12 weeks pregnant
Pregnancy weeks are counted from the start of the last period, not from conception. See how week 12 fits the calendar in the example under the form.
How it works
Due date equals the first day of your last period plus 280 days. Weeks along equals days since that first day divided by 7, rounded down. On this page the pregnancy is 84 days from the first day of the last period.
Worked example
The first day is 9 December. Add 84 days to get 3 March. 84 divided by 7 is 12 weeks. Add the 196 days still to go and the total is 280. The due date is 15 September. The year in this example has 365 days.
Common questions
Which trimester is week 12?
First trimester. The first trimester covers weeks 1 to 13, the second covers weeks 14 to 27 and the third runs from week 28 to the birth. Week 12 is week 12 of the first trimester.
Why do weeks start from the last period and not conception?
Most people know the date of their last period but not the day of conception. Counting from the last period gives everyone the same starting point. A pregnancy is about two weeks longer on paper than from conception. Counted from conception, this would be week 10, which is 70 days.
How accurate is a due date?
Only a small share of babies arrive on the due date. Most arrive within a couple of weeks either side. Treat the date as the middle of a window. Here the due date is 15 September, and 196 days remain from week 12.
What if my cycle is not 28 days?
The 280 day rule assumes ovulation near day 14. If your cycle is much longer or shorter, your doctor may adjust the due date, often with an ultrasound. With a 35-day cycle some clinicians add 7 days, which would move 15 September to 22 September.
Does a leap year change the due date?
It can. A year with an extra day in February moves dates after that day by one. The calculator above uses the real calendar for your dates. If the span from 9 December includes a 29 February, 15 September reads one day earlier, as 14 September.