To calculate how far along you are in pregnancy, count the number of weeks since the first day of your last menstrual period (LMP). That starting point is the standard used by doctors worldwide, even though conception typically happens about two weeks later. So if your last period started 8 weeks ago, you’re considered 8 weeks pregnant, even though the embryo has only been developing for roughly 6 weeks.
The LMP Method Step by Step
The simplest way to figure out your gestational age and estimated due date uses a formula called Naegele’s Rule. Here’s how it works:
- Step 1: Find 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.
For example, if your last period started on March 10, 2025, you’d count back 3 months to December 10, 2024, then add 1 year and 7 days, giving you a due date of December 17, 2025. From there, you can count forward from your LMP date to today to find how many weeks along you are right now.
This formula assumes a regular 28-day cycle with ovulation on day 14. It’s a good starting estimate, but it won’t be perfectly accurate for everyone.
What If Your Cycles Aren’t 28 Days
Naegele’s Rule breaks down when your cycle is significantly shorter or longer than 28 days. If you have a 35-day cycle, for instance, you likely ovulated around day 21 rather than day 14, meaning the LMP calculation would overestimate how far along you are by about a week. The reverse is true for shorter cycles.
You can adjust roughly by adding or subtracting the difference between your actual cycle length and 28 days. If your cycle runs 35 days, add 7 days to your estimated due date (pushing it later). If it runs 24 days, subtract 4 days (pulling it earlier). But if your cycles are irregular or unpredictable, the LMP method becomes unreliable, and an early ultrasound is the better way to pin down your dates.
If You Know Your Conception Date
When you know the exact date you conceived, whether from tracking ovulation or timing intercourse carefully, your gestational age is your conception date plus 2 weeks. That’s because the gestational clock always starts from the LMP, which is roughly 2 weeks before ovulation and fertilization. So if you conceived on January 20, your gestational age is counted as though it started January 6, and all your weekly milestones follow from there.
This distinction matters. When your provider says you’re “10 weeks pregnant,” the embryo has been developing for about 8 weeks. Every pregnancy app, every trimester boundary, and every developmental milestone uses gestational age (from LMP), not fetal age (from conception).
Dating After IVF or Embryo Transfer
If you conceived through IVF, the calculation is more precise because the exact dates of egg retrieval and embryo transfer are known. For a day-5 embryo transfer (the most common type, called a blastocyst transfer), you count back 19 days from the transfer date to get your equivalent LMP date, then calculate from there. For a day-3 transfer, you count back 17 days. Whether the embryo was frozen on day 5, 6, or 7, the due date calculation is the same since they’re all considered blastocyst-stage transfers.
How Ultrasound Refines the Estimate
A first-trimester ultrasound, done before 14 weeks, is the most accurate method to confirm gestational age. The technician measures the embryo from head to rump (crown-rump length), and that measurement is accurate to within 5 to 7 days. If the ultrasound date differs from your LMP-based estimate by more than a week in the first trimester, your provider will typically adjust your due date to match the ultrasound.
Later ultrasounds are less precise. In the second and third trimesters, measurements of the baby’s head and thighbone carry a margin of error of 7 to 10 days, while abdominal measurements can be off by 10 to 14 days. That’s because babies start growing at different rates as pregnancy progresses. So if you’re trying to figure out how far along you are and haven’t had an early ultrasound, getting one sooner rather than later gives the best answer.
Fundal Height as a Quick Check
Starting around 24 weeks, your provider will measure the distance from your pubic bone to the top of your uterus with a tape measure. This is called the fundal height, and it roughly corresponds to your gestational age in centimeters, plus or minus about 3. At 30 weeks, for example, a fundal height of 27 to 33 centimeters is considered normal. This isn’t used for precise dating, but it’s a useful check at routine appointments to confirm the baby is growing on track with your established timeline.
Can Blood Tests Tell You How Far Along You Are
Pregnancy hormone levels (hCG) rise dramatically in early pregnancy, but the ranges overlap so much that they can’t reliably pinpoint your week. At 5 weeks, hCG can be anywhere from 200 to 7,000. At 6 weeks, the range balloons to 200 to 32,000. Two women at exactly the same gestational age can have wildly different hCG levels and both be perfectly healthy. Blood tests confirm that you’re pregnant and that levels are rising appropriately, but they’re not a dating tool.
Understanding the Pregnancy Timeline
A full pregnancy spans 40 weeks from the LMP, divided into three trimesters. The first trimester covers weeks 1 through 13, the second runs from week 14 through 27, and the third goes from week 28 to delivery. But “full term” doesn’t simply mean 40 weeks. The medical definitions are more specific:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Postterm: 42 weeks and beyond
These categories exist because outcomes improve meaningfully between 37 and 39 weeks. A baby born at 39 weeks has better lung function and feeding ability than one born at 37 weeks, even though both fall within what used to be loosely called “term.” Knowing exactly how far along you are helps you and your provider make informed decisions about timing if any complications arise late in pregnancy.
Putting It All Together
For most people, the calculation works like this: start with your LMP date, count forward to today, and that’s your gestational age in weeks and days. If your cycles are irregular, adjust for the difference or rely on an early ultrasound instead. If your LMP-based date and your first-trimester ultrasound agree within a week, you can feel confident in your timeline. If they disagree by more than 7 days, the ultrasound date is more reliable. Your provider will settle on a single “estimated due date” early in your care, and all your pregnancy milestones will be measured against it from that point forward.