Pregnancy is measured from the first day of your last menstrual period (LMP), not from the day you actually conceived. This means you’re already considered about 4 weeks pregnant by the time you miss a period and get a positive test. A full-term pregnancy lasts roughly 40 weeks counted this way.
Why Counting Starts Before Conception
This is the part that confuses most people. Conception typically happens about two weeks after the start of your last period, during ovulation. But because most people don’t know the exact day they conceived, doctors use the period date as a reliable starting point everyone can identify. The result is that “gestational age,” the number you hear at every appointment, always runs about two weeks ahead of how old the embryo or fetus actually is. A pregnancy at 8 weeks gestational age contains a 6-week-old embryo.
This two-week gap isn’t a mistake or a rounding error. It’s baked into every milestone, every growth chart, and every due date calculation. When your doctor says you’re 12 weeks pregnant, they’re measuring from your LMP, and the entire medical system is calibrated to that same clock.
The LMP Method Step by Step
The simplest way to figure out how far along you are is to count the weeks and days from the first day of your last period to today. If your last period started on March 1 and today is April 12, you’re 6 weeks pregnant. Most pregnancy apps do this math automatically once you enter your LMP date.
To estimate a due date from your LMP, doctors use a formula called Naegele’s Rule. According to Johns Hopkins Medicine, the steps are: 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 June 10, you’d count back to March 10, then add a year and seven days, giving you a due date of March 17.
This formula assumes a 28-day menstrual cycle. If your cycles are consistently longer or shorter, the estimate shifts. Someone with a 35-day cycle ovulates about a week later than someone with a 28-day cycle, so their true conception date is later and their due date would be pushed back by roughly a week.
How Ultrasound Dating Works
Ultrasound gives a more precise measurement of gestational age, especially in the first trimester. Between about 7 and 13 weeks, the technician measures the embryo from head to tailbone, a measurement called crown-rump length. At this stage, embryos grow at a remarkably predictable rate, so the length translates reliably into a gestational age. Research published in the American Journal of Obstetrics & Gynecology found that crown-rump length was more precise than both LMP and second-trimester ultrasound for estimating gestational age.
Later in pregnancy, ultrasound dating becomes less accurate because babies start growing at different rates based on genetics and other factors. By the second trimester, measurements of the head, abdomen, and thigh bone are used instead, but these carry a wider margin of error.
When Your Due Date Gets Changed
Sometimes ultrasound measurements don’t match the dates calculated from your LMP. If the gap is small, a few days, your original due date stays. But when the discrepancy hits 7 days or more in the first trimester, doctors will often switch your due date to the ultrasound estimate. About 8.7% of pregnancies show this kind of mismatch between LMP dating and first-trimester ultrasound.
The American College of Obstetricians and Gynecologists has specific thresholds for when to reassign a due date, and these thresholds widen as pregnancy progresses. In the second trimester, for example, the discrepancy generally needs to exceed 10 days before the date is revised. The earlier the ultrasound, the more trustworthy its dating.
How IVF Pregnancies Are Counted
If you conceived through IVF, the math is more straightforward because you know the exact day of embryo transfer. For a day-3 embryo transfer, 263 days are added to the transfer date to find the due date. For a day-5 transfer, it’s 261 days. These calculations back-convert into the same gestational age system so that your week count lines up with everyone else’s milestones and growth charts.
Gestational Age vs. Fetal Age
You’ll sometimes see “fetal age” mentioned alongside gestational age, and the distinction matters. Gestational age counts from your last period. Fetal age (sometimes called embryonic age) counts from conception, making it roughly two weeks shorter. A baby described as 20 weeks gestational age has been developing for about 18 weeks. Nearly all medical conversations, apps, and pregnancy books use gestational age, so unless something specifically says “fetal age,” assume the two-week head start is already built in.
Other Ways Pregnancy Timing Is Tracked
Starting around week 20, your provider may measure fundal height, the distance from your pubic bone to the top of your uterus, with a tape measure. Between weeks 20 and 36, the measurement in centimeters roughly matches the number of weeks pregnant you are, give or take 2 centimeters. At 28 weeks, for instance, a fundal height between 26 and 30 centimeters is considered normal. This isn’t used for precise dating but serves as a quick check that the baby is growing on track.
Blood levels of hCG, the hormone detected by pregnancy tests, rise in a general pattern during early pregnancy but aren’t useful for pinpointing how many weeks along you are. At 5 weeks, for example, hCG can range anywhere from 200 to 7,000. By 7 weeks, the range spans from 3,000 to 160,000. The overlap between weeks is too large for hCG to tell you much beyond confirming that a pregnancy exists and is progressing.
Weeks, Months, and Trimesters
Pregnancy weeks are grouped into three trimesters. The first trimester covers weeks 1 through 13, the second runs from week 14 through 27, and the third spans week 28 through delivery. Converting weeks to months is less precise because calendar months vary in length, which is why doctors stick to weeks. When someone says they’re “three months pregnant,” that loosely translates to about 12 or 13 weeks, but the week count is always the more accurate number to track.