Pregnancy Due Date Calculator - Free Online Tool

Milestones
Milestone	Date
End of first trimester (13 weeks)	2026-08-31
Anatomy scan window (20 weeks)	2026-10-19
End of second trimester (27 weeks)	2026-12-07
Full term begins (37 weeks)	2027-02-15
Estimated due date (40 weeks)	2027-03-08
42 weeks	2027-03-22

About the Pregnancy Due Date Calculator

A due date is calculated with Naegele rule: 280 days, or 40 weeks, from the first day of the last menstrual period. That convention counts pregnancy from before conception actually happened, which is why the first two weeks of the count are technically pre-conception. If you know the conception or ovulation date instead, the tool counts 266 days from there.

Cycle length matters because the standard 280 days assumes ovulation on day 14. A longer cycle shifts ovulation later and pushes the due date back by the difference, which this calculator applies automatically. An early dating ultrasound remains the most accurate estimate and will usually override a date calculated from the calendar.

How to use the Pregnancy Due Date Calculator

Choose whether you are working from the last period, a conception date or a due date you already have.
Enter the date, and your average cycle length if it differs from 28 days.
Read the due date, how many weeks along you are today, and the milestone table.

Frequently asked questions

How is a due date calculated?

Naegele rule adds 280 days to the first day of the last menstrual period, adjusted for cycle length. That is 40 weeks, or roughly nine months and one week.

How accurate is a due date?

Only about 4 percent of babies are born on the estimated date, and around 80 percent arrive within the two weeks either side. Births from 37 to 42 weeks are considered normal term.

What if my cycle is not 28 days?

Ovulation typically happens about 14 days before your next period, so a 32-day cycle shifts everything four days later. Enter your average cycle length and the calculator adjusts the due date.

Why does pregnancy count from the last period?

Because the date of the last period is usually known while the exact moment of conception is not. It adds about two weeks to the count compared with fertilisation age.