Figuring out your current week is simple date math — days since your last period, divided by seven. Here is the method providers use, worked through with real dates you can check yourself.
Count the days from the first day of your last menstrual period to today and divide by 7. The whole number is your completed weeks; the remainder is extra days. Example: 77 days ÷ 7 = exactly 11 weeks. If you only know your due date, subtract the days remaining until it from 280 — that gives the same day count.
280
days in a full-term pregnancy from LMP
÷ 7
converts your day count into weeks
13 / 27
week boundaries of the three trimesters
2 wks
built-in offset between LMP and conception
How Pregnancy Weeks Are Calculated
Every provider, chart, and pregnancy app uses the same starting line: gestational age is measured from the first day of your last menstrual period (LMP), not from conception. The method is deliberately simple — count the calendar days from that date to today, divide by 7, and the result is your pregnancy length in weeks and days.
The remainder matters. If your day count is 73, dividing by 7 gives 10 with 3 left over — so you are 10 weeks and 3 days pregnant, written 10w3d. That precision is not pedantry: screenings, growth charts, and term definitions all key off exact days, which is why providers always speak in weeks-plus-days rather than rounding.
One more thing the formula assumes: a 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter, the raw LMP count will be off by roughly the difference — a 35-day cycle shifts the true start about 7 days later than the formula assumes. We will cover that adjustment below.
Worked Examples: Real Dates, Step by Step
The best way to trust the math is to see it run on real calendar dates. Each example below is fully worked — count the days, divide by 7, read off the weeks.
Example 1: A clean 10 weeks
Say your LMP began on January 1, 2026, and today is March 12, 2026. January 1 to March 12 spans exactly 70 days (31 days of January after the 1st, plus 28 days of February 2026, plus 12 days of March: 30 + 28 + 12 = 70). Divide 70 by 7 and you get exactly 10 — so you are 10 weeks and 0 days pregnant. As a check, January 1 plus 280 days lands on October 8, 2026, your due date.
Example 2: Another exact week
Now suppose your LMP was July 20, 2026 and today is October 5, 2026. From July 20 to October 5 is 77 days (11 days remaining in July, 31 in August, 30 in September, 5 in October: 11 + 31 + 30 + 5 = 77). 77 ÷ 7 = 11 exactly — you are 11 weeks and 0 dayspregnant, and your due date is July 20 plus 280 days: April 26, 2027.
Example 3: Weeks plus leftover days
Most dates do not divide evenly. Take an LMP of April 22, 2026 with today as October 5, 2026: the span is 166 days (8 + 31 + 30 + 31 + 31 + 30 + 5 = 166). Divide 166 by 7 to get 23 with a remainder of 5 (23 × 7 = 161; 166 − 161 = 5). You are 23 weeks and 5 days pregnant — written 23w5d.
Notice the pattern in every example: the arithmetic is just subtraction and division. If you can count days on a calendar, you can date your own pregnancy to the day — no app required, though an app saves you the calendar-counting.
Counting Backward From Your Due Date
What if you know your due date but not your LMP — because an ultrasound set the date, or you simply never tracked your periods? You can work the same math in reverse. A full pregnancy is 280 days, so the days you have already completed equal 280 minus the days remaining until your due date.
Days pregnant = 280 − (due date − today’s date)
Example: your due date is October 8, 2026 and today is September 10, 2026. The due date is 28 days away (20 days remaining in September plus 8 in October). 280 − 28 = 252 days elapsed. 252 ÷ 7 = 36 exactly — you are 36 weeks and 0 days pregnant. The reverse math always agrees with the forward math, because both count the same 280-day span from opposite ends.
This backward method is also handy for answering “how far along was I on that date?” — a question that comes up with travel, illness, or medication timing. Pick any past date, count the days between it and your due date, and subtract from 280. If the mental math gets tedious, the reverse due date calculator runs it instantly from either direction.
Gestational Age vs Fetal Age: The Two-Week Offset
Here is the quirk that confuses nearly every newly pregnant person: at the moment of conception, you are already considered about two weeks pregnant. Gestational age (the clinical count from your LMP) runs roughly two weeks ahead of fetal age (the actual age of the embryo since fertilization).
The offset exists because ovulation typically happens around day 14 of the cycle, and the LMP count starts at day 1. So at 4 weeks gestational age — the classic “missed period” moment — the embryo itself is only about 2 weeks old. At 6 weeks gestational age, when a heartbeat often becomes detectable, the embryo is about 4 weeks along in actual development.
