To calculate how far along you are in pregnancy, start from the first day of your last menstrual period (LMP), not the day you think you conceived. Count the number of weeks and days from that date to today, and that’s your gestational age. A full-term pregnancy is 40 weeks, or 280 days, counted from that starting point.
The LMP Method: Step by Step
The most common way to figure out how far along you are is to count forward from the first day of your last period. This is the date you’d enter into any online pregnancy calculator, and it’s the same starting point your doctor uses.
Here’s why the math feels a little odd: pregnancy is dated from about two weeks before you actually conceived. Ovulation typically happens around day 14 of a 28-day cycle, so by the time a sperm meets an egg, you’re already considered “2 weeks pregnant.” That means gestational age (what your doctor tracks) is roughly two weeks ahead of fetal age (the actual age of the embryo). This convention exists because most people can recall when their period started, but few know the exact day they ovulated.
To count your weeks, simply find the number of days between the first day of your last period and today, then divide by seven. If it’s been 58 days, you’re 8 weeks and 2 days pregnant. You’ll often see this written as “8w2d” in medical shorthand. You’re “in your ninth week” but have completed eight full weeks.
How to Estimate Your Due Date
Your estimated due date gives you the 40-week endpoint, which makes it easier to track where you are in the overall timeline. The standard formula, known as Naegele’s Rule, works in three steps:
- Step 1: Find the first day of your last menstrual period.
- Step 2: Count back three calendar months from that date.
- Step 3: Add one year and seven days.
For example, if your last period started on March 10, 2025, you’d count back three months to December 10, 2024, then add one year and seven days to get December 17, 2025. That’s your estimated due date, and you can count forward from March 10 to today’s date to see how far along you are.
This formula assumes a regular 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter than 28 days, the estimate shifts. Someone with a 35-day cycle, for instance, likely ovulates around day 21 instead of day 14, which pushes the true conception date about a week later than the standard formula assumes. In that case, the LMP-based calculation would overestimate how far along you are by roughly a week.
When Ultrasound Changes the Date
A first-trimester ultrasound (up to 13 weeks and 6 days) is the most accurate method to confirm gestational age. The technician measures the embryo from head to rump and compares that length to established growth charts. Early embryos grow at a remarkably consistent rate, so the measurement reliably translates into a gestational age.
If the ultrasound date and your LMP-based date are close, your due date stays the same. If there’s a meaningful gap between the two, your provider will typically adjust your due date to match the ultrasound. This is especially common when you have irregular cycles, aren’t sure when your last period started, or were on hormonal birth control shortly before conceiving. Once the due date is set, your “how far along” number is recalculated from there.
Later ultrasounds, in the second and third trimesters, are less reliable for dating because babies start growing at different rates. A scan at 30 weeks can reflect the baby’s individual size rather than a precise gestational age. That’s why early dating matters so much.
Calculating After IVF
If you conceived through IVF, the math works a bit differently because you know the exact date the embryo was transferred. For a day-5 embryo transfer (the most common type with frozen embryos), you count back 19 days from the transfer date to get the equivalent LMP date, then count forward from there like everyone else. For a day-3 transfer, you count back 17 days. The difference accounts for how many days the embryo developed in the lab before being placed in the uterus.
Once you have that adjusted LMP date, calculating how far along you are works the same way: count the weeks and days from that date to today.
Where You Are in Each Trimester
Pregnancy is divided into three trimesters, and knowing which one you’re in helps you understand what’s happening developmentally and what to expect at appointments.
- First trimester: First day of your LMP through 13 weeks and 6 days. This is when major organs form. It’s also when dating is most accurate.
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days. The period of rapid growth, when you’ll typically feel movement for the first time and have the detailed anatomy scan.
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days. The baby gains weight and organs mature for life outside the womb.
Full-term pregnancy spans 37 to 42 weeks. A baby born before 37 weeks is considered preterm, while pregnancies that go past 40 weeks are closely monitored but still within the normal range.
Why Your Number Might Seem Off
A few things commonly confuse the count. First, pregnancy apps and your doctor may phrase things differently. Being “in your seventh week” means you’ve completed six full weeks plus some days. If you’re 6 weeks and 3 days, you’re in week seven. This trips people up because it works the same way as age: a baby who is 11 months old is in their first year.
Second, if you know the date you had sex or the date of a positive ovulation test, you might calculate a different number than your doctor. That’s because you’re thinking in terms of fetal age (time since conception), while the medical system uses gestational age (time since last period). The gap is about two weeks. Neither number is wrong, but gestational age is the standard used on charts, in apps, and by your care team.
Third, months don’t map neatly onto weeks. Pregnancy is 40 weeks, but that’s not exactly nine calendar months because months vary in length. Trying to convert weeks to months creates confusion. Stick with weeks and days for the clearest picture of where you are.