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 by the time ovulation and fertilization actually happen. This starting point catches most people off guard, but it’s the standard used by every major medical organization and is how your provider will track your pregnancy from the very first appointment.
Why the Count Starts Before Conception
The reason pregnancy is dated from your last period is practical: most people can remember when their period started, but pinpointing the exact day of conception is nearly impossible. Ovulation can shift by several days from cycle to cycle, and sperm can survive in the body for up to five days before fertilizing an egg. So rather than guess, the medical convention is to start the clock on day one of your LMP and count forward 280 days (40 weeks) to estimate your due date.
This means there’s always roughly a two-week gap between your “gestational age” (the number your provider uses) and the actual age of the embryo. When your doctor says you’re 8 weeks pregnant, the embryo has been developing for closer to 6 weeks. This distinction rarely matters in everyday conversation, but it explains why the math can feel off when you think about when you actually conceived.
How to Calculate Your Week Count
Start with the first day of your last menstrual period. Count the number of days from that date to today, then divide by seven. The whole number is your week, and the remainder is the extra days. If it’s been 52 days since your LMP, you’re 7 weeks and 3 days pregnant.
To estimate your due date, you can use Naegele’s Rule, a simple three-step formula from Johns Hopkins Medicine:
- 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.
So if your last period started on March 10, you’d count back three months to December 10, then add a year and seven days to land on December 17 as your estimated due date. This formula assumes a 28-day cycle with ovulation on day 14, which brings us to a big caveat.
What If Your Cycles Are Irregular
The LMP method works best when your cycles are close to 28 days. If your periods are irregular, the count can be significantly off. A study of 277 women with irregular cycles found that LMP-based dating disagreed with ultrasound dating by more than a week in over half the cases, and by more than two weeks in about a quarter of them. When researchers compared both methods against the actual delivery date, ultrasound was far more accurate: 83.6% of women delivered within two weeks of the ultrasound-predicted date, compared to only 64.8% using LMP alone.
If you have irregular cycles, don’t remember your last period, or had spotting that could be mistaken for a period, your provider will rely on an early ultrasound to establish your dates. First-trimester ultrasounds measure the embryo’s length and are the most precise way to estimate gestational age outside of the LMP. When there’s a significant gap between LMP dating and ultrasound dating, the ultrasound estimate typically becomes the official one used for the rest of your pregnancy.
How IVF Pregnancies Are Dated
If you conceived through IVF, the math is more exact because the conception date is known. Gestational age is calculated by taking your embryo transfer date, subtracting the age of the embryo at transfer, and then adding 266 days to find the due date. For a 5-day embryo transfer on January 20, you’d subtract 5 days to get a conception date of January 15, then count forward 266 days. Your provider will convert this into the standard week count so your care follows the same milestones as any other pregnancy.
Weeks to Months: A Simple Conversion
One of the most confusing parts of pregnancy counting is converting weeks into months. Calendar months vary between 28 and 31 days, but pregnancy months are typically counted as exactly four weeks each. Here’s how the math breaks down:
- Month 1: Weeks 1 through 4
- Month 2: Weeks 5 through 8
- Month 3: Weeks 9 through 12
- Month 4: Weeks 13 through 16
- Month 5: Weeks 17 through 20
- Month 6: Weeks 21 through 24
- Month 7: Weeks 25 through 28
- Month 8: Weeks 29 through 32
- Month 9: Weeks 33 through 36
- Month 10: Weeks 37 through 40
Yes, that’s 10 months, not 9. This is another source of confusion. Nine calendar months and 40 weeks don’t divide neatly, so pregnancy actually spans parts of 10 four-week blocks. When someone asks how far along you are, weeks are always more precise than months.
Trimesters by the Numbers
The American College of Obstetricians and Gynecologists defines the three trimesters as:
- First trimester: First day of LMP through 13 weeks and 6 days
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days
What “Full Term” Actually Means
Not all weeks at the end of pregnancy are equal. The designation of “term” has been broken into four categories that reflect real differences in newborn health outcomes:
- Early term: 37 weeks 0 days through 38 weeks 6 days
- Full term: 39 weeks 0 days through 40 weeks 6 days
- Late term: 41 weeks 0 days through 41 weeks 6 days
- Post-term: 42 weeks 0 days and beyond
These distinctions matter because babies born even a week or two before 39 weeks can have slightly higher rates of breathing and feeding difficulties compared to those born in the full-term window. This is why your week count becomes especially important in the final stretch of pregnancy, and why providers are careful about the accuracy of dating before making decisions about timing of delivery.