“How far along am I?” is one of the first questions almost everyone asks after a positive test — and the answer involves a small counting quirk that trips people up. Once you understand how pregnancy weeks are measured, tracking your progress becomes much clearer, and you can see exactly where you stand at any moment.
Pregnancy is dated from your last period
Pregnancy is counted from the first day of your last menstrual period (LMP) — not from the day you conceived. Because ovulation and conception happen roughly two weeks after your period starts, this means weeks 1 and 2 you’re not yet pregnant at all. You’re technically counted as “two weeks along” around the time you actually conceive.
It feels backward, but the logic is practical: nearly everyone knows when their last period began, while very few know the exact day of conception. Dating from the LMP gives a consistent starting line, which is the same convention behind our pregnancy week calculator and due date calculator.
The 40-week, 280-day timeline
A full pregnancy runs about 40 weeks, or 280 days, from that first day of your last period. That total is why you’ll see pregnancy described in weeks all the way up to around 40, even though the baby has only been growing for roughly 38 of them.
How to count weeks and days
Pregnancy is tracked in completed weeks plus extra days, written like “10 weeks and 3 days” (often shortened to 10+3). That means you’ve finished 10 full weeks and are 3 days into your eleventh week. You don’t round up — being “almost 11 weeks” still reads as 10 weeks and something. Counting the exact days from your LMP to today gives you the number, and a calculator handles the arithmetic instantly.
The three trimesters
Pregnancy is divided into three stretches, each with its own rhythm and milestones.
| Trimester | Weeks | What’s happening |
|---|---|---|
| First | Weeks 1–13 | Major organs and body systems form; early symptoms like nausea and fatigue are common; the baby grows from a cluster of cells to roughly the size of a plum |
| Second | Weeks 14–27 | Often the most comfortable stretch; you may feel first movements; the baby grows quickly and an anatomy scan is usually offered |
| Third | Weeks 28–40 | The baby gains weight and matures its lungs; you prepare for birth; check-ups become more frequent toward the end |
The exact week each trimester begins can be described slightly differently from source to source, but the ranges above are the ones you’ll most commonly see.
How an ultrasound may adjust your dates
Your LMP-based count is a solid estimate, but an early ultrasound can refine it. In the first trimester, measuring the baby gives a very reliable read on gestational age. If the scan shows the baby measuring bigger or smaller than your dates predicted — by more than a few days — your provider may adjust your due date and how far along you’re counted. A first-trimester scan is considered the most accurate way to confirm dating, so it takes priority over calendar math if the two disagree.
Keeping track week to week
Once you know your starting point, following your progress is genuinely one of the joys of pregnancy — each week brings a new development to read about and look forward to. To see exactly how far along you are today, plug your last period into the pregnancy week calculator; it will show your current week and days at a glance.
The bottom line
You’re counted from your last period, which means the first couple of weeks predate conception; a full term is about 40 weeks; and progress is tracked in weeks plus days. An early scan can gently shift the numbers, and that’s completely normal. For anything that feels off or if you’re unsure of your dates, your provider can confirm exactly where you are and what to expect next.