins.
  • The trimester boundaries here are the common ones: first through week 13, second from week 14 to 27, third from week 28. Different sources draw the lines slightly differently, so a one-week discrepancy against another chart is not an error.
Limits

What Pregnancy Due Date Calculator does not do

The honest boundary, so you do not lose time finding it yourself.

  • LMP-based only. It cannot take a conception date, an IVF transfer date, or a scan measurement.
  • Cycle length is clamped to between 20 and 45 days.
  • It assumes a single pregnancy. Twins and higher multiples typically deliver earlier and are not modelled.
  • It is not medical advice, and it does not account for any medical history.
At a glance

Who Pregnancy Due Date Calculator is for

A quick way to understand who this helps, what it solves, and where it connects next.

Best fit

Expecting parents estimating pregnancy dates.

Ideal for

Using the pregnancy due date calculator without installing anything or signing up.

FAQ

Common questions

Short answers for the questions people usually have before trying a utility like this.

How is the due date calculated?

By Naegele's rule with a cycle-length adjustment: 280 days (40 weeks) from the first day of your last menstrual period, plus or minus the difference between your cycle length and 28 days. A 32-day cycle therefore shifts the estimate 4 days later, because ovulation happened later in that cycle.

How likely is the baby to arrive on the due date?

Around 4% of babies are born on their estimated date. Most arrive within a two-week window either side, and that is considered entirely normal. The date is a planning anchor for scans and appointments, not a prediction. Treat it as the middle of a range rather than a deadline.

Why does the calculator say I am pregnant before I conceived?

Because obstetric convention counts gestational age from the first day of your last period, roughly two weeks before ovulation. So on the day of conception you are already called two weeks pregnant. It is a bookkeeping convention, chosen because the date of your last period is something you know and the date of conception usually is not.

My scan gave a different date. Which one is right?

The scan. A first-trimester dating scan measures the fetus directly and is typically accurate to within about five days, whereas any LMP calculation is only as good as the assumption that you ovulated on schedule. If your clinic has revised your date after a scan, use theirs and ignore this.

Is my information stored anywhere?

No. The date you enter never leaves your device. There is no account, no cookie holding it, no request to a server, and nothing logged. The arithmetic is done in JavaScript in your own tab, which is a deliberate choice for a tool where the input is this sensitive.

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