You just got a positive test, and the very next thing you want to know is how far along you are. The catch: pregnancy is measured in weeks of gestation, and the count starts before you were even pregnant. No wonder it confuses people.
Why counting starts from your last period
Doctors count from the first day of your last period (LMP), not from the day you conceived. Ovulation usually happens about two weeks after your period begins, so by the time your period is late you are already "4 weeks pregnant", even though the embryo is only about two weeks old. It feels backwards, but starting from the LMP gives everyone one fixed line to count from. The pregnancy week calculator does the arithmetic for you.
Weeks, months and trimesters
A full-term pregnancy runs 40 weeks from your LMP, which is exactly why "nine months" never quite adds up. Those weeks split into three trimesters: the first covers weeks 1 to 13, the second weeks 14 to 27, and the third runs from week 28 to birth. When someone says they are "12 weeks", they usually mean 12 completed weeks and a few days on top, written as 12+3.
What if my cycle is irregular?
The LMP method assumes a tidy 28-day cycle with ovulation on day 14. Real cycles are rarely that tidy. If yours runs long, short, or unpredictable, the estimate can be off by several days, and an early ultrasound will date things more accurately (here is why the two can disagree). Do not be surprised if your provider shifts your dates after that first scan.
Turning weeks into a due date
Once you know your gestational age, your due date falls right out of it: it is simply the day you reach 40 weeks. The due date calculator shows that date and how many weeks you have left, and the pregnancy week guide walks you through what is happening along the way.
How to count the weeks yourself
If you would rather check by hand, it is simple. Count the days from the first day of your last period up to today, then divide by seven. The whole number is your completed weeks and the remainder is the extra days, so 87 days comes out to 12 weeks and 3 days, written 12+3. It is the same sum the calculator runs, just on paper, and a handy way to sanity-check an app whose number looks off.
Why the exact week matters for your care
Accurate dating is not just trivia. Your week sets the timing of almost everything that follows: the window for first-trimester screening, the anomaly scan at around 20 weeks, glucose testing later on, and the point at which a baby is considered term or overdue. If the dates are off by a week or two, those appointments can land at the wrong time. That is exactly why providers re-date by an early scan when it disagrees with your last period, and why it is worth knowing your week rather than guessing.