The most common methods of dating a pregnancy use the first day of your last menstrual period and the most common length of menstrual cycles, which is 28 days. According to Epigee.org, however, 30 percent of women have menstrual cycles outside of this length. For those women with a 30-day cycle, there is a relatively easy way to determine your due date and your current number of pregnancy weeks.
Determine the first day of your last period.
Take the month of your last period and subtract three months from that month. For example, if your period began in December and you subtract three months, the month will then be September.
For a 30-day cycle, add nine days to the first day of your last period. For example, if you add nine days to Dec. 13, you will get Dec. 22.
Add one year to the year of your last period if your due date will be in the following year. For example, if the first day of your period was Dec. 13, 2010, then your estimated due date will be in the year 2011. So your due date is Sept. 22, 2011.
Determine the current number of pregnancy weeks by adding every day from the first day of your last period to the current date. You are then going to divide the number of days by seven. For example, if your period was Dec. 13, and the date is currently Jan. 30, there will be 49 days between those dates. Divide 49 by seven and the answer is seven. Your current number of pregnancy weeks is seven. Weeks are not determined from the moment of conception, as it is difficult to determine precisely when conception occurred in most cases.