“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.
Gestational age versus fetal age
This gap between “how pregnant you are” and “how old the baby is” has a name. Gestational age is the number counted from your last period — the figure you’ll hear at every appointment, see on your chart, and get from a week calculator. Fetal age (sometimes called conceptional or embryonic age) counts from the moment of conception and runs about two weeks behind. So at 12 weeks gestational age, the developing baby is closer to 10 weeks of actual growth.
Almost everything in mainstream pregnancy care — your due date, your scans, your milestones, the week-by-week updates you read — uses gestational age, so that’s the number worth anchoring to. Fetal age tends to surface mainly in embryology, fertility clinics, or the occasional app that reports both. If you ever see two different “weeks” numbers and they’re roughly two weeks apart, this is almost always why. When in doubt, ask which one a source is using; your provider always means gestational age unless they say otherwise.
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.
A worked example
Suppose your last period began on April 1 and today is June 17. Count the total days between them: 29 remaining days in April, 31 in May, and 17 in June, which comes to 77 days. Now divide by seven — 77 ÷ 7 is exactly 11 weeks, 0 days. If today were June 20 instead, you’d have 80 days, which is 11 weeks with 3 days left over, written 11 weeks and 3 days or 11+3. The whole method is just “total days since your LMP, expressed as weeks plus the remainder.” You never need to do this by hand — our pregnancy week calculator counts the days for you — but seeing the arithmetic once makes the “+3” notation click, and shows why a single date is all the information the count needs.
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.
What the milestones mean
Beyond the broad strokes in the table, a few markers within these stretches tend to carry real weight. The close of the first trimester is a milestone many people feel in their shoulders relaxing — organ formation is largely complete and early symptoms often begin to ease. Somewhere in the second trimester, usually between about 18 and 22 weeks, an anatomy scan takes a careful look at how the baby is developing, and it’s often around this window that first flutters of movement, called quickening, become noticeable. Deeper into pregnancy, viability and lung maturity become the themes: the third trimester is largely about the baby putting on weight and getting ready to breathe air. None of these are hard lines you cross at a stroke — they’re gradual transitions — but knowing roughly when they fall helps you understand what each appointment is checking for and why the pace of care picks up toward the end.
How irregular cycles affect your dates
The tidy counting above leans on a textbook 28-day cycle with ovulation around day 14. Plenty of people don’t run on that schedule, and that’s worth naming. If your cycles are consistently longer — say 34 days — you likely ovulate later than day 14, so an LMP-based count can read a touch further along than the pregnancy truly is; shorter cycles can tilt it the other way. Conditions like PCOS, irregular or unpredictable periods, a recent hormonal birth control method, or simply not being sure when your last period started all loosen the reliability of calendar-based dating.
This isn’t a problem to solve on your own, and it’s not a reason to worry — it’s exactly the situation an early ultrasound is made for. Because a first-trimester scan measures the baby directly, it sidesteps the guesswork about when you ovulated. If your cycles are irregular or your LMP is fuzzy, mention it to your provider; they’ll lean more heavily on a scan to pin down where you are.
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.