Pregnancy Due Date Calculator

Overview

One Timeline, Whichever Date You Have

A due date is the same calculation every time — 280 days, or 40 weeks, from the first day of the last menstrual period — but not everyone arrives at it from the same place. Some people have a reliable period date. Some tracked ovulation. Some have an IVF transfer date, which dates a pregnancy far more tightly than a period ever could. Some have a scan that has already read a gestational age off a measurement.

This calculator takes any of those four and converts it back to the same starting point, so the answers are on one scale and can be compared with each other. The American College of Obstetricians and Gynecologists describes the estimated due date as 280 days from the first day of the last period, and the NHS notes that a scan usually gives a more accurate date than that count does — which is why the ultrasound method here rebuilds the whole timeline rather than nudging the existing one.

Beside the date you get the things people actually want next: how far along the pregnancy is today in weeks and days, which trimester that falls in, the 37 to 42 week window a term birth can land anywhere inside, the estimated conception date, and the dates of the appointments and scan windows that hang off the same timeline.

Methods

Which Dating Method to Pick

Pick the date you are most confident about, not the one that gives the answer you were hoping for. Each method has a different amount of slack in it, and the tool converts each one back to an equivalent last-period date before doing anything else.

Method	What it needs	How the due date comes out
Last period	The first day of your last period, plus your usual cycle length	280 days on, shifted by however far your cycle differs from 28 days.
Conception	The date of conception, from ovulation tracking	266 days on, because conception is about 14 days after the period starts.
IVF transfer	The transfer date and whether the embryo was day 3 or day 5	263 days on for a day-3 embryo, 261 for a day-5 blastocyst.
Ultrasound	The date of the scan and the weeks and days it reported	The scan sets how far along you were on that day, and the rest is counted from there.

If two methods disagree by more than a few days, the more precise one usually wins: a scan or an IVF record beats a remembered period date. That is a conversation for your midwife or doctor rather than a rule this page can apply for you.

Pregnancy due date calculator dated from a day-5 IVF transfer, showing the estimated conception date, the 37 to 42 week term window, the dating method and days remaining above six milestone cards with the passed ones marked in green — Dated from an IVF transfer: the term window and the estimated conception date, then the milestone dates with the ones already passed marked.

Results Guide

Reading the Results

Result	What it shows
Estimated due date	40 weeks from the equivalent last-period date. One date, and the least useful number on the page on its own.
Gestational age	How far along the pregnancy is today, in weeks and days, counted from the same starting point.
Trimester	First up to 13w 6d, second from 14w 0d to 27w 6d, third from 28w 0d. The bar shows where today sits across all three.
Term window 37–42w	The span a term birth can land anywhere inside. This is the honest version of the due date.
Estimated conception	About 14 days after the equivalent last-period date, which is why it moves with the cycle length.
Milestone dates	The dates the first appointment, the dating scan, the second trimester, the anatomy scan, the third trimester and the due date fall on.

Guide

How to Use the Due Date Calculator

Three steps, and nothing to sign up for.

Step 1: Choose the method that matches the date you have

The four buttons at the top change what the page asks for. Last period wants a date and your cycle length; conception wants one date; IVF wants the transfer date and the embryo age; ultrasound wants the scan date and the weeks and days it reported. Only the fields the method needs are on screen, so there is nothing to leave blank.

Step 2: Set the date, and the number beside it

Open the date picker and use the month and year menus to jump straight to the right month rather than paging back one at a time. The cycle length, the embryo age and the scan reading all have their own control, and the whole timeline redraws the moment any of them changes — there is no calculate button to press.

Step 3: Read the window, not just the date

The single date is the shorthand; the 37 to 42 week window is the real answer, and the milestone cards give you the dates worth putting in a calendar. "Copy summary" puts the due date, the gestational age, the window and the conception estimate on your clipboard as plain text. To count forward or back from any of those dates, use Add Days to Date; to measure the gap between two of them, use the Date Difference Calculator.

Support

Frequently Asked Questions

How is a pregnancy due date calculated?

The standard estimate counts 280 days — 40 weeks — from the first day of the last menstrual period. This calculator works the same way whichever method you pick: conception, an IVF transfer and a scan are each converted back to the equivalent last-period date first, and the due date is then 280 days on from that. It means the four methods can be compared against each other instead of each producing an answer on its own scale.

Is the estimated due date exact?

No, and no due date is. It is an estimate: only a small share of babies arrive on the date itself, and a term birth is anything from 37 to 42 weeks. That is why the tool shows the 37 to 42 week window beside the single date — the window is the honest answer and the date is the shorthand. Your midwife or doctor may also revise the official date after a scan.

Why does the cycle length change the answer?

The 280-day rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, you ovulated later than day 14 and the pregnancy started later than the plain rule assumes, so the due date moves back by the difference; a shorter cycle moves it forward. The tool applies the difference between your cycle length and 28 days, which is why a 35-day cycle pushes the estimate a week later.

How does the IVF due date work?

An IVF transfer dates the pregnancy far more precisely than a period does, because the age of the embryo is known. The calculator takes the transfer date, subtracts the embryo age and another 14 days, and gets the equivalent last-period date. In practice that means a day-5 blastocyst transfer is due 261 days after the transfer and a day-3 embryo 263 days after it.

Why can an ultrasound change the due date?

An early scan measures the fetus and reads a gestational age off that measurement, which does not depend on remembering a date or on assuming when you ovulated. If that reading disagrees with the last-period calculation by more than a few days, clinical guidance is generally to re-date the pregnancy from the scan. Enter the scan date and the weeks and days it reported and the tool rebuilds the whole timeline around it.

What is gestational age, and why is it not the same as how long you have been pregnant?

Gestational age is counted from the first day of the last period, which is roughly two weeks before conception. So at the moment of conception the pregnancy is already recorded as two weeks along, and a pregnancy described as 12 weeks is about 10 weeks past conception. Every clinical week number on this page — the trimesters, the scan windows, the 40-week due date — uses gestational age, because that is the scale maternity care runs on.

Are my dates uploaded, or kept after I leave?

Neither. The whole calculation runs in your browser with JavaScript, and nothing about the pregnancy is sent to Randomly.online. It is also not written to your browser's storage: close or reload the page and the fields are empty again, so a shared or borrowed device does not keep your dates.

Can this replace prenatal care?

No. It is a planning and education tool. Estimating a due date from a date you type is not the same as pregnancy dating by a clinician, who has your scan measurements and your history. Use this for calendars, appointments and getting your bearings, and let your midwife or doctor set the date that goes in your notes.

Does the pregnancy due date calculator work on a phone?

Yes. On a narrow screen the two columns stack into one: the method switch and its date fields sit at the top, and the due date, the gestational age, the trimester progress bar and the milestone dates follow directly below. The steppers, the listbox and the date picker are all built for touch and every control meets a 44px target.

Pregnancy due date calculator on a 390px phone: the four dating method buttons and the last-period date stacked in one column above the estimated due date and the gestational age and trimester cells — The pregnancy due date calculator on a phone — one column, with the method and the date above the due date.

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