Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. This means you’re technically “two weeks pregnant” before conception even happens. A full-term pregnancy is 280 days, or 40 weeks, measured from that starting point.
This system can feel counterintuitive, but it exists because most people can pinpoint when their last period started far more reliably than when they ovulated or conceived. Here’s how each method works and which one gives you the most accurate answer.
Counting From Your Last Period
The simplest way to estimate how far along you are is to count the weeks and days since the first day of your most recent menstrual period. If that date was exactly eight weeks ago, you’re eight weeks pregnant, even though your baby was likely conceived around six weeks ago. The two-week gap exists because the standard system assumes ovulation (and conception) happens on day 14 of a 28-day cycle.
To estimate your due date using this same starting point, Johns Hopkins Medicine outlines a three-step method called Naegele’s Rule: take the first day of your last period, count back three calendar months, then add one year and seven days. So 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 cycle, so if your cycles are consistently longer or shorter, the estimate shifts accordingly.
Gestational Age vs. Fetal Age
Two different age systems come up in pregnancy, and they’re offset by about two weeks. Gestational age counts from the first day of your last period. Fetal age (sometimes called conceptional age) counts from the actual moment of conception. A pregnancy that’s 10 weeks gestational age contains an embryo that’s roughly 8 weeks old. When your provider, your app, or your ultrasound report says you’re a certain number of weeks pregnant, they’re almost always using gestational age. The median length of pregnancy is 280 days by gestational age, or 266 days by fetal age.
How Ultrasound Refines the Estimate
A first-trimester ultrasound, done up through 13 weeks and 6 days, is the most accurate method to confirm or adjust how far along you are. During this scan, your provider measures the embryo from head to tailbone (crown-rump length). Between 9 and 13 weeks, the embryo grows rapidly and at a very consistent rate across pregnancies, which makes the measurement highly reliable for pinpointing gestational age.
As pregnancy progresses, dating by ultrasound becomes less precise. In the early second trimester (14 to about 22 weeks), measurements of the head, abdomen, and thighbone are accurate to within about 7 to 10 days. Between 22 and 28 weeks, accuracy drops to plus or minus 10 to 14 days. By the third trimester, babies vary so much in size that ultrasound can only flag a major discrepancy, typically if the estimated age is off by more than 21 days.
This is why an early ultrasound matters so much for dating. If your LMP-based estimate and a first-trimester ultrasound disagree, your provider will usually adjust your due date to match the ultrasound. The later the first scan happens, the wider the margin of error becomes.
When Your Period Doesn’t Give a Clear Answer
LMP-based dating works best when you have regular, predictable cycles and can remember when your last period started. It becomes unreliable in several common situations: irregular cycles, cycles that are significantly longer or shorter than 28 days, bleeding in early pregnancy that gets mistaken for a period, or simply not remembering the date. In all of these cases, a first-trimester ultrasound becomes the primary tool for establishing gestational age rather than just confirming it.
Pregnancy blood tests that measure hCG (the hormone your body starts producing after implantation) can confirm that you’re pregnant and that the pregnancy is progressing, but they can’t tell you exactly how many weeks along you are. The normal ranges are enormous. At week 5, for instance, hCG can be anywhere from 18 to 7,340 mIU/mL. At weeks 7 to 8, the range stretches from 7,650 to 229,000. Two people at the same gestational age can have wildly different levels and both be perfectly normal. What matters with hCG is how quickly the number rises over a few days, not the absolute number at any single point.
Dating an IVF Pregnancy
If you conceived through IVF or another form of assisted reproduction, dating is more straightforward because the conception date is known. The calculation works backward from the embryo transfer: subtract the embryo’s age in days from the transfer date to get the equivalent conception date, then add 266 days to find the due date. For a day-3 embryo transfer, the due date is 263 days from the transfer date. For a day-5 embryo, it’s 261 days from transfer. Your fertility clinic will assign the gestational age using these figures, and ACOG recommends using this ART-derived date rather than relying on ultrasound measurements to adjust it.
Putting It All Together
In practice, your provider pieces together multiple data points. They’ll start with your LMP, factor in your cycle length, and then compare that estimate against your earliest ultrasound. If the numbers line up closely, your due date stays as calculated. If they diverge beyond a certain threshold, the ultrasound measurement takes priority, particularly if it was done in the first trimester.
Keep in mind that your “weeks pregnant” number is always an estimate, and your due date is a target, not a deadline. Only about 4% of babies arrive on their exact due date. The number still matters, though, because it guides the timing of prenatal tests, screens, and milestones throughout your care. Getting it as accurate as possible early on gives you and your provider the clearest picture for the rest of the pregnancy.