To figure out how many weeks pregnant you are, count from the first day of your last menstrual period, not from the day you think you conceived. This is the standard method used in medicine, and it means your “week count” actually starts about two weeks before the egg was fertilized. A missed period typically puts you at around four to five weeks pregnant by this counting system, even though conception happened only two to three weeks earlier.
Why Pregnancy Weeks Start Before Conception
Pregnancy is dated from the first day of your last menstrual period (LMP) because that’s a date most people can identify with reasonable confidence. Ovulation and conception usually happen about two weeks later, but pinpointing the exact day sperm met egg is nearly impossible without fertility tracking or assisted reproduction. So the medical world adds those two weeks to the front end of every pregnancy.
This is why you’ll sometimes see two terms: gestational age and fetal age. Gestational age is the number everyone uses, counted from your LMP. Fetal age (sometimes called embryonic age or conceptional age) is roughly two weeks shorter because it starts at fertilization. When your doctor says you’re “eight weeks pregnant,” the embryo has been developing for closer to six weeks. Every milestone chart, ultrasound measurement, and due date calculation uses gestational age unless specifically stated otherwise.
Counting From Your Last Period
The simplest way to estimate your weeks is to count forward from the first day of your most recent period. If that day was October 1 and today is November 12, you’re about six weeks pregnant. Most pregnancy apps and online calculators do exactly this math. Your estimated due date falls 280 days (40 weeks) from that same start date.
This method works best if your cycles are fairly regular and close to 28 days. If your cycles run longer, say 35 days, you likely ovulated later than day 14, which means the standard count could overestimate your weeks by about a week. If your cycles are shorter, it could underestimate. And if you have no idea when your last period started, or your cycles are very irregular, the LMP method becomes unreliable. In those cases, an early ultrasound is the best way to establish your dates.
Ultrasound Dating: The Most Accurate Method
A first-trimester ultrasound, done before 14 weeks, is the most accurate way to determine how far along you are. The American College of Obstetricians and Gynecologists considers it the gold standard for confirming or adjusting gestational age. The technician measures the embryo from head to rump (called a crown-rump length), and that measurement correlates tightly with specific weeks of development.
The margin of error is remarkably small. Large population studies have found the average difference between ultrasound-estimated age and true gestational age to be about one-third of a day in the first trimester. That precision drops as pregnancy progresses because babies start growing at different rates, which is why early scans are so much more reliable for dating than later ones.
In practice, about 40% of women who receive a first-trimester ultrasound end up having their due date adjusted because the scan disagrees with their LMP-based estimate by more than five days. If your ultrasound date and your period-based date are close, your provider will typically stick with the LMP date. If there’s a meaningful gap, the ultrasound date takes priority. This adjusted date then becomes the official reference point for the rest of your pregnancy.
Blood Tests and hCG Levels
A blood test measuring the pregnancy hormone hCG can give a rough sense of how far along you are, though it’s less precise than ultrasound. hCG rises rapidly in early pregnancy, roughly doubling every two to three days in the first weeks, then peaks between weeks 9 and 12 before tapering off.
Typical ranges by week (counted from your last period):
- Week 3: 5 to 50 mIU/mL
- Week 4: 5 to 426 mIU/mL
- Week 5: 18 to 7,340 mIU/mL
- Week 6: 1,080 to 56,500 mIU/mL
- Weeks 7 to 8: 7,650 to 229,000 mIU/mL
- Weeks 9 to 12: 25,700 to 288,000 mIU/mL
Notice how wide those ranges are. Two people at exactly the same point in pregnancy can have vastly different hCG levels and both be perfectly normal. Because of this overlap, hCG is useful for confirming pregnancy is progressing (your doctor may check whether levels are doubling appropriately) but not reliable for pinpointing a specific week. It’s a supporting clue, not a precise calendar.
Home Pregnancy Tests With a Weeks Indicator
Some digital home pregnancy tests display an estimate like “1-2 weeks,” “2-3 weeks,” or “3+ weeks” since conception. These work by measuring the concentration of hCG in your urine and matching it to expected ranges. Keep in mind these show weeks since conception, not gestational weeks, so you’d add about two weeks to convert to the standard pregnancy count your doctor uses. A reading of “2-3 weeks” on the test translates to roughly 4 to 5 weeks pregnant.
Clinical testing of these devices found about 93% agreement with the actual time since ovulation when normal measurement variability was accounted for. Compared against ultrasound dating, agreement reached as high as 99% when using the appropriate reference formulas. That’s solid for a home test, but the result is still a broad estimate grouped into just three categories. It’s a helpful early indicator, not a substitute for a proper dating scan.
Dating an IVF Pregnancy
If you conceived through IVF, dating is actually more straightforward because you know the exact day of embryo transfer and the age of the embryo at that point. The conception date equals the transfer date minus the embryo’s age in days. For a 5-day embryo (blastocyst) transferred on November 10, the conception date would be November 5. For a 3-day embryo transferred on November 10, the conception date would be November 7.
From there, the due date is calculated as 266 days from the conception date. To convert to gestational weeks (the standard count starting from a theoretical last period), add 14 days to the conception date and count forward. Your fertility clinic will typically provide this calculation, and it tends to be more accurate than LMP-based dating since there’s no guesswork about when ovulation occurred.
When Your Dates Don’t Line Up
It’s common for your own estimate and your ultrasound date to disagree by a few days or even a week or more. This doesn’t necessarily signal a problem. The most frequent reasons are late ovulation, an irregular cycle, or simply misremembering the start date of your last period.
If the discrepancy is small (within about five days in the first trimester), your provider will generally keep your original LMP-based date. Larger differences prompt a switch to the ultrasound-based date. Once an estimated due date is established early in pregnancy, it typically isn’t changed by later ultrasounds, because measurements become less precise as the baby grows and individual size variations widen. The early dating, whether from a reliable LMP or a first-trimester scan, remains the anchor for tracking your pregnancy milestones, scheduling tests, and planning for delivery.