21 weeks pregnant
Week 21 of pregnancy means 147 days since the last period began. Use the form to place your own dates, or read the example below.
How it works
Add 280 days to the first day of your last period to get the due date. To find the current week, count the days since that day and divide by 7. Week 21 is 147 days.
Worked example
Start at 8 September. Count 280 days ahead to reach 15 June. Count 147 days ahead to reach 2 February. 147 divided by 7 is 21 weeks. 133 days remain. A leap year shifts dates after February by one day.
Common questions
Which trimester is week 21?
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 21 is week 8 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 19, which is 133 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 June, and 133 days remain from week 21.
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 June to 22 June.
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 8 September includes a 29 February, 15 June reads one day earlier, as 14 June.