Pregnancy Due Date Calculator (Naegele’s Rule)
Estimate your pregnancy due date from your last menstrual period using Naegele's rule, with weeks and days along. An estimate, not medical advice.
Usually between 22 and 44 days; 28 is the average. A longer cycle moves the due date later by the difference, a shorter one earlier.
Not sure of that date? Pick another starting point under “Calculate from” — a conception date, an IVF transfer, an ultrasound scan, or a due date you were already given. If you have none of these, a midwife or doctor can date the pregnancy with a scan.
Naegele's rule (LMP + 280 days, moved by however far your cycle is from 28 days). 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 and an estimated conception date. A "What comes next" table dates the NHS dating and anomaly scan windows, the start of each trimester and full term from the same starting point — the scan windows are the ones offered in England, so the dates your own clinic gives come first. It assumes a regular 28-day cycle unless you enter your own cycle length. Leave the period date empty and the page, besides asking for one, points to the other starting points under “Calculate from” and, failing those, to a dating scan from a midwife or doctor.
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?
Enter your cycle length and the page moves the date for you: a longer cycle usually means ovulation later than the rule assumes, so the due date lands later by the difference, and a shorter one earlier. 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.
Can I start from a conception date, an IVF transfer or a scan?
Yes. Besides the last period, the estimate can start from the conception date, an IVF transfer, an ultrasound's weeks and days, or a due date you were already given.
Does it show the scans and milestones as well as the due date?
Yes. A "What comes next" table dates the NHS dating and anomaly scan windows, the start of the second and third trimesters, full term and the point past the usual end of pregnancy, all from the same starting date, with the ones already past dimmed. The scan windows are the ones offered in England, so your own clinic’s dates come first. An estimated conception date sits beside the due date.
What if I don't know the date of my last period?
Leave it empty and, under the request for a date, the page points to the other methods in “Calculate from” — a conception date, an IVF transfer, an ultrasound scan, or a due date you were already given. If you have none of those, a midwife or doctor can date the pregnancy with a scan, which is more accurate than any period date anyway.