How is the due date calculated?

With Naegele's rule: add 280 days (40 weeks) to the first day of your last period, which is the same as subtracting 3 months and adding 7 days.

Can the due date change?

Yes. An ultrasound before 14 weeks is the most accurate dating method and may move the date by several days. Later scans are less reliable for dating.

What do the trimesters mean?

The first trimester runs to the end of week 12, the second from week 13 to 27, and the third from week 28 until birth. Each trimester is monitored for different developments and tests.

🤰 Pregnancy Week Calculator

Calculate current pregnancy weeks and due date from last menstrual period.

Last Menstrual Period Start Date

How Pregnancy Weeks Are Counted

Pregnancy is dated from the first day of your last menstrual period, not from conception, because the exact day of ovulation is usually unknown. That convention adds about two weeks before conception, which is why week 4 is roughly when a period is missed. A full-term pregnancy is 40 weeks from the last period, yet only about 4% of babies arrive exactly on the due date — most births fall between 37 and 42 weeks.

Pregnancy Week Calculation

Based on the last menstrual period (LMP), add 280 days (40 weeks) to get the due date.

Due Date = Last Period Start Date + 280 days

Naegele's rule: subtract 3 months from the first day of the last period and add 7 days. For a last period starting on 5 May, the due date is 12 February of the following year. An ultrasound in the first trimester can shift the estimate by a few days.

Naegele's Rule

Subtract 3 from the month and add 7 to the day of the start date.

Due Date = (Month - 3), (Day + 7)

Worked Examples

Last period 5 May → 5 May − 3 months = 5 February, + 7 days = 12 February

At 28 weeks you are in the third trimester with about 84 days to go.

Pregnancy Period Classification

Period	Weeks
Early (Weeks 1-12)	1 ~ 12 weeks
Mid (Weeks 13-27)	13 ~ 27 weeks
Late (Weeks 28-40)	28 ~ 40 weeks

The Due Date Is an Estimate

The estimate assumes a 28-day cycle with ovulation on day 14. With longer or irregular cycles the date shifts, and only an early ultrasound confirms gestational age. Multiple pregnancies, IVF transfers and an unknown last-period date are dated differently, so follow your clinician's figures.

Frequently Asked Questions

How is the due date calculated?: With Naegele's rule: add 280 days (40 weeks) to the first day of your last period, which is the same as subtracting 3 months and adding 7 days.
Can the due date change?: Yes. An ultrasound before 14 weeks is the most accurate dating method and may move the date by several days. Later scans are less reliable for dating.
What do the trimesters mean?: The first trimester runs to the end of week 12, the second from week 13 to 27, and the third from week 28 until birth. Each trimester is monitored for different developments and tests.