The simplest way to estimate how far along you are is to count forward from the first day of your last menstrual period (LMP). Doctors measure pregnancy from that date, not from conception, which means you’re considered about two weeks “pregnant” before you actually conceived. A full-term pregnancy is 280 days, or 40 weeks, from that starting point.
If you don’t remember your last period, have irregular cycles, or just want a more precise answer, there are several other ways to figure it out.
Counting From Your Last Period
The standard method used by virtually every doctor’s office starts with the first day of your most recent period. From there, each week that passes adds one week to your gestational age. If your last period started on January 1 and today is February 12, you’d be about 6 weeks along.
To estimate your due date using the same starting point, you can use a formula called Naegele’s Rule: take the first day of your last period, count back three calendar months, then add one year and seven days. So a last period starting June 10 would give an estimated due date of March 17 of the following year. This formula assumes a 28-day cycle with ovulation on day 14, which is a rough average but not true for everyone.
The catch is that this method only works well if your cycles are fairly regular and close to 28 days. If your cycles run 35 days, for example, you likely ovulated later than day 14, and the LMP method would overestimate how far along you are by about a week.
Why Doctors Add Two Extra Weeks
One thing that confuses almost everyone early in pregnancy: gestational age and the actual age of the embryo are not the same number. Gestational age starts counting from your last period, which is roughly two weeks before ovulation and conception. So when your doctor says you’re “6 weeks pregnant,” the embryo is closer to 4 weeks old.
This matters if you’re comparing what your doctor tells you to what a home pregnancy test says. Some digital tests display results like “1-2 weeks,” “2-3 weeks,” or “3+ weeks,” and those numbers refer to weeks since conception, not gestational weeks. To convert, add two weeks. A test reading “2-3 weeks” means you’re roughly 4 to 5 weeks along in gestational terms.
What a Digital Pregnancy Test Can Tell You
Certain home pregnancy tests include a “weeks indicator” that estimates how far past conception you are. These work by measuring the pregnancy hormone (hCG) in your urine against set thresholds. Below about 153 units, the test displays 1-2 weeks since conception. Between 153 and 2,750 units, it shows 2-3 weeks. Above 2,750, it displays 3+.
In clinical testing, these readings matched the actual time since ovulation about 93% of the time. That’s reasonably accurate for a home test, but the broad categories (each spanning a full week or more) mean you’re getting a ballpark, not a precise answer. And because hCG levels vary enormously from person to person at any given week, the test can occasionally place you in the wrong category. At 6 weeks gestational age, for instance, normal hCG can range from 152 to over 32,000, a spread so wide it could easily land in different week categories for different people.
Using Ovulation Data for a More Precise Date
If you were tracking ovulation before you got pregnant, you have a significant advantage. Knowing when you ovulated narrows down conception to a one- or two-day window, which is far more precise than working backward from your last period.
Ovulation kits detect a surge in luteinizing hormone that triggers the release of an egg. A positive result (where the test line is as dark or darker than the control line) means ovulation is about to happen, typically within 24 to 36 hours. If you recorded that date, add two weeks to convert it to gestational age. So if your positive ovulation test was 5 weeks ago, you’re approximately 7 weeks along in gestational terms.
Basal body temperature tracking works similarly but in reverse. Your resting temperature rises slightly after ovulation, so the date of that temperature shift marks ovulation. The limitation is that by the time you see the rise, ovulation has already happened, making it a better tool for confirming the date after the fact than for pinpointing it in real time.
Ultrasound: The Most Accurate Method
An early ultrasound is the most reliable way to determine how far along you are, and it’s the method your doctor will use to confirm or adjust any estimate you’ve calculated at home. Between 6 and 13 weeks, the technician measures the embryo from head to tailbone (called the crown-rump length), and that measurement correlates closely with gestational age.
First-trimester ultrasounds are so precise that they can be accurate to within a few days. That level of precision matters for screening tests and for setting your official due date. If your ultrasound dating and your LMP-based estimate disagree, your provider will typically go with the ultrasound, especially if the difference is more than a few days. After the first trimester, ultrasound dating becomes less precise because babies start growing at more individual rates.
This is also why a “dating scan” is often scheduled between 8 and 12 weeks. It’s the sweet spot where the embryo is large enough to measure accurately but early enough that growth variation between pregnancies is minimal.
Fundal Height Later in Pregnancy
Once you’re past 24 weeks, there’s a simple physical measurement that can give a rough check on your dates. Your provider will measure the distance from your pubic bone to the top of your uterus, called the fundal height. After week 24, this measurement in centimeters generally matches the number of weeks you are, give or take about 3 centimeters. At 30 weeks, you’d expect a fundal height somewhere between 27 and 33 centimeters.
This isn’t a dating tool for early pregnancy, and it’s not precise enough to change your due date. But it’s a useful check at routine appointments to confirm that the baby’s growth is tracking as expected for your gestational age.
If You Don’t Know Your Last Period
Irregular cycles, recent use of hormonal birth control, or simply not remembering the date can all make the LMP method unreliable. If that’s your situation, an early ultrasound becomes especially important. It can establish your gestational age independently of your cycle history.
Blood tests measuring hCG can confirm that you’re pregnant and that levels are rising appropriately, but they’re not useful for pinpointing exactly how far along you are. The normal ranges at any given week are enormous. At 7 weeks, for example, hCG can be anywhere from about 4,000 to over 153,000 and still be perfectly normal. That kind of spread makes it impossible to reverse-engineer a precise week from a single blood draw. What hCG blood tests are good for is tracking the pattern over two or more draws a few days apart, which can confirm whether the pregnancy is progressing normally.
Quick Reference: Estimating Your Weeks
- You know your last period date: Count the weeks from the first day of that period to today. That’s your gestational age.
- You tracked ovulation: Count the weeks from your ovulation date and add 2. That’s your gestational age.
- You have a digital test reading: Add 2 weeks to the number shown. A “2-3” reading means roughly 4-5 weeks gestational age.
- You’re unsure of dates: An early ultrasound between 6 and 13 weeks will give you the most accurate answer, often within a few days of the true gestational age.