Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means by the time most people get a positive pregnancy test, they’re already about four weeks pregnant. This counting method can feel counterintuitive, but it’s the standard used by virtually every healthcare provider, and understanding it is the first step to figuring out how far along you are.
Why Pregnancy Starts Before Conception
Gestational age, the number your provider uses to track your pregnancy, begins counting from the first day of your last period. Since ovulation typically happens about two weeks into your cycle, roughly two of those “pregnant” weeks pass before an egg is even fertilized. This is why gestational age and fetal age are not the same thing: if you’re 10 weeks pregnant by gestational age, the embryo itself is closer to 8 weeks old.
Providers use gestational age rather than fetal age for one practical reason: most people know when their last period started, but very few know the exact day conception occurred. The system isn’t perfect, but it gives everyone a common starting point.
Counting From Your Last Menstrual Period
The simplest way to estimate your weeks is to count forward from the first day of your last period. Day one of that period is day one of your pregnancy. From there, every seven days adds another week. If your last period started on March 1 and today is April 12, you’re about six weeks pregnant.
To estimate a due date from your LMP, providers often use 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 June 10, you’d count back to March 10, then add a year and seven days, giving you a due date of March 17 the following year. This formula assumes a 28-day cycle, which brings us to an important caveat.
Adjusting for Irregular or Longer Cycles
Naegele’s Rule works best when your cycle is close to 28 days. But cycles vary widely. Research published in the BMJ found women reporting usual cycle lengths anywhere from 19 to 60 days. The length of your cycle directly affects when you ovulate: women with cycles of 27 days or shorter tended to ovulate earlier, while those with longer cycles ovulated later. In fact, among women with short cycles, roughly a third had already reached their fertile window by the end of the first week, compared to only 7% of women with long cycles.
This matters because LMP-based dating assumes you ovulated on day 14. If your cycle is 35 days, you likely ovulated around day 21, meaning a straight LMP calculation would overestimate your gestational age by about a week. If you know your usual cycle length, you can roughly adjust by adding the difference between your cycle and 28 days. For a 35-day cycle, your pregnancy is probably about one week less advanced than a standard LMP calculation suggests. This is one reason ultrasound dating becomes especially important for people with irregular periods.
Ultrasound Dating: The Most Accurate Method
An ultrasound performed in the first trimester is the most reliable way to determine how far along you are. During this scan, the provider measures the embryo from head to rump. Because embryos grow at a remarkably predictable rate in early pregnancy, with very little size variation between individuals, this measurement translates precisely into gestational age.
Accuracy drops as pregnancy progresses. In the first trimester, ultrasound dating is accurate enough that in one large study, 40% of women who received a first-trimester scan had their due date adjusted because it differed from the LMP-based estimate by more than five days. By the third trimester, the margin of error widens to plus or minus three weeks, because babies start growing at different rates depending on genetics, nutrition, and other factors. This is why ultrasound in the first half of pregnancy is considered the gold standard for dating, and a scan performed at 36 weeks can’t reliably tell you whether you’re 34 or 37 weeks along.
Once a due date is established based on early ultrasound, it generally stays put. Changing an estimated due date later in pregnancy is reserved for rare circumstances, because later scans reflect the baby’s individual growth pattern rather than true gestational age.
What Blood Tests Can and Cannot Tell You
A pregnancy blood test measures a hormone called hCG, which rises rapidly after implantation. The levels loosely correspond to how far along you are, but the ranges overlap so much that hCG alone can’t pinpoint your week with any precision. At four weeks, normal levels range from 5 to 426. By six weeks, the range balloons to 1,080 to 56,500. Between weeks 9 and 12, levels can fall anywhere from 25,700 to 288,000.
What matters more than any single number is how quickly hCG rises over time, which is why providers sometimes order two blood draws 48 hours apart. But for the purpose of figuring out exactly how many weeks pregnant you are, blood tests are a rough guide at best. Ultrasound is far more useful for that.
Fundal Height: A Low-Tech Check
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 the number in centimeters should roughly match your weeks of pregnancy, plus or minus two centimeters. If you’re 28 weeks, your fundal height should be around 26 to 30 centimeters. This correlation holds from about weeks 20 through 36, after which the baby’s position in the pelvis can throw off the measurement. Fundal height isn’t precise enough to establish your dates, but it’s a useful check that growth is on track at routine visits.
Dating an IVF Pregnancy
If you conceived through IVF or another form of assisted reproduction, dating is more straightforward because the conception date is known. The calculation starts with your embryo transfer date, then subtracts the age of the embryo at transfer. A day-5 embryo (blastocyst) transferred on March 20 means conception is estimated at March 15. From there, the due date is 266 days after conception. To convert this into gestational age that matches the LMP system everyone else uses, you simply add two weeks to the conception date to get an equivalent LMP date, then count forward from there as usual.
Putting It All Together
In practice, most providers use a combination of methods. Your LMP gives the initial estimate. If you have an early ultrasound and it agrees closely with the LMP date, that due date is confirmed. If there’s a significant discrepancy, particularly in women with irregular cycles or uncertain period dates, the ultrasound measurement takes priority. Later ultrasounds, blood work, and fundal height serve as supporting data points but won’t change the established timeline.
If you’re trying to figure out your weeks at home before your first appointment, counting from the first day of your last period is the best starting point. Just keep in mind that if your cycles run longer or shorter than 28 days, the number may shift by a week or so once you get an early ultrasound. That scan, ideally done in the first trimester, will give you the most reliable answer.