You can estimate how many weeks pregnant you are by counting from the first day of your last menstrual period. That date, not the date you had sex or conceived, is the starting point doctors use. A typical pregnancy lasts 40 weeks from that day, and most people can get a reliable estimate at home before they ever see a provider.
Start With Your Last Period
The simplest way to figure out your week is to count forward from the first day of your most recent period. If that day was eight weeks ago, you’re considered eight weeks pregnant. This is called gestational age, and it’s the number your doctor will use throughout your pregnancy.
Here’s the part that confuses people: gestational age includes about two weeks before you actually conceived. Ovulation and fertilization typically happen around two weeks after the start of your period, so a baby’s true developmental age is roughly two weeks less than the gestational age. When someone says they’re “six weeks pregnant,” the embryo has really been developing for about four weeks. This isn’t a mistake. It’s just how the system works, because most people can remember when their period started but can’t pinpoint the exact day of conception.
This counting method assumes a 28-day menstrual cycle. If your cycles are longer, say 35 days, you likely ovulated a week later than the formula assumes, which means you could be about a week less far along than a simple count suggests. Shorter cycles shift things the other direction. If your cycle length varies a lot or you don’t remember when your last period started, the count becomes unreliable, and an ultrasound will give you a much better answer.
How to Estimate Your Due Date
Once you know the first day of your last period, you can estimate your due date using a formula called Naegele’s Rule. It works in three steps: take the first day of your last period, count back three calendar months, then add one year and seven days. So 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 that due date you can work backward to figure out your current week. Or skip the math entirely and use one of the many free online pregnancy calculators that do the same thing. You plug in the date, and it tells you your week and day.
Why Ultrasound Is More Accurate
A first-trimester ultrasound, done in the first 13 weeks, is the most accurate way to date a pregnancy. At that stage, the provider measures the length of the embryo from head to rump, and that measurement correlates tightly with gestational age. The margin of error is only about five to seven days.
Accuracy drops as the pregnancy progresses. By the third trimester (beyond 28 weeks), ultrasound dating can be off by three to four weeks, because babies grow at increasingly individual rates. That’s why the American College of Obstetricians and Gynecologists recommends establishing a due date as early as possible and rarely changing it later. If you had a first-trimester ultrasound that agreed with your period-based estimate, that date should hold for the rest of your pregnancy, even if a later scan suggests something slightly different.
When there’s a meaningful gap between the date calculated from your period and the date suggested by an early ultrasound, your provider will typically go with the ultrasound. This is especially common when cycles are irregular or when someone isn’t sure of their last period date.
What Digital Pregnancy Tests Can Tell You
Some digital home pregnancy tests display an estimate of how far along you are. These tests measure the level of a hormone called hCG in your urine and sort results into broad categories: 1 to 2 weeks, 2 to 3 weeks, or 3+ weeks since conception. Note that these are weeks since conception, not gestational weeks, so you’d add about two weeks to convert to the number your doctor would use.
Studies show these estimates agree with ultrasound dating more than 90% of the time, which is reasonably good for a test you can take at home. They’re less reliable if you test very early (before enough hCG has built up), if you’ve recently used fertility medications containing hCG, or if you have certain rare medical conditions. Think of these tests as a rough bracket, not a precise week count.
How hCG Levels Relate to Weeks
If you’ve had a blood test, you might be wondering whether your hCG number can pinpoint your week. The short answer: not precisely. hCG levels rise rapidly in early pregnancy but vary enormously from person to person. At five weeks (counting from your last period), normal levels range from 18 to 7,340 mIU/mL. By weeks seven to eight, the range balloons to 7,650 to 229,000. These ranges overlap so much that a single hCG reading can’t reliably distinguish week five from week seven.
What matters more is how quickly hCG rises over consecutive blood draws. Your provider uses that trend to assess whether the pregnancy is progressing normally, not to assign a specific week. For dating purposes, ultrasound is far superior.
Dating an IVF Pregnancy
If you conceived through IVF or another assisted reproduction method, dating is actually more straightforward because you know exactly when the embryo was transferred. The calculation depends on the age of the embryo at transfer. For a Day 5 blastocyst transfer, the conception date is considered five days before the transfer date. From there, you add 266 days to get the due date. Your clinic will do this math for you, but it means IVF pregnancies tend to have the most precisely dated due dates of all.
Fundal Height Later in Pregnancy
Starting around 20 weeks, your provider may measure the distance from your pubic bone to the top of your uterus with a tape measure. This is called fundal height, and between weeks 20 and 36 the measurement in centimeters roughly matches the number of weeks pregnant you are, plus or minus two centimeters. At 28 weeks, for example, a fundal height of 26 to 30 centimeters is considered normal.
This isn’t used to establish your week in the first place. It’s a quick check during routine appointments to confirm that the baby’s growth is tracking with the dates already set by your period or early ultrasound. Before 20 weeks the uterus isn’t high enough to measure reliably, and after 36 weeks it starts to drop as the baby settles lower into the pelvis.
Putting It All Together
If you know the date of your last period and have regular cycles, count forward from that date. That number is your gestational age in weeks. If your cycles are irregular, you’re unsure of the date, or you want confirmation, a first-trimester ultrasound will give you the most reliable answer. Once an early due date is established, it rarely changes. Everything in your prenatal care, from screening test timing to delivery planning, anchors to that date, so getting it right early matters.