Pregnancy Week Calculator

Which week of pregnancy you are in, in the notation midwives and obstetricians use. It counts from the first day of your last period, not from conception.

You are

—

You are—

Trimester—

Estimated due date—

Days to go—

Weeks to go—

Progress—

Estimated conception—

How this was worked out

Indicative result. The real gestational age is set by the first-trimester scan, which is more reliable than any calculation from dates. This estimate does not replace medical care.

The formula

weeks = days since the last period ÷ 7

Where it comes from

Medical convention counts pregnancy from the first day of the last period because it is the only date known for certain: the exact moment of conception is rarely known. That adds about two “free” weeks at the start, and explains why a positive test already means four or five weeks.

How to work it out by hand

Count the days since the first day of your last period
Divide by seven: the whole part is the weeks
The remainder is the odd days
It is written as “12+3”: twelve weeks and three days

What is worth knowing

The plus-sign notation is standard in clinic: “12+3” means twelve completed weeks and three days. If your cycle is not 28 days, ovulation shifts and the whole count with it, which is why entering your real length matters. And something that surprises people: at week 4, when many women find out, the embryo has been developing barely two weeks.

Frequently asked questions

When do the weeks of pregnancy start counting?

From the first day of your last period, not from conception. That is about two weeks of difference.

What does 12+3 weeks mean?

Twelve completed weeks and three days. It is the notation midwives and obstetricians use.

How many weeks does a pregnancy last?

Forty from the last period. It counts as at term between 37 and 42.

What if my cycle is not 28 days?

Ovulation shifts and the whole count with it. Enter your real cycle length and the calculation adjusts.