The moment a pregnancy test turns positive, one question takes over: how far along am I? It sounds like it should have a simple answer, but the way pregnancy is measured trips up almost everyone at first. You might assume the clock starts at conception, the day egg and sperm met. It doesn't. By long-standing medical convention, your pregnancy is dated from the first day of your last menstrual period (LMP), which is usually about two weeks before you actually conceived.
That quirk is why a doctor might tell you that you're "6 weeks pregnant" when conception happened only about four weeks ago. This guide breaks down exactly how the weeks are counted, where the trimesters begin and end, and why an early ultrasound sometimes nudges your due date in one direction or the other. By the end you'll be able to pinpoint how far along you are with confidence.
This article is for general informational and educational purposes only and is not medical advice. Always confirm your dating and any concerns with your obstetrician, midwife, or other qualified healthcare provider.
How Pregnancy Weeks Are Counted
Human pregnancy lasts about 40 weeks, or roughly 280 days, when measured from the first day of your last period. The American College of Obstetricians and Gynecologists (ACOG) uses this LMP-based system as the standard starting point for estimating a due date.
Here's the part that confuses people. Counting from your last period means the first two weeks of your "pregnancy" happen before you're even pregnant. During week 1 you're menstruating; around the end of week 2 you ovulate and conception can occur. So the calendar runs roughly like this:
- Week 1: The first day of your last period (no conception yet).
- Week 2: Ovulation and conception happen around the end of this week.
- Week 3: The fertilized egg travels and implants in the uterus.
- Week 4: A missed period; a home pregnancy test may turn positive.
This is why doctors and apps almost always speak in LMP terms. It gives everyone a consistent, easy-to-track starting point, since most people know when their last period began but can rarely pinpoint the exact day of conception. If you want the math done for you, our how far along am I calculator takes your last period date and returns your current week and day instantly.
Weeks Plus Days: Why "7+3" Matters
You'll quickly notice that providers don't just say "7 weeks." They say something like "7 weeks and 3 days," often written as 7+3 or 7w3d. That precision exists because a single week is a meaningful span in early development, and milestones, screening windows, and measurements are tied to exact gestational age.
The system works on a simple rule: you are a given number of completed weeks plus the extra days. If your last period started exactly 52 days ago, that's 7 full weeks (49 days) plus 3 extra days, so you're 7+3. You officially "turn" 8 weeks once that 56th day arrives. Think of it like a baby's age in the first months of life, where weeks matter far more than they will later.
This granularity becomes important for time-sensitive care. The first-trimester combined screening, for example, is typically done between 11 and 14 weeks, and certain blood tests and the anatomy ultrasound have their own windows. Being off by even a few days can affect whether a test falls inside its recommended range, which is one reason accurate dating matters so much.
The Three Trimesters: Where the Lines Fall
Pregnancy is traditionally divided into three roughly equal stretches called trimesters. Each comes with its own developmental themes and typical symptoms. Using the standard LMP-based weeks, here is where the boundaries sit:
| Trimester | Gestational Age | What's Happening |
|---|---|---|
| First trimester | 0 to 13 weeks 6 days | Major organs form; morning sickness and fatigue common; highest miscarriage risk, which drops sharply after a heartbeat is confirmed. |
| Second trimester | 14 weeks to 27 weeks 6 days | Often the "easier" stretch; you may feel the baby move; anatomy scan around 18 to 22 weeks; bump becomes visible. |
| Third trimester | 28 weeks to birth | Rapid weight gain and growth; more frequent checkups; preparing for labor and delivery. |
A few clarifications worth keeping straight. The first trimester runs through the end of week 13 (that is, up to 13 weeks and 6 days), and the second trimester begins at 14 weeks 0 days. The second trimester ends at 27 weeks 6 days, and the third trimester starts at 28 weeks 0 days and continues until your baby is born. While the exact cutoffs can vary slightly between sources, the 0 to 13w6d, 14 to 27w6d, and 28w-plus framework is the one most U.S. providers use.
