How are pregnancy weeks counted?

From the first day of your last period, not from conception. That is why you are already about two weeks pregnant on paper on the day you conceive, and why a full pregnancy is counted as 40 weeks.

It means 14 completed weeks and 6 days. You are then in your 15th week of pregnancy, and tomorrow you will be 15+0. Midwives, doctors and scan reports all use this notation.

When does each trimester start?

The first trimester runs from week 0 to the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 until the birth. Some books and countries draw the lines a week earlier or later, so do not worry if your app says something slightly different.

My scan gives a different week. Which one is right?

Go by the scan. An early scan measures the baby and is more reliable than the date of a last period, especially with irregular cycles. Choose "Due date" in the calculator and enter the date from your scan to see the matching week.

Why do baby sizes differ between apps and books?

Until about 20 weeks babies are measured from head to bottom, because the legs are curled up; after that, usually from head to heel, which gives a much bigger number. Sources also use different averages, and healthy babies vary, so the sizes here are a rough guide.

Pregnancy week calculator: how far along am I?

How pregnancy weeks are counted

Pregnancy is counted from the first day of your last period, not from the day you conceived. Almost everyone knows when their last period started, while the exact day of conception is rarely known, so midwives and doctors everywhere use the same starting line. This is called gestational age.

Conception usually happens around two weeks after that first day, when you ovulate. So on paper you are already two weeks pregnant at conception, and four weeks pregnant around the time your next period would have been due. A full pregnancy is counted as 280 days, or 40 weeks, ending on the estimated due date.

If you already have a due date, for example from a dating scan, the calculator works backwards: it takes the due date, subtracts 280 days to find the starting point, and counts from there. A scan date is usually more accurate than a period date, so use it when you have one.

What the result tells you

The result shows how many completed weeks and days you are today. Your notes will write this as weeks plus days, for example 14+6. That means 14 full weeks and 6 days, and you are in your 15th week. This is a common source of confusion: "14 weeks pregnant" covers the seven days from 14+0 to 14+6.

You also see:

Your trimester and how far through the 40 weeks you are, as a bar.
The days left until your due date, or how many days past it you are.
Your due date, or the date your pregnancy started on paper if you entered a due date.
The time since conception, which is about two weeks less than the pregnancy week. Some countries and older books count from conception, so this helps when two sources seem to disagree.
A short note about this week: what is developing and roughly how big the baby is.

A worked example

Say the first day of your last period was 1 January 2026, and today is 15 April 2026. That is 104 days later. 104 days is 14 weeks and 6 days, so you are 14+6, in the second trimester. The due date is 280 days after 1 January, which is 8 October 2026, and 176 days are left. Conception most likely happened around 15 January, so about 12 weeks ago.

If you enter 8 October 2026 as your due date instead, you get exactly the same answer.

The three trimesters

First trimester, weeks 0 to 13: the baby's organs, muscles, limbs and bones all form. By the end of week 12 the baby is fully formed and from then on mainly grows. Nausea and tiredness are often at their strongest.
Second trimester, weeks 14 to 27: the baby grows fast, starts to hear, and its movements become stronger. Most parents first feel the baby move somewhere between 16 and 24 weeks, and a detailed anomaly scan is usually offered around 18 to 22 weeks.
Third trimester, week 28 until birth: the baby gains most of its weight, the lungs and brain mature, and most babies turn head down. From 37 weeks a baby is early term, from 39 weeks full term, and from 41 weeks late term.

About the weekly notes and sizes

The weekly notes follow the milestones described in week-by-week guides such as the NHS guide. They are written to be careful: development is a gradual process, so a milestone in one week often starts a little earlier or finishes a little later.

Sizes are averages. Until about 20 weeks, babies are measured from the top of the head to the bottom (crown to rump), because the legs are folded. From about 20 weeks, guides usually switch to head to heel, which is why the length seems to jump. Weights are estimates of average fetal growth and differ a little from one source to another. Your own baby may be bigger or smaller and still perfectly healthy; your scans and your midwife are the best guide.

From around the middle of pregnancy, get to know your baby's pattern of movements. If the movements slow down, stop or change, contact your midwife or maternity unit straight away, at any time of day or night. Do not wait until the next day.

Limits of this calculator

The calculator assumes a regular cycle with ovulation about two weeks after your period started. If your cycles are long or irregular, if you are unsure of the date, or if you had bleeding in early pregnancy, the period-based week can be off by a week or more. Your dating scan settles this, and your midwife or doctor will tell you which week you are in and what to expect at each check-up.