Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means by the time you get a positive pregnancy test, you’re typically already considered about four weeks pregnant. This counting method can feel counterintuitive, but it’s the standard used by virtually every doctor, midwife, and pregnancy app.
Why Counting Starts Before Conception
A full-term pregnancy is 280 days, or 40 weeks, counted from the first day of your last period. This convention assumes a 28-day cycle with ovulation happening around day 14. Since fertilization happens around ovulation, you weren’t technically pregnant during the first two weeks of your “pregnancy.” But because most people can remember when their period started more reliably than when they ovulated, the LMP became the universal starting point.
This also means your gestational age (the number your doctor uses) is always about two weeks ahead of your baby’s actual developmental age. If you’re told you’re 8 weeks pregnant, the embryo has been developing for roughly 6 weeks.
How to Calculate Your Weeks at Home
The simplest method: count forward from the first day of your last period. If your last period started on March 1 and today is April 12, that’s 42 days, which puts you at 6 weeks. Most pregnancy apps do this math automatically once you enter your LMP date.
This calculation works best if your cycles are fairly regular and close to 28 days. If your cycles run longer or shorter, the estimate can be off. Someone with a 35-day cycle, for example, likely ovulated around day 21 instead of day 14, meaning they’d be about a week less far along than a simple LMP calculation suggests.
If You Tracked Ovulation
If you were monitoring ovulation with test strips, cervical mucus tracking, or basal body temperature (BBT), you may have a more precise conception window. BBT rises by about half a degree Fahrenheit after ovulation, and when that temperature stays elevated for three or more days, ovulation has likely occurred. Sperm can survive up to five days in the reproductive tract, so conception could have happened anywhere in that window.
To convert a known ovulation or conception date into gestational weeks, simply add 14 days to that date and count from there. This gives you the equivalent LMP date, which keeps your timeline in sync with the standard system your doctor uses.
If You Conceived Through IVF
For pregnancies conceived through fertility treatment, the math is more precise because the date of embryo transfer is known exactly. The conception date equals the transfer date minus the age of the embryo at transfer. So for a 5-day embryo transfer on April 10, the conception date would be April 5. Your due date is then 266 days from that conception date. Your clinic will handle this calculation, but it’s useful to understand why your dates may differ slightly from what a standard LMP calculator produces.
How Ultrasound Dating Works
A first-trimester ultrasound is the most accurate way to confirm how far along you are. Between 6 and 14 weeks, a measurement of the embryo from head to rump correlates closely with gestational age. The earlier in this window the ultrasound is done, the more precise it tends to be.
If the ultrasound date and your LMP-based estimate are close, your doctor will generally stick with the LMP date. If there’s a significant gap between the two, the ultrasound measurement takes priority. This is especially common when cycles are irregular, when you’re unsure of your LMP, or when the dates just don’t line up.
Later ultrasounds, in the second and third trimesters, are less reliable for dating because babies start growing at different rates. A first-trimester scan is the gold standard.
What About Blood Tests?
Pregnancy blood tests measure hCG, the hormone your body produces after implantation. While hCG levels do rise on a general schedule, the ranges at any given week are so wide that they’re not useful for pinpointing how far along you are. At 6 weeks, for instance, normal hCG can fall anywhere between 200 and 32,000. At 7 weeks, the range stretches from 3,000 to 160,000. Two people at exactly the same point in pregnancy can have wildly different hCG levels and both be completely normal.
Doctors use hCG trends (whether levels are rising appropriately over 48 to 72 hours) to assess whether a pregnancy is progressing, not to determine the week. So if you’ve had bloodwork done, don’t try to reverse-engineer your gestational age from the number.
Fundal Height: A Physical Check After 20 Weeks
Starting around the midpoint of pregnancy, your provider will measure fundal height at each visit. This is the distance in centimeters from your pubic bone to the top of your uterus. Between weeks 20 and 36, this measurement in centimeters roughly matches your week of pregnancy, give or take 2 centimeters. If you’re 28 weeks along, a fundal height of 26 to 30 centimeters is considered normal.
This isn’t a tool for figuring out your dates early on, but it’s how your provider tracks whether growth is on schedule once your pregnancy is well established.
When Your Dates Feel Uncertain
If you don’t remember the exact start of your last period, if your cycles were irregular, or if you were on hormonal birth control recently, your LMP-based estimate may not be reliable. In these situations, an early ultrasound becomes especially important. Many providers will schedule a dating scan between 7 and 10 weeks specifically to establish a clear timeline.
It’s also worth knowing that your due date and week count may shift after that first ultrasound. This is normal and doesn’t mean anything went wrong. It just means the measurement gave a more accurate picture than your period date alone could provide. Once a final due date is set, typically after the first-trimester ultrasound, it generally stays fixed for the rest of your pregnancy.