The most common way to figure out how far along you are in pregnancy is to count from the first day of your last menstrual period (LMP). This date, not the day you actually conceived, is the starting point doctors use to measure pregnancy in weeks. If you know your LMP and have a regular cycle, you can estimate your gestational age right now with simple math. If you don’t remember the date or your cycles are irregular, an ultrasound is the most accurate tool available.
Why Pregnancy Starts Before Conception
One detail trips up almost everyone: gestational age starts about two weeks before you actually conceived. That’s because the medical convention counts from the first day of your last period, not from ovulation or fertilization. Conception generally occurs around two weeks after that date. So when you’re told you’re “6 weeks pregnant,” the embryo is closer to 4 weeks old. This system exists because most people can recall when their period started but can’t pinpoint the exact day of conception.
This means a full-term pregnancy is counted as 40 weeks, even though fetal development spans roughly 38 weeks. Every week count you see on apps, in doctor’s offices, and on pregnancy trackers follows this same convention.
Calculating From Your Last Period
If you remember the first day of your last menstrual period, you can estimate both how far along you are and your due date. To find your current gestational age, count the number of weeks and days from that date to today. Most pregnancy apps and online calculators do this instantly.
To estimate your due date, doctors traditionally 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. If your last period started on March 10, you’d count back to December 10, then add a year and seven days to land on a due date of December 17. This formula assumes a 28-day menstrual cycle with ovulation on day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts accordingly. A 35-day cycle, for instance, means you likely ovulated later, pushing your actual gestational age back by about a week compared to what the standard formula suggests.
When an Ultrasound Is More Accurate
An ultrasound in the first trimester is the most precise way to date a pregnancy. Early in pregnancy, embryos grow at a remarkably predictable rate, so measuring the length of the embryo (called crown-rump length) gives a reliable gestational age, typically accurate to within a few days. This is especially useful if you don’t remember your last period, have irregular cycles, or conceived while on birth control.
As pregnancy progresses, ultrasound dating becomes less exact because babies start growing at different rates. In the second trimester, doctors may measure the baby’s head, abdomen, and thigh bone to estimate gestational age, but there’s more room for variation. The American College of Obstetricians and Gynecologists uses specific thresholds to decide whether to adjust a due date based on ultrasound. For pregnancies between 16 and 22 weeks, for example, the ultrasound estimate needs to differ from the LMP-based date by more than 10 days before the due date is formally changed.
If you’ve already had a dating ultrasound and received an estimated due date, that’s generally the most reliable number to work from. You can count backward from it to figure out your current week.
What Fundal Height Tells You Later On
Starting in the second half of pregnancy, your provider will likely measure the distance from your pubic bone to the top of your uterus using a tape measure. This is called fundal height. After week 24, the measurement in centimeters roughly matches your gestational age in weeks, plus or minus about 3 centimeters. At 30 weeks, for example, you’d expect a fundal height of around 27 to 33 centimeters.
Fundal height isn’t used to pinpoint exactly how far along you are. It’s a quick screening tool to check whether the baby is growing as expected. A measurement that’s significantly off may prompt an ultrasound to take a closer look.
Why Blood Tests Don’t Pin Down Your Week
You might assume that the pregnancy hormone hCG, the same one detected by home tests, could tell you exactly how far along you are. In practice, hCG levels vary enormously from person to person at the same gestational age. At 6 weeks, for instance, normal hCG can range anywhere from 200 to 32,000 units per liter. By 7 weeks, the range stretches from 3,000 to 160,000. With that much overlap between weeks, a single blood draw can confirm you’re pregnant and give a rough sense of early versus later first trimester, but it can’t reliably distinguish between week 5 and week 7.
Doctors sometimes order two hCG tests 48 hours apart to check whether levels are rising appropriately, which helps assess whether a pregnancy is progressing normally. But this is about viability, not dating.
Putting It Together
The quickest way to estimate how far along you are depends on what information you have:
- You know your LMP and have regular cycles: Count the weeks and days from the first day of your last period to today. That’s your gestational age.
- You’ve had a dating ultrasound: Use the due date from that scan. Subtract your due date from 40 weeks to find the gestational start date, then count forward to today.
- You’re unsure of your LMP or have irregular cycles: A first-trimester ultrasound will give you the most accurate answer. If you’re past the first trimester, a later ultrasound can still estimate gestational age, just with a wider margin of error.
- You know when you conceived: Add two weeks to get your gestational age, since the standard pregnancy count begins two weeks before conception.
Whichever method you use, keep in mind that a “due date” is really the middle of a window. Only about 5% of babies arrive on their exact due date. Most full-term births happen between 39 and 41 weeks, so your gestational age is best understood as an estimate, not a countdown to a specific day.