Accurate dating of pregnancy is one of the most important steps in your prenatal care. Every decision you make and many tests that are run are directly related to or interpreted from where you are in your pregnancy. Using the first day of your last menstrual period as the starting point is the standard convention for dating pregnancies ¯ that's because it's a date most women can recall, whereas the actual date of conception is a little harder to pinpoint.
It can be confusing, though. For example, an embryo that is twenty-six days from conception would be about forty days from the first day of the last menstrual period for a woman with twenty-eight-day cycles. Therefore, she could be called "almost six weeks," even though she is only almost four. A "due date" is typically calculated based on 280 days from the first day of your last menstrual period. Actually, the normal gestation of a human fetus is 266 days from conception.
Confused yet? The relevance of menstrual dates is that most books use them to discuss the progress of your pregnancy and further divide it into three trimesters. If you get a book on fetal development, however, it will be using actual dates that start with conception.
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