18 weeks pregnant
Enter the first day of your last period to get a due date and the current week. The example here follows a person who is at week 18.
How it works
Start on the first day of your last period. Move ahead 280 days for the due date. Divide the days that have passed by 7 to get weeks. The remainder is extra days. At 126 days along, 154 days remain.
Worked example
Use 27 June as the first day. Add 280 days for a due date of 3 April. Week 18 starts 126 days in, on 31 October. 126 divided by 7 is 18. Subtract 126 from 280 to get 154 days to go. This example assumes a 365-day year.
Common questions
Which trimester is week 18?
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 18 is week 5 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 16, which is 112 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 3 April, and 154 days remain from week 18.
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 3 April to 10 April.
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 27 June includes a 29 February, 3 April reads one day earlier, as 2 April.