Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means you’re already considered about two weeks pregnant at the time of ovulation. A full-term pregnancy is 280 days, or 40 weeks, counted from that starting point. There are several ways to figure out where you fall on that timeline, ranging from a simple calendar count to an ultrasound measurement.
The Standard Calendar Method
The quickest way to estimate how far along you are is to count the weeks and days since the first day of your last period. If your last period started on March 1 and today is May 10, you’re roughly 10 weeks pregnant. Online calculators automate this, but the math is the same.
Your estimated due date uses a formula known as Naegele’s rule: take the first day of your last period, add nine months and seven days. That gives you 280 days, the standard length of pregnancy. This formula assumes a 28-day cycle with ovulation on day 14, so it works best for people with regular cycles. If your cycles are longer, shorter, or unpredictable, the estimate can be off by a week or more.
One detail that confuses a lot of people: gestational age (the number your doctor uses) runs about two weeks ahead of the baby’s actual age. You weren’t pregnant during the first two weeks of your “pregnancy” because you hadn’t ovulated yet. But the medical convention counts from the LMP anyway, so when a doctor says you’re eight weeks along, the embryo is closer to six weeks old.
Why Ultrasound Is More Accurate
An early ultrasound is the most reliable way to pin down how far along you are. In the first trimester, the technician measures the embryo from head to rump (called a crown-rump length). At this stage, embryos grow at a very predictable rate, so the measurement translates precisely to gestational age. First-trimester ultrasounds are accurate to within about five to seven days.
That accuracy drops the further along you get. By the second trimester, babies start growing at slightly different rates depending on genetics, nutrition, and other factors. A 20-week ultrasound might be off by up to two weeks. In the third trimester, the margin of error widens further, making late ultrasounds poor tools for establishing a due date. This is why your provider will typically set your due date based on the earliest ultrasound available rather than updating it later.
For people with irregular periods, ultrasound dating is especially important. Research comparing ultrasound estimates to LMP-based estimates in women with irregular cycles found that more than half had dates that disagreed by over a week. When the ultrasound date was used instead, the percentage of pregnancies accurately predicted (birth within 14 days of the due date) jumped from about 65% to 84%. In short, if your periods aren’t regular, the calendar method alone isn’t reliable enough.
When Your Doctor Changes Your Due Date
It’s common to go into your first ultrasound with one due date and leave with a different one. Providers follow specific guidelines for when to switch from the LMP-based date to the ultrasound-based date. The general principle: the earlier the ultrasound, the smaller the discrepancy needed to justify a change. If a first-trimester scan disagrees with your LMP date by more than about five to seven days, your due date will typically be adjusted. Later in pregnancy, a larger gap (10 to 14 days or more) is required before your provider would redate.
Once a due date is established based on an early ultrasound, later scans won’t change it. A baby measuring large or small at 32 weeks doesn’t mean the due date was wrong. It means the baby is growing above or below average for that gestational age.
Home Pregnancy Tests With a Weeks Indicator
Some digital pregnancy tests display an estimate of how many weeks since conception (typically “1–2,” “2–3,” or “3+”). These work by measuring the concentration of hCG, the hormone your body produces after implantation. In a study of this type of test, results matched the actual time since ovulation about 93% of the time when accounting for normal variability in hormone levels.
Keep in mind these tests show weeks since conception, not gestational age. So a reading of “2–3 weeks” means you’re roughly 4–5 weeks pregnant in the way your doctor counts. They’re a reasonable ballpark, but hCG levels vary enormously from person to person. At any given week, normal hCG can range from very low to very high. At 9 weeks of gestational age, for example, normal levels span from about 24,000 to 126,000 units. Because of that wide range, hCG-based estimates can’t tell you your exact week the way an ultrasound can.
Fundal Height: A Low-Tech Check
Starting around the midpoint of pregnancy, your provider will likely measure the distance from your pubic bone to the top of your uterus with a tape measure. After 24 weeks, this measurement in centimeters roughly equals the number of weeks pregnant you are, give or take about 3 centimeters. So at 30 weeks, a fundal height of 27 to 33 centimeters is considered normal.
This isn’t a precise dating tool, and it becomes less reliable after 36 weeks as the baby drops lower in the pelvis. It’s more useful for tracking growth over time than for figuring out your exact gestational age. But if you’re curious during a checkup, it’s a quick reference point.
Dating an IVF Pregnancy
If you conceived through IVF, your dates are more precise than a natural conception because the day of embryo transfer is known exactly. For a day-5 embryo transfer (blastocyst stage), your gestational age on the day of transfer is already 2 weeks and 5 days, since the calculation backs up to what would have been your “LMP.” Your provider counts from there. The same principle applies to day-6 or day-7 transfers at the blastocyst stage, as they’re all treated the same for dating purposes.
Practical Ways to Track Your Weeks
If you know the first day of your last period and have fairly regular cycles, counting weeks from that date gets you a solid starting estimate. An early ultrasound will either confirm or adjust that number, and from that point forward you have your working due date. Here’s how the common methods stack up:
- LMP calculation: Free, instant, works well with regular 28-day cycles. Less reliable with irregular cycles or uncertain dates.
- First-trimester ultrasound: Most accurate method, within about 5 to 7 days. This is the gold standard.
- Digital pregnancy test with weeks indicator: Useful as an early ballpark (about 93% agreement with actual timing), but only distinguishes broad categories, not exact weeks.
- Fundal height: Practical after 24 weeks, accurate within about 3 weeks. Better for growth monitoring than dating.
If you have no idea when your last period was, don’t stress about calculating at home. An ultrasound in the first trimester will give you and your provider a reliable gestational age to work with. The earlier it’s done, the more precise it will be.