How many weeks pregnant am I?
4 min read · Updated
Pregnancy is measured in completed weeks and days, counted from the first day of the last menstrual period. It is a simple system that trips almost everyone up at first.
Here is how the counting works, and what the numbers mean once you have them.
The arithmetic
Count the days from the first day of your last period to today. Divide by seven. The whole number of weeks is your gestational week, and whatever is left over is the number of days.
So 85 days is 12 weeks and 1 day, written 12w1d in clinical notes. The week number only advances once a full seven days have passed, so you spend a whole week at each number.
Twelve weeks pregnant is not the twelfth week
Saying you are 12 weeks pregnant means twelve full weeks are behind you and you are living through the thirteenth. It works exactly like age: a 12-year-old has completed twelve years and is in their thirteenth.
This is why "I'm 12 weeks" and "I'm in week 12" describe points a week apart. Maternity notes always use completed weeks, so that is the form worth getting used to.
Where the trimesters fall
Trimesters divide the 40 weeks into three roughly equal stretches. The most common division puts the first trimester through the end of week 13, the second from week 14 to the end of week 27, and the third from week 28 onwards.
You will see slightly different boundaries in different places, because the split is a convenience rather than a biological event. Nothing changes on the day one trimester becomes the next.
- First trimester: 0w0d to 13w6d
- Second trimester: 14w0d to 27w6d
- Third trimester: 28w0d onwards
Why weeks, not months?
Because months are not four weeks long. Forty weeks is a little over nine months, and the mismatch compounds, so any weeks-to-months conversion is a convention rather than arithmetic.
The calculators here show a month alongside the week count, using the common nine-month table below. Treat it as a rough guide: other sources shift these boundaries by a week, and no two agree exactly. For anything that matters, weeks are the unit your care team will use.
- Month 1: weeks 1-4
- Month 3: weeks 9-13
- Month 5: weeks 18-22
- Month 7: weeks 28-31
- Month 9: weeks 36-40
When the numbers disagree
If a scan has revised your due date, work from the revised due date rather than your last period. Counting from an old last period date will keep producing a week count that no longer matches your notes.
The pregnancy week calculator accepts either input, so entering the due date your care team gave you is the quickest way to stay in step with them.