Pregnancy Due Date Calculator
Estimate your pregnancy due date and current gestational age from the first day of your last menstrual period.
- Estimated gestational age today
- 0 weeks, 0 days
- Trimester
- First trimester
This calculator provides estimates only. It is general information, not medical advice or a diagnosis, and doesn't account for your full personal health circumstances. Speak with a doctor or qualified health professional before making health decisions.
Put this calculator on your own website
Embed it as a lead magnet and capture visitor emails.
How this calculator works
This calculator uses Naegele's rule, the standard clinical estimate: due date = first day of your last menstrual period (LMP) + 280 days (40 weeks), assuming a typical 28-day cycle with ovulation around day 14.
Enter the first day of your last period, and it works out your estimated due date, how far along you are today (in weeks and days), and which trimester you're in — first (weeks 0-13), second (weeks 13-27), or third (weeks 27 onward).
This is a population-average estimate, not a measurement of your actual pregnancy — cycle length varies between people, and only around 1 in 20 births happen exactly on the estimated due date. An ultrasound dating scan, usually done early in pregnancy, gives a more individually accurate estimate and is what your care provider will typically rely on.
Frequently asked questions
- How accurate is a due date calculated this way?
- It's a useful starting estimate, but only around 5% of babies are born on their exact estimated due date — most arrive within about two weeks either side. An early ultrasound dating scan is generally more accurate, especially if your cycle isn't a typical 28 days.
- What if I don't know the first day of my last period?
- Your due date can still be estimated from an ultrasound scan (measuring the fetus) — that's what most care providers use when LMP dates are uncertain. Speak with your doctor or midwife for a more precise estimate.
- Why 280 days and not 9 months?
- 280 days (40 weeks) from your last period works out to a bit over 9 calendar months because pregnancy is conventionally dated from the last period, not from conception, which typically happens roughly two weeks later.