Traditionally, doctors have measured pregnancy from a number of convenient points, including the day of last
menstruation,
ovulation, fertilization, implantation and chemical detection. This has led to some confusion about the precise length of human pregnancy, as each measuring point yields a different figure.
The American Medical Association has stated that hormonal contraception that may affect implantation "cannot terminate an
established pregnancy".<1> Similarly, the British Medical Association has defined an "established pregnancy" as beginning at implantation.<2> The legal definition in the United Kingdom is not clear.<3> Other definitions exist. The American Heritage® Stedman''s Medical Dictionary defines "pregnancy" as "from conception until birth."<4> There has been some debate on the matter of definition in medical journals.<5>
Finally the standard historical method of counting the duration of pregnancy begins from the last
menstruation and this remains common with doctors, hospitals, and medical companies.<6> This system is convenient because it is easy to determine when menstruation ended, while both fertilization and implantation occur out of sight. An interesting consequence is that a pregnancy measured this way begins two weeks before ovulation.