Pregnancy Calculator

An estimated due date is produced by counting, not by observing. Nobody watches conception happen, so the standard method starts from a date you can actually recall — the first day of your last menstrual period — and counts forward a fixed number of days. That is why a pregnancy is described as 40 weeks long even though the biological process is closer to 38, and why your calculated date can shift once an ultrasound gives a clinician something more direct to measure.

Why pregnancy is dated from your last period

Gestational age is measured from the first day of your last menstrual period (LMP), roughly two weeks before ovulation. For that first fortnight of the count you are not yet pregnant — the clock simply starts at a landmark that is easy to identify and consistent between patients.

Fertilisation age (also called embryonic or conception age) starts at ovulation instead, so it runs about two weeks behind. When a book, an app or a clinician says “12 weeks”, they almost always mean gestational age. This calculator reports gestational age throughout: the weeks-and-days figure, the trimester, the progress bar and the timeline dots are all counted from LMP.

Naegele's rule and the 280-day count

The classical formula, Naegele's rule, states the due date as the first day of the LMP plus one year, minus three months, plus seven days. That works out at roughly 280 days, and this calculator uses the plain 280-day form directly:

• LMP mode: estimated due date = LMP + 280 days (40 weeks); estimated conception = LMP + 14 days.
• Conception mode: estimated due date = conception date + 266 days (38 weeks); LMP is back-calculated as conception minus 14 days.
• Due date mode: works backwards — LMP = due date − 280 days, conception = due date − 266 days.

Notice the assumption sitting underneath all three: a 28-day cycle with ovulation on day 14. That is a textbook average, not a description of any individual. If your cycles run longer, ovulation tends to fall later, and an LMP-based estimate will read early by roughly the difference. There is no cycle-length field here to correct for that, so treat the output as a rough midpoint if your cycles are long, short or irregular. The ovulation calculator and menstrual cycle tracker work from your own cycle length instead.

The four input modes and what each assumes

• Last Menstrual Period. The default and most widely used starting point. Enter the first day of bleeding, not the last.
• Conception Date. For a known insemination, transfer or single intercourse date. Everything else is derived from it.
• Due Date. Enter a date you were already given and the tool reconstructs the LMP, the conception estimate and your current progress. Use this once a clinician has confirmed or revised your dates.
• Ultrasound (weeks + days). Enter the gestational age as of today. This is the input that trips people up: the calculator reads those weeks and days as where you are right now, not where you were at a scan last week. If a scan ten days ago reported 8w 2d, enter 9w 5d.

Whichever mode you pick, the “conception date” card is a derived estimate rather than a record of an event — it is simply the calculated LMP plus 14 days.

Why ultrasound dating can differ from your LMP date

Ultrasound dates the pregnancy by measuring it rather than by counting from a remembered date, and in the first trimester the crown–rump length measurement is generally regarded as the more reliable of the two. Several things pull the estimates apart: a cycle length away from 28 days, uncertainty about the exact LMP date after hormonal contraception or with irregular cycles, early bleeding mistaken for a period, and normal variation in early growth.

When the two disagree by more than a small margin, clinicians commonly redate the pregnancy to the ultrasound figure. The dating your clinician records supersedes any calculator, including this one. If you have been given a revised due date, switch to Due Date mode and enter it so the trimester and progress figures line up with your care.

Trimester boundaries and what the date really means

Trimesters are administrative conventions rather than biological switches, and different sources draw the lines a week apart. This calculator uses these boundaries:

• First trimester: up to 12 weeks and 6 days.
• Second trimester: 13 weeks 0 days to 26 weeks 6 days.
• Third trimester: 27 weeks 0 days onward.

You will see 14-week and 28-week boundaries used elsewhere; neither convention is wrong, they are just different cut-offs for the same continuum. The same applies to the due date itself: it marks the centre of a range, and birth rarely falls exactly on it. The progress bar and the “days remaining” figure are there to give you a sense of position, not a countdown to a fixed appointment.

How to use the pregnancy calculator

1. Choose a calculation method from the dropdown — LMP, conception date, due date, or ultrasound weeks and days.
2. Enter the date in the field beside it. Selecting the ultrasound method swaps that field for two number boxes, weeks and days.
3. Press Use Today's Date if you want to reset the date field to today before adjusting it.
4. Read the six result cards: estimated due date, estimated conception date, gestational age in weeks and days, days remaining, current trimester, and completed weeks.
5. Check the progress bar and the five timeline dots beneath it, which light up as you pass LMP, the first, second and third trimesters and the due date.
6. Change the method or the date at any time; every figure recalculates immediately.

Two things to keep in mind while reading the output. Gestational age and days remaining are both calculated against your device's current date, so a wrong system clock produces a wrong answer. And “days remaining” stops at zero rather than going negative, so a pregnancy past its estimated date will simply show 0 days.

Frequently Asked Questions

Why does the calculator say I am pregnant for two weeks before conception?

Because gestational age is counted from the first day of your last period, which precedes ovulation by roughly two weeks. It is a counting convention shared by clinicians worldwide, not a claim about when the pregnancy began.

My cycles are not 28 days. Does that change the result?

Yes. Every LMP-based calculation here assumes ovulation on day 14. Longer cycles usually mean later ovulation, which makes an LMP-based date read earlier than reality; shorter cycles have the opposite effect. Early ultrasound dating is not affected by cycle length, which is one reason it is often preferred.

Which mode should I use if I have both an LMP date and a scan?

Use whichever your clinician has adopted as your official dating — usually the scan-based one. Enter it through the Due Date mode if you have been given a specific date, or the Ultrasound mode if you know your current gestational age.

Why did my due date change after my scan?

Redating after an early ultrasound is common and does not by itself indicate a problem. The scan measures the pregnancy directly, while the LMP method relies on a remembered date plus an assumed ovulation day. Your clinician can explain the change in the context of your own history.

Is 40 weeks how long a pregnancy lasts?

Forty weeks from LMP is the counting convention behind the estimate, which corresponds to about 38 weeks from conception. Actual duration varies between pregnancies, which is why the result is labelled an estimated due date.

What does the “conception date” card actually tell me?

It is an arithmetic estimate: the calculated LMP plus 14 days. It is not derived from any test or measurement, so it should not be read as a precise date.

Is anything I enter stored or sent anywhere?

No. Every calculation runs in your browser, nothing is uploaded, and nothing is kept once you close or reload the page.

Sources

• Naegele's rule — the classical due-date formula: first day of the last menstrual period, plus one year, minus three months, plus seven days. Implemented here in its equivalent 280-day form.
• The 40-week (280-day) gestational age convention, measured from the first day of the last menstrual period, with fertilisation age running approximately two weeks behind it.
• Crown–rump length (CRL) ultrasound dating — the first-trimester measurement clinicians use to confirm or revise an LMP-based due date.