Why the count starts before conception
Pregnancy is assumed to start 2 weeks after the first day of the LMP, so an extra 2 weeks is counted at the beginning, when you are not actually pregnant. The 40 weeks of pregnancy include those 2 weeks.1
The 2 weeks come from the convention behind the count, which assumes a regular 28-day cycle with ovulation on the 14th day.3 That is an assumption, not a measurement: ovulation, and so conception, often happens earlier or later than day 14, even in regular cycles.4 The first day of bleeding, by contrast, is a clear, specific date, and starting the count there gives everyone the same reference point.
On the convention's 28-day cycle, the next period would be due 28 days after the LMP, which is exactly 4 weeks. So by the time a period is missed, a pregnancy is already counted as about 4 weeks along.
Completed weeks and days
Gestational age, how far along a pregnancy is, is usually given in completed weeks plus extra days.51 The World Health Organization's ICD-11 Reference Guide sets out the counting:5
- The first day of gestation is day 0, written as 0+0 (zero completed weeks, zero days).
- Six days later is 0+6. The next day starts week 1: 1+0.
- Completed weeks are the days since the first day divided by 7, as a whole number, with the remainder as days. Day 252 is 36+0, and day 258 is 36+6.
ACOG writes the same thing as a fraction: "36 and 3/7 weeks" means 36 weeks and 3 days of pregnancy.1
- 36+3
- 36 completed weeks and 3 days
- 36 3/7 weeks
- The same, written as a fraction
- "37th week"
- The week from 36+0 to 36+6: during it, 36 weeks are completed and the 37th is under way. Ordinal weeks are one ahead of completed weeks.
How to count your weeks and days
You need two dates: the first day of your LMP and the date you want to count to. Then:
- Count the calendar days between the two dates. The LMP day itself is day 0.
- Divide by 7. The whole number is your completed weeks.
- The remainder is the extra days.
Worked example
First day of the LMP: August 1, 2026. Counting to October 2, 2026: 62 days.
62 ÷ 7 = 8 completed weeks (56 days) with 6 days left over: 8+6, or 8 weeks, 6 days. One day later, on October 3, it is 63 days: exactly 9+0.
Counting by hand is where most mistakes happen, usually by starting the count at 1 instead of 0 or by miscounting the days in a month. The Pregnancy Weeks Calculator does this count for today's date.
Where the trimesters fall
The 40 weeks are often grouped into three trimesters, counted from the first day of the LMP:1
- First trimester
- First day of the LMP to 13 weeks and 6 days
- Second trimester
- 14 weeks and 0 days to 27 weeks and 6 days
- Third trimester
- 28 weeks and 0 days to 40 weeks and 6 days
Because the trimesters are counted from the LMP too, the first trimester includes the two weeks before conception. Weeks after 37 have their own names (early, full, late and postterm); see what counts as a full-term pregnancy.
Gestational age and the age of the embryo
Development is often described from fertilization instead of from the LMP: the embryo stage lasts up to 8 weeks after fertilization, and from 9 weeks after fertilization it is called a fetus.1 On the standard convention, gestational age is about 2 weeks more than the time since fertilization. So "8 weeks pregnant" and "an 8-week-old embryo" are not the same thing.
The two-week gap is an average; LMP vs. conception date explains why the real gap varies.
Who sets the official count
In clinical care, gestational age is based on the best obstetric estimate. Its starting point is usually the first day of the LMP if an early ultrasound scan confirms it, or the early scan where the two differ.5 In England, the dating scan checks how many weeks pregnant you are and works out your due date.6
So the week count your doctor or midwife uses can differ from a count based on your LMP alone. Their count is the one that applies to your care; ultrasound dating vs. LMP dating explains how the two are compared.
How many weeks are you today?
Enter the first day of your last period and the Pregnancy Weeks Calculator counts the weeks and days to today.
Common questions
Am I pregnant during the first two weeks?
No. The first 2 weeks of the count come before conception is assumed to happen; they are counted even though you are not actually pregnant yet.1
What is the difference between "37 weeks" and "the 37th week"?
"37 weeks" (37+0) means 37 weeks are completed. "The 37th week" is the week before that, from 36+0 to 36+6, while the 37th week is under way.
Why does my week count differ from my doctor’s?
Clinical dating uses the best obstetric estimate, which follows an early ultrasound when it differs from the LMP.5 A count from your LMP alone will not reflect that.
Sources
- American College of Obstetricians and Gynecologists (ACOG). How Your Fetus Grows During Pregnancy. patient FAQ, reviewed 2026.
- NHS (England), Best Start in Life. Week 4 (week-by-week guide to pregnancy). accessed October 2026.
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- BMJ. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. 321(7271):1259–62. 2000.
- World Health Organization (WHO). ICD-11 Reference Guide. accessed October 2026.
- NHS (England). 12-week scan. reviewed November 2023.