Free · No Login Required
Work out your estimated due date, how many weeks along you are, and what is coming next — from your last period, a conception date or an ultrasound.
Better cycle
Awareness
for a healthier
you
Choose the date you actually have. We will always show you which method produced your answer.
Enter your details and press Calculate to see your estimated dates here.
From your last menstrual period it applies Naegele’s rule — adding 280 days, which is the same as adding seven days, subtracting three months and adding a year. From a known conception date it adds 266 days. From an ultrasound it uses the due date on your report unchanged.
That question belongs to the period calculator. This page deals only with pregnancy dating, which runs from the first day of your last period rather than from a predicted next period.
Cycle length commonly falls between 21 and 35 days, though longer cycles are normal for some people. A useful average is what you see across several cycles, not a textbook figure.
You can, but expect the date to be less accurate, because the rule assumes ovulation happened about two weeks before your due date. If your cycles are irregular, a first-trimester scan is what actually settles the date — and most clinics re-date at that appointment anyway.
They are mathematical estimates, not confirmed medical dates. Clinical dating may differ, and your care team has the final say. A first-trimester ultrasound is the most reliable dating method available, though later scans can adjust it slightly.
No. The calculation runs entirely in your browser. Nothing is sent to a server, written to a database or attached to an account, and the inputs clear when you leave or refresh the page.
Understand your cycle, track important dates, and take charge of your reproductive health.
Explore Health HubThree methods, one result. Pick the one that matches the date you can actually vouch for.
The calculator always shows the method it used. That is deliberate: two different inputs can produce dates a fortnight apart, and knowing which rule produced your number matters more than the number itself.
Trimester boundaries are clinical conventions rather than hard biological changes — but they are what appointments and tests are organised around.
Week counting is deliberately unfussy: you are however many whole weeks and days have passed since the reference date, so “11 weeks, 4 days” is a perfectly normal answer to give a clinician.
These sound like they should be the same date. They are not, and knowing why removes a lot of confusion.
Conception happens in a narrow window around ovulation, and you cannot observe exactly when it occurred. So pregnancy dating does not pretend to start there. It counts from the first day of your last period — roughly two weeks earlier — because that is the only date reliably known in advance.
That is why a home test can be positive before you would describe any change in your body, and why a scan taken at what you thought was six weeks may read as eight. It is a labelling convention, not a finding that anything happened before conception.
The whole method rests on one assumption: that ovulation happened about fourteen days before your next period would have arrived. When that assumption is off, the dates are off with it.
In practice this is why a first-trimester ultrasound matters more than any calculator. It measures the pregnancy directly rather than inferring it, and re-dating based on that measurement is routine.
A calculator can do date arithmetic. It cannot examine you, take a blood test, or rule anything out.
Use our additional tools to support your health journey.
