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Pregnancy Week Calculator

Answer "how far along am I?" in seconds. Enter the first day of your last period, your due date or your conception date to see how many weeks and days pregnant you are, your trimester, due date and a dated list of milestones.

Calculate from

First day of last periodDue dateConception date

Date

Last period: the first day it started.

Average cycle length

days

Used with the last period date (20–45).

How far along on

Enter your values to see the result.

How to use the Pregnancy Week Calculator

Choose what you know: the first day of your last period, your due date or your conception date.
Pick the date, and set your usual cycle length if you use the last-period method.
Leave "How far along on" as today, or choose another date to plan ahead.
Press Calculate weeks to see weeks and days, trimester and due date.
Check the milestone table for the dates of common scans and checkups, and download it as CSV.

How the calculation works

From your last period (Naegele's rule): due date = LMP + 280 days, adjusted by (cycle length − 28) days.
From a due date: pregnancy start = due date − 280 days.
From conception: pregnancy start = conception − 14 days, so the due date is conception + 266 days.
Weeks pregnant = (date − pregnancy start) ÷ 7; the remainder is the extra days.

Example: a last period starting March 1 with a 28-day cycle gives a due date of December 6 (in a non-leap year). On May 24 that is 84 days, or exactly 12 weeks 0 days.

Trimesters at a glance

Stage	Weeks	Common checks
First trimester	0 – 13w6d	Confirmation, dating scan, first prenatal visit, optional NIPT and nuchal scan
Second trimester	14w0d – 27w6d	Anatomy scan (18–22 weeks), glucose screening (24–28 weeks)
Third trimester	28w0d – birth	Growth checks, group B strep test (36–37 weeks), birth planning
Early term	37w0d – 38w6d
Full term	39w0d – 40w6d
Late term	41w0d – 41w6d
Post-term	42w0d and later

Term definitions follow ACOG. Screening schedules vary by country and by provider.

Frequently asked questions

How are pregnancy weeks counted?

Pregnancy is dated from the first day of your last menstrual period (LMP), about two weeks before conception. So at the moment of conception you are already counted as about 2 weeks pregnant, and a full-term pregnancy is 40 weeks (280 days) from the LMP.

What does 12w3d mean?

It means 12 completed weeks and 3 days of gestation. You are in the 13th week of pregnancy at that point.

When does each trimester start?

The first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks until birth.

Why does my cycle length matter?

The 280-day rule assumes ovulation on day 14 of a 28-day cycle. With longer cycles ovulation tends to happen later, so the calculator moves the dates by the difference: a 32-day cycle shifts the due date 4 days later.

My ultrasound gives a different date. Which is right?

An early ultrasound, especially in the first trimester, is usually the most accurate way to date a pregnancy. If your provider has changed your due date based on a scan, use the due date option with that date.

Is this calculator medical advice?

No. It gives estimates for information only. Always follow the dates and advice of your midwife or doctor.

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