Pregnancy Due Date Calculator

How this due date calculator works

Enter the first day of your last menstrual period, or your conception date if you know it, and this calculator estimates your due date, your current week of pregnancy, and your trimester using the same standard method doctors have used for decades.

The formula: Naegele’s rule

From last menstrual period (LMP): Due date = LMP + 280 days (40 weeks)
From conception date: Due date = Conception date + 266 days (38 weeks)

Naegele’s rule assumes a typical 28-day menstrual cycle with ovulation around day 14. It’s the standard estimation method used in obstetrics, though it’s an estimate, not a guaranteed date.

Step-by-step

Identify the first day of your last menstrual period (or your conception date, if known).
Add 280 days to the LMP date (or 266 days to the conception date).
That date is your estimated due date.
To find your current week, divide the number of days since your LMP by 7.

Worked example

Last menstrual period started on January 1st:

Due date: January 1st + 280 days = October 8th
At 10 weeks pregnant (70 days since LMP): still in the first trimester (weeks 1-13)
At 20 weeks: second trimester (weeks 14-27)
At 30 weeks: third trimester (weeks 28-40+)

Common misunderstandings

Treating the due date as exact. Only a small percentage of babies are born on their exact estimated due date; most arrive within two weeks before or after.
Assuming a standard 28-day cycle when yours is shorter or longer, which shifts the real due date earlier or later than this simple calculation suggests.
Ignoring your doctor’s ultrasound-based estimate, which is generally more accurate than LMP-based calculations, especially in early pregnancy.

Tips

Use your doctor’s ultrasound dating as the authoritative estimate once you’ve had a dating scan.
Track your own cycle length if it differs from 28 days, and mention this to your provider for a more personalized estimate.
Remember this is a planning tool, not a medical diagnosis; always follow your healthcare provider’s guidance.

How accurate is a due date calculated this way?

It’s a widely used estimate, but only about 5% of babies are born exactly on their calculated due date. Most arrive within a two-week window on either side.

Why might my doctor give a different due date than this calculator?

Doctors often adjust the estimate based on an early ultrasound, which measures the fetus directly and can be more accurate than a calculation based solely on your last period, especially if your cycle isn’t exactly 28 days.

Can I calculate my due date without knowing my last period?

Yes, if you know your conception date, use that mode instead, it adds 266 days rather than 280.

What’s the difference between gestational age and fetal age?

Gestational age is counted from your last menstrual period and is what doctors typically use. Fetal age is counted from actual conception, roughly two weeks less than gestational age.