13 weeks pregnant
Week 13 of pregnancy means 91 days since the last period began. Use the form to place your own dates, or read the example below.
How it works
Forty weeks is 280 days. Add 280 days to the first day of your last period for the due date. Divide the days that have passed by 7 for weeks and days. At 91 days along, 189 days remain.
Worked example
Start at 12 January. Count 280 days ahead to reach 19 October. Count 91 days ahead to reach 13 April. 91 divided by 7 is 13 weeks. 189 days remain. A leap year shifts dates after February by one day.
Common questions
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 19 October to 26 October.
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 12 January includes a 29 February, 19 October reads one day earlier, as 18 October.
How many months is 13 weeks pregnant?
About 3.0 months. A pregnancy is 40 weeks, a little over 9 calendar months. Months are only a rough guide. At 13 weeks there are 189 days to the due date.
Which trimester is week 13?
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 13 is week 13 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 11, which is 77 days.