How to Count Weeks

There are a wide number of free calculators available that help you count the number of weeks you have been pregnant and determine your due date. Most health care professionals use a formula called Naegle's Rule, to calculate the number of weeks based on a set point -- the date of you started your last menstrual period -- and count forward 40 weeks for a due date. While this assumes, based normal pregnancy duration of 38 weeks, that you became pregnant exactly two weeks after your period, this is the most accurate way to count weeks, until further along in your pregnancy.

Things You'll Need

• Calendar

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Instructions

• 1

  Find, or estimate, the date of the first day of your last period (LMP).

• 2

  Count backward three months from that date, then add seven days. This gives you your approximate due date. Even though you are counting backward, the date is the next occurrence of that date. For example, a LMP in February should give you an approximate due date in November.

• 3

  Use your approximate due date to the count backward, week by week, to count which week of pregnancy you will be in during which set of dates.