The standard way to count pregnancy weeks starts from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy is 40 weeks by this count, which equals 280 days. To find your current week, count the number of days between the first day of your last period and today, then divide by seven.
The LMP Counting Method
This is the simplest way to estimate your weeks at home, and it’s the same starting point your doctor uses. Find the date your last period began, then count forward to today. If your last period started 49 days ago, you’re 7 weeks pregnant. If it started 70 days ago, you’re 10 weeks.
This method assumes a 28-day menstrual cycle with ovulation happening around day 14. That means “week one” of pregnancy actually starts before you ovulated or conceived. It sounds counterintuitive, but it’s the universal convention in obstetrics. The actual embryo is roughly two weeks younger than your gestational age. So when someone says they’re 8 weeks pregnant, the embryo has been developing for about 6 weeks since fertilization.
Why Your Period Date Might Be Off
The LMP method has a well-known limitation: it only works cleanly if your cycles are regular and close to 28 days. If your cycles run 35 days, you ovulated later than day 14, which means you’re probably not as far along as the simple count suggests. The reverse is true for shorter cycles. The standard formula also can’t account for fuzzy memory. If you’re not sure exactly when your last period started, even being off by a few days shifts your estimated week.
If your cycles are irregular or you don’t remember the exact date, an early ultrasound becomes the most reliable way to pin down your gestational age.
How Ultrasound Dating Works
In the first trimester, ultrasound estimates gestational age by measuring the embryo from head to tailbone, called the crown-rump length. This measurement is most accurate before 14 weeks because embryos grow at a very predictable rate during that window, regardless of genetics or other factors that cause size variation later on.
The measurement isn’t perfect. Clinical data show that errors of 2 millimeters or more are common in practice, and the 95% probability range for crown-rump length measurements can be as wide as plus or minus 5 millimeters. In practical terms, that translates to a margin of about 3 to 5 days in early pregnancy. Still, a first-trimester ultrasound is considered more accurate than LMP-based dating for most people, and your provider will often adjust your due date to match the ultrasound if the two estimates differ by more than a week.
Later ultrasounds (second and third trimester) become progressively less precise for dating because babies start growing at different rates. A 30-week scan can be off by two to three weeks, so it’s not useful for figuring out how far along you are if that hasn’t already been established.
The Due Date Formula
Once you know your weeks, you can also estimate your due date using a formula called Naegele’s Rule. The steps are simple: take the first day of your last period, count back three calendar months, then add one year and seven days. If your last period started on March 10, 2025, you’d count back to December 10, then add a year and seven days to get December 17, 2025.
From your due date, you can always work backward to find your current week. Subtract today’s date from the due date to see how many weeks remain, then subtract that number from 40.
What Digital Pregnancy Tests Tell You
Some home pregnancy tests display a “weeks since conception” estimate, typically showing 1–2, 2–3, or 3+. These work by measuring the concentration of the pregnancy hormone hCG in your urine and matching it to expected ranges. One important detail: these results show weeks since ovulation, not gestational age. To convert, add two weeks. So a result of “2–3 weeks” means roughly 4 to 5 weeks pregnant in standard gestational terms.
In a clinical study, these tests agreed with ultrasound-confirmed dating about 93% of the time when normal measurement variation was accounted for. They’re a reasonable rough estimate in very early pregnancy, but they use hormone thresholds rather than direct measurement, and hCG levels vary enormously between individuals. At 9 weeks, for example, normal hCG can range from about 24,000 to over 125,000 units. That kind of spread makes hormone levels useful for confirming pregnancy but unreliable for pinpointing an exact week.
Dating After IVF or Fertility Treatment
If you conceived through IVF, you have the advantage of knowing the exact day the embryo was transferred. The calculation works by subtracting the embryo’s age at transfer from the transfer date to find the equivalent conception date, then adding 266 days for the due date. For a day-5 blastocyst transfer on January 20, conception is counted as January 15. To convert to gestational age (the same system everyone else uses), add 14 days to that conception date, making your LMP equivalent January 1.
This makes IVF dating the most precise method available, since there’s no guesswork about when ovulation or fertilization happened.
Gestational Age vs. Fetal Age
You’ll sometimes see “fetal age” or “embryonic age” referenced in apps or pregnancy books, and the two-week gap can cause real confusion. Gestational age, counted from your last period, is what your doctor uses for all milestones, appointments, and growth charts. Fetal age, counted from fertilization, is two weeks shorter. When your provider says you’re 12 weeks, and a book describes what a “10-week embryo” looks like, they’re talking about the same pregnancy.
Keeping this distinction clear matters because developmental milestones are pegged to gestational age. A heartbeat can typically be detected on vaginal ultrasound around week 6. By week 12, the fetus has all its basic organ structures in place. The anatomy scan, where the baby’s physical development is checked in detail, happens around week 20. All of these milestones use the gestational age clock that started with your last period.