Goldstein, Steven R; Md, Snyder; Jon, R. Md; Watson, Carol B; Danon, Milton D
(http://journals.lww.com/greenjournal/Abstract/1988/08000/Very_Early_Pregnancy_Detection_With_Endovaginal.12.aspx, 1998)
There are many situations in which the earliest possible detection of an intrauterine pregnancy would enhance clinical management. Current radioimmunoassays for hCG can detect pregnancy as early as eight to 12 days ...