It’s one of the first things you want to know after a positive test — and one of the most confusing, because pregnancy isn’t counted the way you’d expect. Let’s clear it up.
How are pregnancy weeks counted?
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. Since ovulation (and conception) happens roughly two weeks after your period starts, this means you’re considered “2 weeks pregnant” around the time you actually conceive.
It sounds odd, but there’s a good reason: almost everyone knows when their last period began, while very few know the exact day of conception. Counting from the LMP gives a consistent starting line — and it’s the same method your doctor or midwife uses.
You’ll also see pregnancy written as weeks and days, like “8+4”. That means 8 completed weeks plus 4 days. Each new “week” of pregnancy ticks over on the same weekday as your last period began.
How do I count how many weeks pregnant I am?
To work it out yourself:
- Find the first day of your last period.
- Count the number of whole weeks from that day until today.
- That number is how many weeks pregnant you are; any leftover days are the “+days”.
For example, if your last period started on 1 March and today is 12 April, that’s 6 weeks and 0 days — so you’re 6 weeks pregnant. If today were 15 April, you’d be 6+3.
Counting by hand is easy to get slightly wrong, so our Pregnancy Week Tracker does it instantly from your last period — and tells you what’s happening with your baby that week.
Does pregnancy start at week 0 or week 1?
This trips a lot of people up. The very first day of your last period is the start of week 1, not week 0 — so during that first week you’re technically “1 week pregnant” even though conception hasn’t happened yet. By the time you actually conceive, around two weeks later, you’re already counted as 2 weeks pregnant.
That’s why the first positive test usually shows up at around 4 weeks (right when your period would be late), and why those earliest weeks feel like they’re counted before anything has really begun. It’s not an error — it’s just the shared starting line that keeps everyone’s dates consistent.
How many weeks vs months pregnant am I?
Weeks don’t divide neatly into calendar months, which is why providers use weeks. But here’s a rough guide to translate between the two:
| Weeks pregnant | Roughly | Trimester |
|---|---|---|
| 4 weeks | 1 month | First |
| 8 weeks | 2 months | First |
| 13 weeks | 3 months | First |
| 18 weeks | 4 months | Second |
| 22 weeks | 5 months | Second |
| 27 weeks | 6 months | Second |
| 31 weeks | 7 months | Third |
| 35 weeks | 8 months | Third |
| 40 weeks | 9 months | Third |
So a “40-week” pregnancy works out to a little over nine calendar months — not the “nine months” you often hear, which is part of why weeks are the clearer unit.
Why is my baby’s age different from the pregnancy week?
Because of LMP dating, your baby’s true age is about two weeks behind the pregnancy week:
- 6 weeks pregnant → embryo is ~4 weeks old
- 12 weeks pregnant → ~10 weeks old
This is completely normal and nothing to worry about — it’s just how the counting works. When a scan reports your baby’s size “in weeks,” it’s usually still using this same LMP-based system, so the numbers line up with what your provider tells you.
Which trimester am I in?
Pregnancy is split into three trimesters, each with its own milestones:
First trimester
Early development; many feel tired or queasy.
Second trimester
The “easier” middle; first kicks and the anatomy scan.
Third trimester
Baby grows quickly; you prepare for birth.
Most pregnancies last about 40 weeks from the LMP. A baby born between 37 and 42 weeks is considered full term, so your due date is the middle of a window — not a fixed deadline. If you’d like to see exactly when that window falls, our Due Date Calculator will estimate it.
How can I know exactly how far along I am?
For most people with regular cycles, counting from the last period is accurate enough. But the estimate is less reliable if:
- Your cycles are irregular or longer/shorter than 28 days.
- You’re not sure of your LMP date.
- You conceived soon after stopping hormonal birth control.
In those cases, an early dating ultrasound (usually between 8 and 14 weeks) is the most precise way to confirm how many weeks pregnant you are. It measures your baby directly, and if the scan differs from your LMP estimate by more than a few days, your provider will usually re-date the pregnancy from the scan. That’s routine and simply means the estimate has been refined.
In early pregnancy, this measurement is very reliable because all babies grow at a similar rate in the first trimester — so a scan at, say, 9 weeks can pin down your dates to within a few days. Later scans are less precise for dating, which is why the early scan is the one that sets your official weeks and due date.
Want to follow along week by week? The Pregnancy Week Tracker shows your current week, your trimester, and a friendly note on what’s developing.
This article is for general information only and isn’t medical advice. Your healthcare provider is the best source for questions about your pregnancy.