Enter the first day of your last menstrual period — or your conception date if you know it — to estimate when your baby is due. The calculator also shows how many weeks along you are today and when each trimester begins. It is an educational estimate, not medical advice, and everything is worked out in your browser.
Enter a date to estimate the due date.
This is an educational estimate, not medical advice. Only about 4% of babies are born on the exact due date. Your doctor or midwife will confirm your due date, usually with an ultrasound.
How it works
The default method is Naegele’s rule: a due date of 280 days (40 weeks) after the first day of your last menstrual period. This assumes a 28-day cycle with ovulation around day 14, which is why the tool lets you adjust the cycle length — a longer cycle pushes the due date later by the same number of days.
If you know your conception or ovulation date, switch modes: the due date is then 266 days (38 weeks) after conception, and the last-period date is estimated two weeks earlier. Either way, gestational age is counted in weeks and days from the last-period date — the convention doctors use, which is why you are considered “2 weeks pregnant” at conception.
The result includes the start of the second trimester (14 weeks) and third trimester (28 weeks), and your current progress as of today. These are estimates: only about 4% of babies arrive exactly on the due date, and a dating ultrasound is more accurate, especially with irregular cycles.
Practical examples
From a last period on 1 January
With a 28-day cycle, the estimated due date is about 8 October (280 days later), conception is around 15 January, and the third trimester begins in early July.
A longer 32-day cycle
Same last period of 1 January but a 32-day cycle moves the due date four days later, to roughly 12 October — because ovulation, and so conception, happens later in a longer cycle.
From a known conception date
If an ovulation tracker put conception on 1 March, switch to conception mode: the due date is about 22 November (266 days later), independent of cycle length.
Frequently asked questions
How is the due date calculated?
By Naegele’s rule: 280 days (40 weeks) from the first day of your last menstrual period, assuming a 28-day cycle. From a conception date it is 266 days. The tool adjusts for your cycle length in last-period mode.
Why count from my last period and not conception?
Conception is hard to pin down, but the last period is a clear, memorable date. Obstetric convention counts pregnancy from the last period, so gestational age is about two weeks more than the time since conception — you are “4 weeks pregnant” roughly two weeks after conception.
How accurate is the estimate?
It is a guide, not a guarantee. Only around 4% of babies are born on their exact due date, and most arrive within a week or two either side. A first-trimester dating ultrasound is more precise, particularly if your cycles are irregular.
What if my cycle isn’t 28 days?
Enter your average cycle length. Ovulation is assumed to occur about 14 days before your next period, so a 32-day cycle shifts the due date four days later and a 25-day cycle three days earlier. For very irregular cycles, rely on an ultrasound date.
What does gestational age mean?
It is how far along the pregnancy is, expressed in weeks and days counted from the last menstrual period. “12 weeks + 3 days” means 12 full weeks plus three days have passed since that date.
When do the trimesters start?
By the common convention this tool uses, the first trimester runs to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 to birth. Different sources draw the lines slightly differently.
Can it tell me the sex of the baby or anything medical?
No. It only estimates dates from the information you enter. It cannot assess the health, sex or development of the baby — those require medical tests and scans.
Is my information kept private?
Yes. The dates you enter never leave your device — the calculation runs entirely in your browser and nothing is stored or uploaded. Reload the page and it is cleared.
Should I rely on this instead of seeing a doctor?
No. This is an educational estimate to give you a rough timeline. Your midwife or doctor will confirm your due date, usually with an ultrasound, and monitor the pregnancy — always follow their guidance.
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