How many weeks pregnant am I?

Enter the first day of your last period, your due date, or your conception date and the calculator counts the weeks and days from the start of the 40-week pregnancy timeline to today.

Why is 20 weeks and 3 days called week 21?

Your completed weeks and the week you are currently in differ by one. After 20 completed weeks you are in your 21st week, in the same way that a baby who is 20 months old is in their 21st month of life.

How many weeks is 9 months of pregnancy?

A full-length pregnancy is about 40 weeks. That is more than nine calendar months of 4 weeks each, because the average calendar month is a little over four weeks long, so pregnancy months are only an approximation.

Can I calculate my pregnancy week from my due date?

Yes. Choose Due date, enter your estimated due date, and the calculator counts back 40 weeks to find where you are in the pregnancy.

What does full term mean?

Under commonly used definitions, 37 to 38 weeks and 6 days is early term, 39 weeks to 40 weeks and 6 days is full term, 41 weeks to 41 weeks and 6 days is late term, and 42 weeks onward is post-term.

Pregnancy & Fertility

Pregnancy Week Calculator

Enter your last period, due date, or conception date to see exactly how many weeks and days pregnant you are, which week and trimester you are in, and how long until your due date.

Your calculation is performed in your browser.

Enter your last period, due date, or conception date to see how many weeks pregnant you are.

This result is an estimate for informational and educational purposes. It isn’t a diagnosis and isn’t a substitute for advice from a doctor, registered dietitian, or other qualified professional. See our methodology and sources for the formula used and its limitations.

How it works

Pregnancy is counted in weeks from the first day of your last menstrual period (LMP), and a full-length pregnancy is 40 weeks. The calculator works out your due date from whichever date you enter, then counts the days from the start of that 40-week timeline to today, or to any date you choose, and converts them into weeks and days.

Formula & methodology

The calculator measures gestational age from the start of the 40-week timeline that ends on your due date. For a 28-day cycle that start is your LMP. The week you are currently in is always one more than your completed weeks, so 20 weeks and 3 days is the 21st week of pregnancy. Pregnancy months use a common week-to-month convention, so they are approximate.

Gestational ageDays pregnant = Date checked − (Due date − 280 days)

Weeks and daysWeeks = ⌊ Days pregnant ÷ 7 ⌋, Days = Days pregnant mod 7

Current weekCurrent week = Completed weeks + 1

Due date (from LMP)Due date = LMP + 280 days + (cycle length − 28)

Due date (from conception)Due date = Conception date + 266 days

What each term means

LMP: The first day of your last menstrual period.
Cycle length: Days from the first day of one period to the first day of the next. Defaults to 28.
Mod: The remainder left after dividing by 7.

Trimesters by gestational age

Trimester	Weeks
First	0 weeks 0 days – 13 weeks 6 days
Second	14 weeks 0 days – 27 weeks 6 days
Third	28 weeks 0 days – birth

Approximate pregnancy month by week

Month	Pregnancy weeks
1	1 – 4
2	5 – 8
3	9 – 13
4	14 – 17
5	18 – 22
6	23 – 27
7	28 – 31
8	32 – 36
9	37 – 40

Worked example

If the first day of your last period was January 1, 2026 and your cycle is 28 days, then on May 24, 2026 you are 20 weeks and 3 days pregnant. That is the 21st week of pregnancy, in the second trimester and roughly month 5. Your due date is October 8, 2026, which is 19 weeks and 4 days away.

Common uses

—Finding out exactly how many weeks and days pregnant you are today.
—Checking which week you will be in on a future date, such as a scan, appointment, or trip.
—Working out your pregnancy week when you only know your due date.
—Seeing which trimester you are in and how long until the next one.

Important notes

—Pregnancy weeks are counted from your last period, so you are already considered about two weeks pregnant when conception happens.
—Pregnancy months are approximate. A pregnancy is about 9 calendar months but 40 weeks, so a pregnancy month is not the same as a calendar month, and sources divide the weeks slightly differently.
—If an early ultrasound gives a different due date, your doctor or midwife will usually use the ultrasound dating. Follow the dates your care provider gives you.
—This calculator is not intended for pregnancies conceived with IVF. Your clinic will give you dating based on your embryo transfer date.