Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means you count forward from that date in weeks, and a full-term pregnancy is about 40 weeks, or 280 days. If you know when your last period started, you already have the most common starting point for figuring out how far along you are.
Counting From Your Last Period
The standard method, known as Naegele’s rule, assumes a 28-day menstrual cycle with ovulation on day 14. Since conception can’t happen until about two weeks after your period starts, the pregnancy is technically “older” than the embryo by roughly two weeks. This can feel confusing, but it’s the system every provider uses: gestational age always runs about two weeks ahead of the actual age of the developing baby.
To estimate your due date at home, take the first day of your last period, add seven days, then count back three months. That lands you at roughly 40 weeks from your LMP. Online due date calculators do the same math automatically. Once you have a due date, you can count forward from your LMP to today and see where you fall in weeks and days.
This method works best when your cycles are regular and close to 28 days. If your cycles run longer or shorter, ovulation likely happened earlier or later than day 14, which shifts the real conception date and makes the LMP calculation less reliable. Cycles that are very irregular, or not remembering the exact start date of your last period, are two of the most common reasons this approach gives an inaccurate number.
Ultrasound Dating: The Most Accurate Method
An early ultrasound is the gold standard for pinning down how far along you are. In the first trimester, the technician measures the baby from head to rump (called the crown-rump length). At six weeks, that measurement is only about 5 millimeters. By 10 weeks it’s around 32 millimeters, and by 13 weeks it reaches roughly 70 millimeters. Because embryos grow at a very predictable rate in those early weeks, these tiny measurements translate into a gestational age that’s accurate to within just a few days.
Accuracy drops as pregnancy progresses. In the early second trimester (14 to about 22 weeks), ultrasound dating is accurate to within 7 to 10 days. By the third trimester, babies vary much more in size, so an ultrasound can be off by two to three weeks. This is why providers prefer to date the pregnancy as early as possible. If your first-trimester ultrasound gives a date that differs from your LMP calculation by more than a few days, the ultrasound date typically becomes the official one.
Gestational Age vs. Fetal Age
You’ll hear both terms, and the difference is simple. Gestational age counts from the first day of your last period. Fetal age (sometimes called fertilization age) counts from the day the egg was actually fertilized, which is roughly two weeks later. So if your provider says you’re 8 weeks pregnant (gestational age), the embryo is closer to 6 weeks old in real developmental time. Nearly all pregnancy apps, milestone charts, and medical records use gestational age, so that’s the number to pay attention to.
If You Conceived Through IVF
When pregnancy results from in vitro fertilization, the math is more precise because you know the exact date of the embryo transfer. Providers calculate backward from the transfer date rather than relying on a last menstrual period. For a day-5 blastocyst transfer, for example, you’d add 263 days (or about 37.5 weeks) from the transfer date to estimate the due date. Day-5, day-6, and day-7 frozen blastocyst transfers all use the same calculation because the embryo’s developmental stage at the time of transfer is the same.
What Blood Tests Can and Can’t Tell You
A blood test measuring hCG (the pregnancy hormone) can confirm you’re pregnant and give a very rough sense of timing, but it’s not precise enough to date a pregnancy on its own. The normal ranges at each week are enormously wide. At 5 weeks, for instance, hCG can be anywhere from 200 to 7,000. At 6 weeks, the range balloons to 200 to 32,000. Two women at the exact same gestational age can have wildly different hCG levels and both be perfectly healthy. Providers use hCG trends (whether the number is rising appropriately over 48 hours) more than any single value, and they’ll rely on ultrasound for actual dating.
Physical Signs That Suggest How Far Along You Are
Your body offers a few clues, though none are as precise as an ultrasound. In very early pregnancy, breast tenderness and nausea typically start around weeks 5 to 6 and peak between weeks 8 and 12. Feeling the baby move for the first time usually happens between 16 and 22 weeks, with first-time mothers tending toward the later end of that window.
After about 24 weeks, your provider will start measuring fundal height, which is the distance in centimeters from your pubic bone to the top of your uterus. This measurement roughly matches the number of weeks you are, give or take about 3 centimeters. So at 27 weeks, you’d expect a fundal height of around 27 centimeters. It’s a quick bedside check rather than a precise dating tool, and it becomes less reliable after 36 weeks as the baby starts to drop lower into the pelvis.
Putting It All Together
If you’re trying to figure out how far along you are right now, start with your last period. Count the weeks from the first day of that period to today. If you’ve already had an ultrasound, the gestational age listed on that report is your most reliable number, especially if it was done before 14 weeks. If your cycles were irregular or you’re unsure of your LMP date, an early ultrasound is the single best way to get an accurate answer. Everything else, from hCG levels to physical symptoms to fundal height, fills in the picture but can’t replace those two core methods.