Pregnancy Due Date Calculator (Naegele’s Rule)
Estimate your pregnancy due date from your last menstrual period, and see how many weeks along you are. An estimate, not medical advice. In your browser.
🔒 Calculated in your browser — nothing is uploaded. Naegele's rule (LMP + 280 days), assuming a 28-day cycle. An estimate only; an ultrasound date from your provider may differ, and this is not medical advice.
Estimate your due date
Pick the first day of your last menstrual period (LMP). The tool adds 280 days (Naegele’s rule) for an estimated due date and shows how many weeks and days along you are, plus the trimester. It assumes a regular 28-day cycle.
An estimate only
This is a rough estimate, not a diagnosis. A dating ultrasound from your healthcare provider is more accurate, especially with irregular cycles, and only about 1 in 20 babies arrive on the exact due date. This tool is for general information and is not medical advice. Your inputs never leave your device.
What Naegele's rule assumes
The date is the first day of your last period plus 280 days — forty weeks. That arithmetic carries three assumptions inside it: that your cycle is 28 days, that you ovulated on day 14, and that the egg was fertilised that day. Pregnancy is actually counted from a point about two weeks before conception, which is why you are already "four weeks pregnant" a fortnight after a missed period. If your cycles run longer or shorter than 28 days, the estimate is off by roughly the same number of days in the same direction.
How the weeks are counted here
Gestational age is shown as completed weeks plus days — 12+3 means twelve full weeks and three days since the first day of your last period, which is how a clinic writes it. The trimester boundaries this page uses are week 13 for the second and week 27 for the third. Those cut-offs vary a little between sources; some put the third trimester at 28 weeks. Nothing medical hangs on which convention a calculator picked.
Frequently asked questions
How is a due date calculated?
Naegele’s rule adds 280 days (40 weeks) to the first day of your last period, assuming a 28-day cycle. Ultrasound dating can adjust it.
How accurate is the due date?
About one baby in twenty arrives on the estimated date. Most births fall in a five-week window around it, and a dating scan in the first trimester is more accurate than any calculation from a period date — it measures the baby rather than assuming a cycle length.
My cycle is not 28 days. What then?
Shift the estimate by the difference. A 32-day cycle usually means ovulation about four days later than the rule assumes, so the date lands roughly four days later too. A midwife or doctor will adjust properly from a scan.
Is this medical advice?
No. It is a date arithmetic tool. It cannot know anything about you or the pregnancy, and it is not a substitute for antenatal care.