Pregnancy Due Date Calculator
Estimate your due date and current week of pregnancy from your last menstrual period or ovulation date. Everything runs in your browser — your dates are never sent anywhere. This is an estimate, not medical advice.
The dates you enter are processed entirely in your browser and are never sent anywhere.
Enter a date and press "Calculate".
- Estimated due date
- Current gestational age
- Days until due date
The baseline date (week 0, day 0) is after today, so the current gestational age can't be calculated yet. Only the due date is shown.
The due date has passed. Due dates are estimates, and being two weeks early or late is common.
This result is a statistical estimate, not medical advice. For your actual gestational age and due date, please rely on your obstetrician's diagnosis, which typically includes an ultrasound.
How to use
- Choose "From last menstrual period" or "From ovulation date". If your cycle is irregular, the ovulation method is more accurate.
- Enter the first day of your last period (or your ovulation date). If you know your average cycle length, you can change it from the default of 28 days.
- Press "Calculate". Your estimated due date, current gestational age, and trimester appear immediately.
How it works
This is an estimate, not a diagnosis
A due date is a statistical estimate — only about 5% of babies are born on their exact due date, and most arrive within about two weeks before or after it. This result is not a substitute for medical advice. For your actual gestational age and due date, please rely on your obstetrician's diagnosis, which typically includes an ultrasound.
Last menstrual period method (Naegele's rule)
The due date is calculated as the first day of your last menstrual period (LMP) plus 280 days (40 weeks). This is known as Naegele's rule and is the standard method used in obstetric practice worldwide. Gestational age is counted from the first day of the LMP as "week 0, day 0" — conception hasn't actually happened yet at that point, but this is the conventional way pregnancy is dated everywhere, including Japan.
Adjusting for cycle length
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your average cycle is longer or shorter, this calculator shifts both the calculation baseline and the due date by the difference between your cycle length and 28 days. For example, if your cycle is 32 days, ovulation likely happens about 4 days later than the 28-day assumption, so both the baseline and the due date move 4 days later.
Ovulation date method
If you know your ovulation date (or conception date), the due date is calculated as 266 days after that date. This is mathematically consistent with the LMP method (since ovulation is assumed to happen 14 days after the LMP), and tends to be more accurate for people with irregular or unknown cycle lengths.
Current gestational age and trimester
The number of weeks and days shown is the time elapsed from the calculation baseline ("week 0, day 0") to today. Trimesters follow the widely used clinical definition: weeks 0–13+6 are the first trimester, weeks 14–27+6 the second trimester, and week 28 onward the third trimester.
About your data
All calculations happen entirely in your browser. None of the dates you enter are ever sent to a server.
FAQ
Are the dates I enter sent anywhere?
No. All calculations happen entirely in your browser, and no data is ever sent to any external server or API.
How accurate is this due date?
It's a statistical estimate. Only about 5% of babies are born on their exact due date, and most arrive within about two weeks before or after it. For your actual gestational age and due date, please rely on your obstetrician's diagnosis, which typically includes an ultrasound. This result is not a substitute for medical advice.
My cycle is irregular. Which method should I use?
If you know your ovulation date, use "From ovulation date" — it doesn't assume anything about your cycle length, so it tends to be closer to reality for irregular cycles. The LMP method assumes a 28-day cycle, so if your cycle differs a lot from that, use it together with your actual average cycle length, or check the ovulation method instead.
My doctor gave me a different due date. Which one should I trust?
Trust your doctor's estimate. Obstetricians typically refine the due date using ultrasound measurements of the fetus, which is more precise than a date calculated from your last period or ovulation date alone. This tool's result is a simple estimate based only on the dates you enter.
How much does cycle length affect the result?
With the last-menstrual-period method, the due date shifts by the same number of days that your cycle differs from 28. For example, a 32-day cycle shifts the estimated due date about 4 days later than the default. If you're not sure of your cycle length, or it's irregular, consider using the ovulation method, or compare this estimate with your obstetrician's diagnosis.
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