How to Calculate Due Date Based on Current Week
A woman's pregnancy due date is generally 40 weeks after her last known menstrual period, although there is great variance in when a baby will arrive; some arrive several weeks early, and some are a few weeks late. The due date is typically calculated from the first date of your last menstrual period, but a due date can also be deduced by the current week of pregnancy with basic calculations.
Things You'll Need
- Calculator
- Calendar
Instructions
Calculate the current week of pregnancy. This is found by counting the number of weeks since the first date of the last menstrual period, and then subtracting 2. This is because it is assumed that conception takes place two weeks after menstruation. For instance, if it has been 20 weeks since the last menstrual period, the current pregnancy week is 18. Take the current pregnancy week number and subtract it from 40, which is the total number of weeks in an average pregnancy. For instance, if the woman is 18 weeks pregnant, subtract 18 from 40, which is 22. Look at a calendar and, starting from the current date, count the number of weeks left in Step 2. For example, the woman who is 18 weeks pregnant would add 22 weeks, or about five-and-a-half months, from the current date. This is the approximate due date of her baby.
