There are two basic schools of thought that you can use to determine how many weeks pregnant you are. The first bases your pregnancy's progression on the date of your last menstrual period. The other uses the conception date to determine how far along you are. Most health care providers calculate your due date and your shift from one trimester to the next based on your last menstrual period. This, according to the American Pregnancy Association, is because it's more difficult to pinpoint the exact date of conception and many women don't remember the exact date. It's also tricky to pinpoint which conception date resulted in pregnancy if a couple had intercourse more than once.
Write down the date of your last menstrual period on a calendar. Count forward the days, starting with the date of your last menstrual period and ending with today. Divide that number by seven to get the number of weeks since your last period. This will also serve as the number that health care providers use to determine how many weeks pregnant you are.
Mark your conception date on the calendar, if you know it. If not, mark the most likely conception dates, which are those that happened just before, during and just after ovulation. Note that ovulation typically occurs two weeks before your menstrual period. Choose the date of intercourse you suspect most likely resulted in conception and count forward on the calendar to the current day. Divide that number of days by seven to get your weeks pregnant based on conception.
Ask your doctor to determine how many weeks you are during your first ultrasound if you're unsure of the conception date and/or if you have in irregular period. Your doctor can use fetal growth and development milestones to determine how many weeks your baby has been developing.