Pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you conceived. That means by the time most people get a positive test, they’re already considered about 4 weeks pregnant. The standard full-term pregnancy is 40 weeks, or 280 days, counted from that LMP date.
The Basic Calculation
To figure out how far along you are, start with the first day of your last period. Count forward from that date to today, and that’s your gestational age in days. Divide by 7 to get weeks. If your last period started 49 days ago, you’re 7 weeks pregnant.
This method assumes a 28-day menstrual cycle with ovulation happening on day 14. Since conception can’t actually happen until ovulation, about two weeks of your “pregnancy” occur before the egg is even fertilized. Your baby’s actual age is roughly two weeks less than your gestational age. But every milestone, test, and trimester marker you’ll hear about uses the gestational age counted from your LMP, so that’s the number that matters in practice.
How to Find Your Due Date
Your estimated due date falls 280 days (40 weeks) after the first day of your last period. The quickest way to calculate it by hand uses a method called Naegele’s Rule, which Johns Hopkins Medicine breaks into three steps:
- Step 1: Write down the first day of your last menstrual period.
- Step 2: Count back 3 calendar months from that date.
- Step 3: Add 1 year and 7 days.
So if your last period started June 1, 2025, you’d count back three months to March 1, then add a year and 7 days to get March 8, 2026. That’s your estimated due date. From there, you can count backward to today to see exactly how many weeks remain, or count forward from your LMP to see how far along you are right now.
Adjusting for Irregular or Longer Cycles
The 28-day cycle assumption bakes in a specific ovulation date: day 14. If your cycles are consistently longer or shorter, your ovulation day shifts, and the standard calculation will be off. A 35-day cycle means you likely ovulated around day 21 instead of day 14, so you’d be about a week less far along than the basic LMP math suggests.
To adjust, figure out your average cycle length by counting from the first day of one period to the day before the next one starts. If your average cycle is 35 days, add 7 extra days to your due date (since ovulation happened about a week later than the formula assumes). If your cycle is 21 days, subtract 7 days. The principle is simple: for every day your cycle differs from 28, shift your due date by one day in the same direction.
If your periods were highly irregular before pregnancy, the LMP method becomes unreliable, and an early ultrasound will give a more accurate picture.
How Ultrasound Dating Works
An ultrasound in the first trimester measures the embryo’s length and compares it against standard growth curves to estimate gestational age. Early in pregnancy, embryos grow at nearly identical rates regardless of genetics, which makes this measurement very precise. The earlier the scan, the more accurate it is.
If the ultrasound estimate and your LMP calculation are close, your provider will typically stick with the LMP date. If there’s a meaningful gap between the two, the ultrasound date usually takes priority, and your due date (and how far along you are) gets officially adjusted. This is especially common when cycle length is uncertain or periods were irregular.
Dating an IVF Pregnancy
IVF pregnancies are dated differently because the exact timing of fertilization and transfer is known. Instead of counting from a last period, the calculation starts from the date the embryo was transferred into the uterus. A 5-day-old embryo (the most common type transferred) is already 5 days past fertilization at the time of transfer, and that’s factored into the gestational age. Your fertility clinic will give you an adjusted LMP date that translates this into the standard week count so all your prenatal milestones line up the same way.
Converting Weeks to Months and Trimesters
Pregnancy weeks don’t map neatly onto calendar months because months aren’t exactly four weeks long. Here’s a general breakdown:
- Month 1: Weeks 1 through 4
- Month 2: Weeks 5 through 8
- Month 3: Weeks 9 through 13
- Month 4: Weeks 14 through 17
- Month 5: Weeks 18 through 21
- Month 6: Weeks 22 through 26
- Month 7: Weeks 27 through 30
- Month 8: Weeks 31 through 35
- Month 9: Weeks 36 through 40
The three trimesters split the 40 weeks into roughly equal portions. The first trimester covers weeks 1 through 14 (about 3.5 months), the second trimester runs from week 14 through 27 (another 3.5 months), and the third trimester spans weeks 28 through 40 (about 3 months). You’ll hit the halfway mark at week 20.
Why the Number Can Change
It’s common for your “how far along” number to shift by a few days or even a week after your first ultrasound. This doesn’t mean anything went wrong. It simply means the embryo’s measured size suggests a slightly different conception date than the one your LMP predicted. Once your provider settles on a final estimated due date, that becomes the anchor for all your prenatal care, and your gestational age is counted from there going forward.
Keep in mind that a due date is an estimate, not an appointment. Only about 5% of babies arrive on the exact predicted day. Full-term birth anywhere from 39 to 41 weeks is typical, so “how far along you are” is best understood as a useful approximation rather than a precise countdown.