Due Date Calculator

Estimate a pregnancy due date with Naegele’s rule: 280 days, or 40 weeks, from the first day of the last menstrual period. The calculator also shows how far along you are on a chosen reference date as weeks plus days, which trimester that falls in, and a milestone timeline from the end of the first trimester through viability and full term to the estimated date itself. It is a planning aid, not a diagnosis, and it runs entirely in the browser.

How to Use

• Enter the first day of your last menstrual period in the LMP field. The calculator defaults to roughly three months ago so you can see a result immediately.
• Leave the “as of” date at today, or change it to see how the numbers looked or will look on any other day.
• Read the three headline values: the estimated due date, the current week as weeks and days, and the trimester.
• Scan the milestone list. Entries already passed are marked with a check, and each carries its week number and calendar date.
• The yellow disclaimer below the results is always visible, because the estimate is not a substitute for medical advice.

The 280-Day Rule Step by Step

Naegele’s rule starts from the first day of the last menstrual period, adds one year, subtracts three months, and adds seven days. The calculator does the arithmetic directly instead: due date equals LMP plus 280 days. The convention exists because the actual date of conception is usually unknown, while the LMP is a date most people can recall. Assuming ovulation near day 14 of a 28-day cycle puts conception about two weeks after the LMP, leaving roughly 266 days of gestation. The tool never estimates conception, because many viable pregnancies and all IVF transfers would make that figure misleading.

Worked Example

Take an LMP of 5 January 2026. Adding 280 days lands on 12 October 2026. Counting forward, 31 days brings us to 5 February, 59 to 5 March, 90 to 5 April, and so on; the 280th day is 12 October. If the reference date is 5 April 2026, the interval since the LMP is 90 days. The calculator reports 12 weeks 6 days and places the pregnancy in the first trimester. If the reference date is 5 May 2026 instead, the interval is 120 days, or 17 weeks 1 day, and the trimester reads second. The due date itself does not move; only the progress figures change.

Milestones and Trimester Boundaries

Each milestone is simply the LMP plus a fixed number of weeks. Week 13 marks the end of the first trimester, week 24 is the conventional point of viability, week 27 ends the second trimester, week 39 is full term, and week 40 is the estimated due date. The trimester indicator, however, is computed from the total number of days rather than the milestones: it stays at first trimester until 14 completed weeks, switches to second until 28 completed weeks, and third from then on. That means on the single day between week 13 and week 14 you can see “end of first trimester” already checked while the trimester label still reads first. The two conventions are both common, so the small overlap is intentional rather than a bug.

Accuracy and Limits

The estimate is a center point, not an appointment. Only a small percentage of babies are born on the exact due date; normal delivery spans roughly weeks 37 to 42. The tool has no cycle-length input, so a person with a regularly long or short cycle will get an estimate that is consistently late or early. It also does not use ultrasound dating, which is the most reliable early method, does not handle multiple pregnancies, and does not accept an embryo-transfer date. It supports any dates the browser’s date input allows and will simply show the warning if the LMP is after the reference date. Treat the result as a starting point to discuss with a midwife or doctor, especially if your cycle is irregular or your care team’s estimate differs.

Tracking Progress Over Time

Pregnancy progress is usually discussed in completed weeks plus days, and the calculator follows that convention. On the reference date it divides the days since the LMP by seven: 100 days reads as 14 weeks 2 days, 200 days as 28 weeks 4 days, and 280 days as exactly 40 weeks. The progress bar expresses the same interval as a fraction of 280 days and is clamped between 0 and 100 percent, so it stops at full once the due date passes. Changing the reference date without touching the LMP moves only the week, day, trimester, and progress values, which makes it easy to see what a future appointment date corresponds to.

Practical Tips

Treat the milestone dates as planning anchors rather than deadlines. The end of the first trimester, the viability date, and full term are useful for booking antenatal appointments, arranging leave, and answering the “when are you due” question with a concrete date. Because the tool recalculates live, you can enter the LMP once and then move the reference date through the calendar to watch the week count change. If you already have an ultrasound-adjusted due date, work backwards by subtracting 280 days to recover the LMP date your provider is using, then compare the two. The calculator stores nothing, so if you want a record, write the dates down before closing the page.

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