Pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you conceived. A full-term pregnancy lasts 40 weeks, or 280 days, from that starting point. This means you’re already considered “2 weeks pregnant” at the time of ovulation and roughly “4 weeks pregnant” by the time you miss your period.
Why Counting Starts Before Conception
This is the part that confuses almost everyone. Pregnancy weeks are measured by what’s called gestational age, and the clock starts ticking on day one of your last period, about two weeks before an egg is even fertilized. Doctors use this system because most people can recall when their period started, but very few know the exact day they ovulated or conceived.
That two-week gap means gestational age and the actual age of the embryo are never the same number. When your provider says you’re 8 weeks pregnant, the embryo has really been developing for about 6 weeks. Every week count you hear at appointments, see on an app, or read on a lab report uses gestational age unless someone specifically says otherwise.
How to Calculate Your Due Date
The standard formula, called Naegele’s Rule, takes three steps:
- 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.
So if your last period started on March 10, you’d count back to December 10, then add a year and 7 days to land on December 17 as your estimated due date. This formula assumes a 28-day cycle with ovulation on day 14, which is why it doesn’t work perfectly for everyone.
To figure out how many weeks along you are right now, count the number of weeks (and extra days) from the first day of your last period to today. If your LMP was 9 weeks and 3 days ago, you’re 9 weeks and 3 days pregnant. Your provider will write this as “9w3d.”
What If Your Cycles Aren’t 28 Days
Naegele’s Rule assumes you ovulated on day 14 of a 28-day cycle. If your cycles are consistently longer, say 35 days, you likely ovulated around day 21 instead of day 14. That means an LMP-based calculation would overestimate your gestational age by about a week. Shorter cycles push the estimate in the other direction.
This is one of the main reasons providers rely on first-trimester ultrasound to confirm or adjust your dates. An ultrasound done before 14 weeks is the most accurate method of establishing gestational age. The technician measures the embryo from head to rump, and because early embryos grow at a highly predictable rate, the measurement pins down your dates more reliably than period recall alone. If the ultrasound date and the LMP date differ by more than a few days, your provider will typically shift your due date to match the ultrasound.
How Weeks Translate to Months and Trimesters
People will ask “how many months along are you?” constantly, but the math never lines up neatly because calendar months aren’t exactly four weeks long. In obstetric terms, each “pregnancy month” equals exactly 4 weeks, which is why 40 weeks works out to 10 pregnancy months rather than the “9 months” everyone talks about. Here’s a simplified breakdown:
- Weeks 1 through 4: Month 1
- Weeks 5 through 8: Month 2
- Weeks 9 through 12: Month 3
- Weeks 13 through 16: Month 4
- Weeks 17 through 20: Month 5
- Weeks 21 through 24: Month 6
- Weeks 25 through 28: Month 7
- Weeks 29 through 32: Month 8
- Weeks 33 through 36: Month 9
- Weeks 37 through 40: Month 10
The three trimesters divide more cleanly. The first trimester runs from the start of pregnancy through 13 weeks and 6 days. The second trimester covers 14 weeks 0 days through 27 weeks and 6 days. The third trimester begins at 28 weeks and stretches to delivery, typically around 40 weeks.
Counting Weeks After IVF
If you conceived through IVF, the count works a little differently because you know the exact age of the embryo at transfer. For a day-5 embryo transfer (a blastocyst), you add 261 days to the transfer date to get your due date. For a day-3 transfer, you add 263 days. On the day of transfer, you’re already considered pregnant by the number of days the embryo has been developing. A 5-day transfer, for example, puts you at 2 weeks and 5 days of gestational age on transfer day, because you add back the standard two-week pre-ovulation window plus the embryo’s age.
Your provider will often convert this to an equivalent LMP date so that all your appointments and lab work follow the same 40-week timeline as any other pregnancy.
Your Due Date Is an Estimate, Not a Deadline
Only a small fraction of babies arrive on the exact date predicted. Among first-time mothers, about half give birth by 40 weeks and 5 days, with the other half delivering after that point. For people who’ve given birth before, the midpoint is slightly earlier, around 40 weeks and 3 days. A large study tracking natural onset of labor found that 25% of first-time mothers delivered by 39 weeks and 5 days, 50% by 40 weeks and 5 days, and 75% by 41 weeks and 2 days.
In practice, a “due date” really represents the middle of a window. Full term is defined as 39 weeks 0 days through 40 weeks 6 days, but delivery anywhere from 37 weeks onward is common. Thinking of your due date as a due week, or even a due two-week window, is more realistic than circling one day on the calendar.