Pregnancy weeks are counted from the first day of your last menstrual period, not from the day you actually conceived. That means you’re already considered “two weeks pregnant” at the time of ovulation, before fertilization even happens. This counting method, called gestational age, is the standard used by virtually every doctor, pregnancy app, and due date calculator. A full-term pregnancy is 40 weeks, or 280 days, from that starting point.
Counting From Your Last Period
The simplest way to figure out how far along you are is to count the weeks and days since the first day of your most recent period. If that date was exactly nine weeks ago, you’re nine weeks pregnant. You can do this with a calendar, or use any online due date calculator that asks for the same information.
To estimate your due date from this number, doctors use a formula called Naegele’s Rule. Here’s how it works: 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, giving you a due date of December 17. This formula assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is consistently longer or shorter, your actual ovulation date (and therefore your real conception date) may shift by several days in either direction.
One detail that trips people up: gestational age runs about two weeks ahead of the actual age of the embryo. Since counting starts at your last period, and most people ovulate around two weeks later, the embryo is roughly two weeks younger than the “weeks pregnant” number suggests. So at eight weeks pregnant, the embryo has been developing for closer to six weeks. Doctors know this, and all growth charts and milestones are built around gestational age, so you don’t need to adjust anything yourself.
When an Ultrasound Is More Accurate
An early ultrasound is the most precise way to determine how many weeks pregnant you are, especially if your cycles are irregular, you don’t remember the exact date of your last period, or you were using hormonal birth control that may have affected your cycle timing. The ideal window for a dating ultrasound is between 7 and 9 weeks. At that stage, embryos grow at a remarkably consistent rate, so measuring the length from head to rump gives a reliable gestational age.
Accuracy drops as pregnancy progresses. In the first trimester, ultrasound dating is typically accurate to within about a week. By the second trimester, the margin of error widens because babies start growing at more individual rates, influenced by genetics and other factors. A third-trimester ultrasound can be off by several weeks, which is why doctors prefer to establish your dates early and stick with them.
If your ultrasound date and your last-period date disagree by more than a week in the first trimester, your provider will usually go with the ultrasound. If they’re close, the last-period date typically stands.
Digital Home Pregnancy Tests With a Weeks Indicator
Some digital pregnancy tests display an estimate of how many weeks since conception (not gestational weeks). These work by measuring the concentration of hCG, the hormone your body produces after a fertilized egg implants. The test uses hormone thresholds to sort results into categories: 1 to 2 weeks, 2 to 3 weeks, or 3-plus weeks since conception. To convert to the gestational age your doctor uses, add two weeks. So “1 to 2 weeks” on the test corresponds to roughly 3 to 4 weeks pregnant.
In clinical testing, these indicators agreed with the time since ovulation about 93% of the time, and showed 99% agreement with ultrasound dating when standard measurement adjustments were applied. They’re a reasonable ballpark, but not a substitute for a proper dating scan. hCG levels vary enormously from person to person. At 6 weeks, for instance, normal blood levels can range anywhere from 200 to 32,000 units per liter. That wide spread means a hormone-based estimate can easily land in the wrong category for some people.
Fundal Height: A Low-Tech Check Later in Pregnancy
Starting around the midpoint of pregnancy, your provider may measure fundal height, the distance from your pubic bone to the top of your uterus, using a simple tape measure. After 24 weeks, this measurement in centimeters roughly matches the number of weeks pregnant you are, give or take about 3 centimeters. At 30 weeks, for example, a fundal height of 27 to 33 centimeters would be considered normal.
This isn’t used to establish your dates the way an early ultrasound is. It’s more of a quick check that the baby is growing on track with the gestational age already established. If the measurement is significantly off, your provider will typically follow up with an ultrasound to look more closely at growth.
What to Do With Irregular Cycles
If your periods come every 35 to 45 days, or if they’re unpredictable, counting from your last period becomes unreliable. A 35-day cycle means you likely ovulated around day 21 instead of day 14, so a last-period calculation would overestimate your gestational age by about a week. The longer or more variable your cycle, the bigger the potential error.
In these cases, an early ultrasound between 7 and 9 weeks is the most reliable way to pin down your dates. If you were tracking ovulation with test strips or basal body temperature, that data can also help. Knowing your ovulation date lets you calculate gestational age by adding two weeks to that date and counting forward to today.
Quick Reference for Counting Your Weeks
- Known last period, regular cycles: Count the weeks from the first day of that period. That number is your gestational age.
- Known ovulation or conception date: Add two weeks to that date, then count forward to today.
- IVF or assisted reproduction: Your clinic will calculate your gestational age based on the embryo transfer date and the age of the embryo at transfer.
- Unknown last period or irregular cycles: Request a dating ultrasound as early as possible, ideally between 7 and 9 weeks.
Whichever method establishes your dates first, that’s generally the one your provider will use for the rest of pregnancy. Changing the due date later on creates confusion with growth tracking, screening test timing, and delivery planning. If you’re unsure of your dates, bringing it up at your first prenatal visit gives your provider the best chance to get an accurate number early.