Pregnancy Due Date Calculator
Estimate a pregnancy due date from the first day of the last menstrual period, with optional cycle-length adjustment, plus estimated gestational age today, trimester dates and an estimated conception window.
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About the Pregnancy Due Date Calculator
Pregnancy Due Date Calculator estimates a due date from the first day of the last menstrual period, using the standard 280-day (40-week) method for a 28-day cycle. If your usual cycle is longer or shorter, enter its length and the dates are adjusted by the difference from 28 days. Using today’s date on your device, it also shows the estimated gestational age, the estimated stage, estimated trimester dates and an estimated conception window.
Every date is an estimate. An ultrasound scan and clinical assessment may give different dates, and those take precedence. The tool does not provide medical advice, risk information, fertility predictions or emergency guidance — prenatal care should always be guided by a qualified healthcare professional. Nothing you enter leaves your browser.
How to use
- Enter the first day of your last menstrual period.
- Optionally change the usual cycle length (20–45 days; default 28).
- Click Calculate.
- Review the estimated due date, gestational age today, trimester dates and conception window.
- Copy or download the summary if you want to keep it, or Reset to start again.
Key features
- Standard 280-day due-date estimate from the last menstrual period.
- Optional cycle-length adjustment from 28 days.
- Estimated gestational age in weeks and days on your device’s current date.
- Estimated trimester date ranges and current stage.
- Estimated conception window, clearly marked as an estimate only.
- Validation of invalid dates, future dates and unrealistic cycle lengths.
Best practices
- Use the first day of full bleeding as the first day of your period.
- Enter your usual cycle length if you know it; otherwise leave 28.
- Remember that an ultrasound-based date from your care provider takes precedence.
- Use the dates for general planning, not for medical decisions.
- Contact a healthcare professional for prenatal care and any concerns.
Benefits
- A quick estimate of key pregnancy dates.
- Shows the method used so you can follow it.
- Handles leap years and month lengths correctly.
- Private — calculated in your browser with no account.
Frequently asked questions
How is the due date estimated?
By adding 280 days (40 weeks) to the first day of the last menstrual period. If your usual cycle is not 28 days, the difference is added or subtracted — for example, a 30-day cycle adds 2 days.
How accurate is the estimated due date?
It is an estimate. Many births happen in the weeks around the due date rather than on it, and an ultrasound scan or clinical assessment may change the estimated dates.
What is gestational age?
The length of the pregnancy counted from the first day of the last menstrual period (adjusted for cycle length here), shown in weeks and days. It is calculated from today’s date on your device.
How are trimesters defined?
This tool uses a common convention: weeks 0–13, weeks 14–27, and week 28 to the due date. Clinics may divide them slightly differently.
Is the conception window accurate?
It is an estimate only, centred on likely ovulation about (cycle length − 14) days after the start of the last period. The actual date can fall outside it. The tool does not make fertility predictions.
Does this replace prenatal care?
No. It provides general information only and is not medical advice. Prenatal care, test results and any concerns should be discussed with a qualified healthcare professional.