One of the first questions you're asked when you announce a pregnancy is about how far along you are and the expected due date. The traditional method of determining pregnancy weeks is to count the first day of your last menstrual cycle as the start of pregnancy. If you don't have regular periods, however, this method might not be accurate. Early pregnancy tests and ultrasound have made dating pregnancy more precise.
Traditional Pregnancy Dating
Although you probably think pregnancy lasts 9 months, doctors count pregnancy as lasting 40 weeks, or 10 months, by counting the first day of the last menstrual cycle as Day 1 of pregnancy. At the time of your first missed period, your doctor will say you are 4 weeks pregnant. In reality, if you have a 28-day cycle, you just ovulated 2 weeks before and weren't actually pregnant -- the embryo didn't implant in the uterus -- until around week 3.
Newer Dating Methods
Most women have menstrual cycles ranging from 21 to 34 days, so using a 28-day cycle to date their pregnancy might not be accurate. Ultrasound and pregnancy tests can detect pregnancy within 1 to 2 weeks after implantation. These methods of early pregnancy detection can pinpoint the time of conception more accurately. Adding 2 weeks to that date determines how many weeks along you are, based on the 40-week calendar.