Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you conceived. That means you’re already considered about two weeks pregnant at the time of conception. A full-term pregnancy lasts roughly 40 weeks from that starting point, which is why the count feels like it’s running ahead of the actual timeline.
The LMP Method, Step by Step
To figure out how many weeks pregnant you are right now, find the first day of your most recent period. Count forward from that date to today, and divide by seven. If your last period started 49 days ago, you’re 7 weeks pregnant. If it started 63 days ago, you’re 9 weeks. Most pregnancy apps do this math automatically once you enter the date.
This system exists because most people can remember when their last period started, while the exact day of conception is harder to pin down. Ovulation, fertilization, and implantation all happen at slightly different times, so the medical world standardized on the LMP as a reliable starting line. The trade-off is that the first two weeks of “pregnancy” happen before you’re actually pregnant. Conception generally occurs around two weeks after the start of gestation.
Estimating Your Due Date
Once you know your LMP, you can estimate your due date using a formula called Naegele’s Rule. Johns Hopkins Medicine describes it in three steps: 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, 2025, you’d count back to December 10, then add a year and seven days to land on December 17, 2025.
This formula assumes a 28-day menstrual cycle. If your cycles are consistently longer or shorter, your ovulation day shifts, and your actual due date may be a few days off from what Naegele’s Rule predicts. A 35-day cycle, for instance, means you likely ovulated about a week later than someone with a 28-day cycle, pushing your true gestational age back by that amount.
When an Ultrasound Changes Your Dates
An early ultrasound is the most accurate way to confirm how far along you are. Before 14 weeks, the measurement of the embryo (from head to tailbone) can pin down gestational age to within five to seven days. That’s more precise than the LMP method, which depends on your memory and on having regular cycles.
In one study cited by the American College of Obstetricians and Gynecologists, 40% of women who received a first-trimester ultrasound had their due date adjusted because the scan disagreed with their LMP-based date by more than five days. If this happens to you, your provider will typically go with the ultrasound date going forward. That means your week count may jump forward or back slightly after that first scan, which is completely normal.
Later ultrasounds are less reliable for dating because babies start growing at different rates. A scan in the second or third trimester reflects the baby’s individual size more than its exact age.
Counting Weeks After IVF
If you conceived through IVF, the math is more precise because you know exactly when the embryo was transferred. The standard approach is to subtract the embryo’s age from the transfer date to get a conception date, then add 266 days for the due date. For a day-5 blastocyst transfer on January 20, the estimated conception date would be January 15, and your gestational age would be calculated as if your LMP was about two weeks before that, around January 1. Your fertility clinic will usually provide this calculation, and it tends to be the most accurate dating method available.
Trimesters and Milestones by Week
Knowing your week count places you in one of three trimesters, each with a distinct phase of development:
- First trimester: LMP through 13 weeks and 6 days. This is when fertilization occurs and all major organs begin forming.
- Second trimester: 14 weeks through 27 weeks and 6 days. The period of rapid growth, when you’ll typically start feeling movement.
- Third trimester: 28 weeks through 40 weeks and 6 days. The baby gains weight and organs mature in preparation for birth.
Not all weeks at the end carry the same meaning. The National Institutes of Health defines “full term” as 39 weeks through 40 weeks and 6 days. Babies born at 37 or 38 weeks are considered “early term,” while those arriving at 41 weeks are “late term” and 42 weeks or beyond are “post-term.” These distinctions matter because outcomes improve with each additional week up to 39.
Why Your Week Count May Feel Off
One of the most confusing things about pregnancy dating is the two-week gap between gestational age and the baby’s actual age. When your provider says you’re six weeks pregnant, the embryo has only been developing for about four weeks. Both numbers are correct, they’re just measuring from different starting points. Medical records, lab results, and developmental milestones all use gestational age (the LMP-based count), so that’s the number to track.
If you have irregular periods or can’t remember your last period, an early ultrasound becomes especially important. Without a reliable LMP, there’s no accurate way to count weeks on your own. The first-trimester scan will give you a gestational age to work from, and that becomes your official starting point for everything that follows.
Pregnancy weeks are expressed in a “weeks plus days” format. If you’re 8 weeks and 3 days, you’ll see it written as 8+3 or 8w3d. You’re in your ninth week of pregnancy but haven’t completed it yet, similar to how a one-year-old is in their second year of life. When someone asks how far along you are, the completed weeks (just “8 weeks” in this example) is the standard answer.