A baby is considered full term at 39 weeks 0 days through 40 weeks 6 days, early term at 37 to 38 weeks 6 days, and post-term once you reach 42 weeks, according to ACOG terminology. So the popular "due date" at 40 weeks is the midpoint of a normal full-term window, not a hard deadline.
Gestational Age vs. Fetal Age: They're Not the Same
Here's where a lot of confusion creeps in. There are two different ways to count, and they differ by about two weeks:
- Gestational age is measured from the first day of your last period. This is the number your doctor, your chart, and nearly every pregnancy app use. When someone says you're "10 weeks pregnant," they almost always mean gestational age.
- Fetal age (or conceptional age) is measured from the actual day of conception. Because conception typically happens about two weeks after your period starts, fetal age is roughly two weeks less than gestational age. A baby with a gestational age of 10 weeks has a fetal age of about 8 weeks.
Neither number is "wrong"; they simply answer different questions. Gestational age is the practical standard for scheduling care because it relies on a date you actually know. Fetal age describes how long the developing baby has truly existed. The key takeaway: when you read that an embryo's heart starts beating "around 6 weeks," that figure is almost always given in gestational age. If you're curious about the conception side of the equation, our conception date calculator estimates the likely window based on your cycle.
Why an Ultrasound May Change Your Dating
The LMP method assumes a textbook 28-day cycle with ovulation on day 14. Real bodies don't always cooperate. Cycles can run short or long, ovulation can happen early or late, and some people simply aren't sure when their last period started. That's why an early ultrasound is the most accurate tool for confirming or correcting your dates.
In the first trimester, a sonographer measures the crown-rump length (CRL), the distance from the top of the baby's head to the bottom of the rump. Because embryos grow at a remarkably predictable rate in these early weeks, this single measurement can date a pregnancy to within a few days. ACOG considers a first-trimester ultrasound (up to and including 13 weeks 6 days) the most reliable method for establishing or confirming gestational age.
So what happens when your LMP-based date and your ultrasound date disagree? Providers follow a redating rule of thumb: if the ultrasound estimate differs from the LMP estimate by more than a set threshold, the ultrasound date is used instead. ACOG guidance generally redates when the discrepancy exceeds:
- More than 5 days in the early first trimester (up to about 8w6d).
- More than 7 days later in the first trimester (9w0d to 13w6d).
- More than 10 to 14 days in the second trimester, where ultrasound dating becomes less precise.
If your provider "moves" your due date after a scan, this is usually why. It's a normal, evidence-based adjustment, not a sign that anything is wrong. Once your due date is set early in pregnancy, it generally shouldn't be changed by later scans, since babies that are measuring large or small later on may reflect growth differences rather than a dating error.
Working Out Your Own Due Date
The classic shortcut for estimating a due date from your last period is Naegele's rule: take the first day of your last period, add one year, subtract three months, and add seven days. For a last period starting on April 11, that lands you on January 18 of the following year, about 40 weeks later.
For example, if your last period began on March 1, you'd add a year, subtract three months to reach December 1, then add seven days for an estimated due date of December 8. That said, the rule assumes a regular 28-day cycle, so it's a starting estimate rather than a guarantee. Only about 1 in 20 babies actually arrives on the predicted due date, and a normal full-term delivery can land anywhere in a roughly five-week window around it.
You don't have to do this by hand. Our due date calculator applies Naegele's rule for you and shows the result in seconds, and you can cross-check it against any ultrasound dating your provider gives you. If your cycles are irregular, lean on the ultrasound estimate, since it doesn't depend on knowing your ovulation day.
The Bottom Line
Figuring out how far along you are comes down to a few clear rules. Pregnancy is counted in weeks and days from the first day of your last period, not from conception, which is why the standard gestational-age number runs about two weeks ahead of the true fetal age. The first trimester spans 0 to 13w6d, the second covers 14w to 27w6d, and the third runs from 28 weeks until birth. And if an early ultrasound shifts your dates, that's a normal, more accurate correction based on the baby's measured size.
When in doubt, plug your last period date into our how far along am I calculator, confirm the estimated due date, and bring any questions to your obstetrician or midwife. They can fine-tune your timeline with an ultrasound and make sure your prenatal care stays right on schedule.
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