27 weeks pregnant
Pregnancy weeks are counted from the start of the last period, not from conception. See how week 27 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. Week 27 runs from day 189 to day 195 of the 280.
Worked example
The first day is 26 March. Add 189 days to get 1 October. 189 divided by 7 is 27 weeks. Add the 91 days still to go and the total is 280. The due date is 31 December. The year in this example has 365 days.
Common questions
Which trimester is week 27?
Second 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 27 is week 14 of the second 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 25, which is 175 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 31 December, and 91 days remain from week 27.
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 31 December to 7 January.
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. The span from 26 March does not cross February. A leap year changes nothing for 31 December.