Everyday Tools
Pregnancy Due Date Calculator
Enter the first day of your last menstrual period to see the estimated due date, how far along you are, and the trimester milestones.
Calculation inputs
The standard estimate adds 280 days (40 weeks) to the first day of the last period.
Results
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Complete the fields and select Calculate to show results here.
Estimates only, for general information. Not medical advice — confirm dates with your healthcare provider.
About this calculator
This tool estimates a pregnancy due date and current gestational age from the first day of your last menstrual period. It gives a general timeline that many people use to track trimester milestones and plan ahead.
How the calculation works
- Due date = first day of last period + 280 days (40 weeks)
- Cycles longer or shorter than 28 days shift the due date by the difference
- Gestational age counts from the first day of the last period, not from conception
Notes and assumptions
Only about 4% of babies arrive on the estimated due date; most births fall within two weeks either side.
How it works in plain English
The most common way to estimate a due date is Naegele's rule, which adds 280 days, or 40 weeks, to the first day of the last menstrual period. This assumes a fairly typical 28-day cycle with ovulation occurring around day 14, which is why the calculator lets you adjust for a longer or shorter average cycle length.
If your cycle runs longer or shorter than 28 days, ovulation likely happened correspondingly later or earlier, so the calculator shifts the due date estimate by the difference between your entered cycle length and the 28-day baseline.
Gestational age, or how far along the pregnancy is, is calculated as the number of days elapsed since the last period, converted into weeks and days. The pregnancy is then grouped into trimesters, which are simply calendar-based thirds of the pregnancy used to track typical developmental stages and common milestones.
The formula
- Due date = last menstrual period + 280 days + (average cycle length - 28)
Worked example
If the first day of your last period was March 1st and your cycle averages 28 days, the calculator adds 280 days to land on an estimated due date around December 6th. If your cycle instead runs 32 days, four days longer than the 28-day baseline, the estimate shifts to about December 10th. Checking today's date against the last period also tells you how far along you are — for example, 98 elapsed days works out to 14 weeks 0 days, placing you at the start of the second trimester.
Typical trimester weeks in a pregnancy
| Trimester | Approximate weeks |
|---|---|
| First trimester | Weeks 1 through 12 |
| Second trimester | Weeks 13 through 26 |
| Third trimester | Weeks 27 through 40 |
Trimester ranges follow commonly used obstetric conventions based on a 40-week pregnancy.
Frequently asked questions
How accurate is a due date calculated this way?
It is a general estimate, not a guarantee. Only about 1 in 20 babies arrive exactly on the calculated due date, and full-term delivery is generally considered anywhere from about three weeks before to two weeks after that date.
Why does an ultrasound sometimes give a different due date?
Early ultrasound measurements of fetal size can be more precise than a last-period estimate, especially if your cycle is irregular or you are unsure of the exact start date. A healthcare provider will typically use whichever estimate is considered more reliable for your situation.
What if I don't remember the exact first day of my last period?
The estimate depends on that date being reasonably accurate, so an approximate guess will shift the result by a similar margin. A healthcare provider can help refine the estimate using an ultrasound if the last period date is uncertain.
Does irregular cycle length make this estimate less reliable?
Yes. The calculation assumes ovulation happens at a predictable point in the cycle, so irregular or unusually long or short cycles make the last-period method less precise than it would be for someone with regular cycles.
Is this tool a substitute for prenatal care?
No. It offers a general timeline for planning purposes only, based on a simple date calculation rather than any clinical exam or imaging. Pregnancy monitoring, dating, and health decisions should be handled with a qualified healthcare provider who can account for your individual medical history, cycle patterns, and any relevant test or ultrasound results over the course of the pregnancy.