๐คฐ Pregnancy Week Calculator
Enter your last period date to calculate
This estimate uses Naegele's rule and is for reference only. Follow your doctor's guidance for actual care.
Enter your last menstrual period to calculate your current gestational age and an estimated due date. Useful as a reference alongside your prenatal checkup schedule or when thinking about when to share the news.
How to use
- Enter the first day of your last menstrual period.
- Your current gestational age and trimester are shown automatically.
- Check your estimated due date and the number of days remaining.
How the calculation works
Pregnancy is dated from the first day of the last menstrual period, counted as week 0, day 0. Conception usually happens about two weeks later, but since that date is hard to pin down, medicine conventionally counts from the last period instead. The due date is 280 days after the start of the last period (40 weeks, 0 days), known as Naegele's rule. It assumes a 28-day cycle with ovulation on day 14. The current gestational age is the number of days since the last period divided by 7: the quotient is weeks and the remainder is days. Trimesters are shown as early pregnancy for weeks 0โ15, mid pregnancy for 16โ27, and late pregnancy from week 28, a common Japanese convention. Other sources draw the boundaries differently; medical definitions often end the first trimester at 14 weeks.
Worked example
Last period starting 1 January 2026 Due date: 1 January 2026 + 280 days = 8 October 2026 On 1 March 2026, 59 days have passed: 59 รท 7 = 8 remainder 3, so 8 weeks 3 days (early pregnancy).
Things to be aware of
- If your cycle is longer or shorter than 28 days, or irregular, the real gestational age will differ.
- Clinics often adjust gestational age and the due date based on ultrasound measurements. Follow your doctor's assessment.
- Relatively few babies arrive on the due date itself; a few weeks either side is considered normal.
- This is an estimate. If you have concerns about your health, consult a medical professional.
FAQ
How is the due date calculated?
Using Naegele's rule โ 280 days (40 weeks 0 days) after the first day of your last menstrual period.
What if my doctor gave me a different due date?
This tool uses a general formula as a reference. An ultrasound-based estimate from your doctor is more accurate, so always defer to your healthcare provider's assessment.
How are the trimesters defined?
It follows the common convention of weeks 0-15 as the first trimester, 16-27 as the second, and 28 onward as the third.
Is this accurate if my cycle isn't 28 days?
This formula assumes a standard 28-day cycle. If your cycle differs significantly, the estimate may drift from your actual gestational age, so confirm with your healthcare provider.
Can I use this to plan prenatal checkup timing?
Yes โ knowing your current gestational age gives you a rough sense of upcoming checkup timing, though you should always follow your provider's actual schedule.