Pregnancy is a time of expectation and waiting. Knowing how many weeks pregnant you are makes it easier to understand the developmental stage of your baby. Take it week by week and enjoy having the time to plan. By calculating the number of weeks you've been pregnant, you will also know your due date and which trimester you are in. The first trimester ends at week 12 and the second trimester at week 26; your due date will be the first day of week 40.
Find the date of the first day of your last menstrual period. This is week zero using the U.S. method.
Count the number of weeks since that date.
Ignore the fact that the first 2 weeks of pregnancy occur before conception (because ovulation occurs approximately 2 weeks after your period). This paradox allows for the uncertainty of calculating the exact date of conception.
Use the ovulation date method if you are confident of your conception date but have an irregular cycle.
Use the date of conception as your starting date.
Add two weeks to this date in order to reference pregnancy milestones used in the U.S. method.
Mark the 40th week on your calendar. This is your expected due date.
Only 5 per cent of babies are born on their due date, according to the American College of Obstetricians and Gynecologists.
This system assumes that your period is a perfect 28 days; however, women can have cycles from 22 to 44 days. If you know that you have a different cycle length, you may want to manually adjust this number or use an online pregnancy calculator.