Why keep a system that counts two weeks before conception? Because the LMP is observable and recordable, while the instant of fertilization almost never is. The entire medical framework — growth charts, screening windows, term definitions — is calibrated to gestational age, so the offset is baked into every milestone. When your provider says “8 weeks,” they mean 8 weeks gestational; the embryo’s developmental age is about 6 weeks, and every chart they use already accounts for that.
The practical rule is simple: always use gestational age (from LMP) when reading pregnancy information, and never subtract the two weeks yourself. Every guide, calculator, and milestone on this site — including the week-by-week guide — speaks gestational.
Trimesters and What Each Stage Means
Once you know your week, the next question is usually “which trimester am I in?” The conventional split divides the 40 weeks into three roughly equal parts:
Trimester
Weeks
What defines it
First trimester
Weeks 1–13
Implantation, organ formation, first screenings
Second trimester
Weeks 14–27
Growth, anatomy scan, feeling movement
Third trimester
Weeks 28–40+
Rapid growth, birth preparation, delivery
The boundaries are conventions, not biological switches — nothing magical happens overnight between week 13 and week 14. But they are useful shorthand: the first trimester covers formation of all major organs, the second covers rapid growth and the anatomy scan, and the third covers finishing growth and preparation for birth. Knowing your trimester tells you which screenings, symptoms, and preparations are relevant right now.
Common Mistakes When Counting Weeks
The math is simple, which makes the mistakes all the more common. The classic error is counting from conception or from a positive test instead of from the LMP — that silently shaves about two weeks off your count and makes every milestone look “early.” If your numbers disagree with your provider’s by roughly two weeks, this is almost certainly why.
The second classic is the off-by-one: using the last day of your period, or the day your period ended, instead of the first day it began. The count always starts on day one of bleeding. A related slip is forgetting that February has 28 days (29 in a leap year) when counting across months by hand — small calendar errors that compound into a wrong week.
The third is ignoring cycle length. If your cycles run 35 days like clockwork, raw LMP math will put you about a week further along than you really are. Adjust by subtracting the excess (35 − 28 = 7 days) from your day count — or better, let a first-trimester ultrasound settle it. And if your cycles are irregular, treat any LMP-based count as provisional until a scan confirms it.
A fourth, subtler slip is mixing up the due date with the count itself: some people subtract their weeks from 40 to get “weeks remaining” and then treat that as the current week. Weeks remaining and weeks completed are complements, not substitutes — at 23w5d you have 16 weeks and 2 days remaining (280 − 166 = 114 days; 114 ÷ 7 = 16 remainder 2), which is useful for planning but is not your gestational age. Keep the two numbers separate and both stay useful.
When Your Provider’s Number Differs From Yours
It is completely normal for your home count and your provider’s chart to disagree at first — that is exactly what the dating ultrasound is for. If a first-trimester scan measures a gestational age more than about 7 days different from your LMP count, your provider will generally adopt the scan-based date instead, and your “weeks pregnant” shifts along with it.
This can feel like losing or gaining a week of pregnancy overnight. You did not — the pregnancy is the same; the ruler just got sharper. The ultrasound measures the embryo directly at the stage when growth is most uniform between pregnancies, so its number is the more trustworthy one. After about 20 weeks, providers generally stop adjusting, because later scans measure growth (which varies) rather than age.
If the two numbers agree within a few days, your provider will usually keep the LMP-based date — the scan has confirmed it rather than replaced it. Either way, once a date is locked in, use it consistently: it is the number your screenings, growth charts, and term assessments will all reference from here on.
Quick answer
What if I know my due date but not my LMP? Count the days from today to your due date and subtract that from 280 — the result is your days pregnant, which you divide by 7 for weeks. Example: due date 28 days away → 280 − 28 = 252 days → exactly 36 weeks. The reverse calculator does this in one step.
This calculator provides estimates for educational purposes and is not medical advice. Always confirm with your healthcare provider.
Frequently Asked Questions
Common questions answered by our medical team
Count the days from the first day of your last menstrual period to today, then divide by 7. Whole weeks are your completed weeks; the remainder is extra days. For example, 73 days equals 10 weeks and 3 days. If you know your due date instead, subtract the days remaining from 280 — that gives the same day count.