Pregnancy due date calculator

Formula

How it works

Naegele's rule adds 280 days to the first day of the last period (adjusted for a cycle length other than 28 days) to estimate the due date; the current week of pregnancy is the number of days since the last period divided by 7.

Worked example

The same calculation with the default values:

• Last period: day 5
• Last period: month 6
• Last period: year 2,026
• Average cycle length (days) 28
• As of: day 23
• As of: month 10
• As of: year 2,026

1. due date = last period date + 280 days + (28 − 28) = 280 days after the last period
2. weeks pregnant = 140 ÷ 7 = 20 weeks 0 days
3. trimester = 2

Result: Estimated due date: 2027-03-12 · Weeks pregnant: 20 · Extra days: 0

Common mistakes

• Treating the estimated due date as exact - only a small minority of births happen on the estimated date itself.
• Using an irregular or uncertain last-period date without adjusting the cycle length, or at all.
• Skipping clinical confirmation - ultrasound dating is generally more accurate than the last period alone, especially for irregular cycles.