The most common way to figure out how far along you are is to count from the first day of your last menstrual period (LMP). Pregnancy is dated at 280 days, or 40 weeks, from that date. This means your “week count” actually starts about two weeks before you conceived, which can feel confusing, but it’s the standard every doctor, midwife, and pregnancy app uses.
Several methods can pin down your gestational age, from a simple calendar calculation to an ultrasound measurement. Here’s how each one works and how reliable it is.
The Last Menstrual Period Method
Start with the first day of your most recent period. Add seven days, then add nine months. That formula, known as Naegele’s rule, gives you an estimated due date. Count backward from that due date to today, and you’ll know roughly how many weeks pregnant you are. Most online due date calculators do exactly this math for you.
The catch is that this formula assumes a textbook 28-day cycle with ovulation on day 14. If your cycles run longer, shorter, or irregularly, the estimate can be off by a week or more. It also relies on remembering your last period accurately, which isn’t always easy. For these reasons, the American College of Obstetricians and Gynecologists (ACOG) treats LMP dating as a starting point rather than a final answer.
Gestational Age vs. Fetal Age
You’ll hear two different “ages” thrown around, and it helps to know the difference. Gestational age counts from the first day of your last period. Fetal age (sometimes called conceptional age) counts from the day you actually conceived, which is roughly two weeks later. So if you’re told you’re 8 weeks pregnant, your baby has really been developing for about 6 weeks. Nearly all medical records, apps, and pregnancy books use gestational age, so when someone asks “how far along are you?” the answer is the gestational number.
How Ultrasound Dating Works
An early ultrasound is the most accurate tool for confirming how far along you are. Between about 6 and 13 weeks, the technician measures the baby from the top of the head to the bottom of the spine, a measurement called crown-rump length. That number in millimeters maps to a specific gestational age. At 9 weeks, for example, the average measurement is around 27 mm. By 12 weeks it’s roughly 61 mm. These measurements follow a predictable growth curve, so even a few millimeters of difference translates to a specific number of days.
First-trimester ultrasound dating is accurate to within about 3 to 8 days in most cases. In pregnancies where the exact date of conception is known (such as with IVF), studies have found the error to be as small as plus or minus 1.5 days. Accuracy drops as pregnancy progresses because babies start growing at more individual rates. That’s why an early scan is more reliable for dating than one done at 20 or 30 weeks.
If ultrasound dating and your LMP disagree significantly, your provider will typically adjust your due date to match the ultrasound. ACOG has specific thresholds for when that adjustment should happen, and the earlier the scan, the more weight it carries.
When Your Periods Are Irregular
If your cycles are longer than 35 days, unpredictable, or you’re not sure when your last period started, the LMP calculation becomes unreliable. The standard formula assumes ovulation on day 14, but with a 40-day cycle, you may have ovulated around day 26, which would make you nearly two weeks less pregnant than the LMP math suggests. In these situations, a first-trimester ultrasound is the preferred way to establish your dates. Your provider may also ask about other clues: when you got a positive pregnancy test, when symptoms like nausea started, or whether you were tracking ovulation.
Dating an IVF Pregnancy
If you conceived through IVF or another assisted reproduction method, dating is more straightforward because the transfer date is known. The calculation works like this: subtract the embryo’s age at transfer from the transfer date to estimate conception, then add 266 days for the due date. A day-3 embryo transfer means you subtract 3 days; a day-5 blastocyst transfer means you subtract 5. This gives a more precise due date than the LMP method since there’s no guesswork about when ovulation or fertilization happened.
What hCG Levels Can (and Can’t) Tell You
Human chorionic gonadotropin, the hormone detected by pregnancy tests, rises rapidly in early pregnancy and follows a general pattern week by week. At 3 weeks from your last period (around the time of implantation), levels typically range from 5 to 50 mIU/mL. By week 5, they jump to roughly 18 to 7,340 mIU/mL. Between weeks 7 and 8, levels can reach 7,650 to 229,000 mIU/mL, and they peak somewhere between weeks 9 and 12 at 25,700 to 288,000 mIU/mL.
Those ranges are enormous, and that’s the limitation. Two women at exactly 6 weeks pregnant can have hCG levels of 1,080 and 56,500, and both are perfectly normal. So while hCG trends can confirm a pregnancy is progressing (levels should roughly double every 48 to 72 hours in early weeks), a single blood draw can’t reliably tell you which week you’re in. Your provider may order hCG tests early on, but they won’t use the number alone to date your pregnancy.
Fundal Height in Later Pregnancy
Starting around 20 weeks, your provider will measure the distance from your pubic bone to the top of your uterus with a tape measure. This is called fundal height, and it follows a convenient rule: the measurement in centimeters roughly equals your week of pregnancy, give or take 2 cm. At 24 weeks, for instance, a normal fundal height falls between about 22 and 26 cm. By 20 weeks the top of the uterus sits right at your belly button, and it continues to rise from there.
This correlation holds between weeks 20 and 36. After 36 weeks, the baby may drop lower into the pelvis, making the measurement less predictive. Fundal height isn’t precise enough to establish a due date on its own, but it’s a quick, low-tech way for your provider to confirm that growth is tracking as expected at each prenatal visit. A measurement that’s significantly off can prompt a follow-up ultrasound.
Early Milestones That Help Confirm Timing
Certain pregnancy milestones appear on a fairly consistent schedule, which can help confirm your dates. A fetal heartbeat is detectable by transvaginal ultrasound as early as 6 weeks 0 days. With an abdominal (external) ultrasound, it typically isn’t picked up until around 7 weeks. If your provider sees a heartbeat and the embryo measures in line with 7 weeks, that’s strong confirmation of your gestational age.
Other physical signs can offer rough clues. Many people notice their first wave of nausea between weeks 6 and 8, breast tenderness as early as week 4, and visible fetal movement (quickening) between weeks 16 and 25 for a first pregnancy. None of these are precise enough to date a pregnancy, but they can support or raise questions about your estimated timeline.
Putting It All Together
In practice, your provider uses a combination of methods. The LMP gives an initial estimate. An early ultrasound either confirms that estimate or adjusts it. From there, fundal height and fetal growth measurements at later appointments track whether your baby is growing on schedule. If all these line up, you can feel confident in your dates. If they don’t, the first-trimester ultrasound generally wins because it’s the most accurate single data point available.
Keep in mind that a due date is an estimate, not an appointment. Most babies arrive within a window of about two weeks on either side of the predicted date. Knowing how far along you are matters for scheduling screenings, tracking development, and planning, but the exact day is always a best guess.