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There is no any difference between Abdominal and vaginal scan but vaginal scan has more effective vision then abdominal scan.
In every cycle size and time of follicle formation is different because it always depends upon your hormonal
level.So don't worry 8 mm. thickness is ok but follicle rupture is necessory to get conceive.Go for usg till confermation of follicle rupture.
Consult gynecologist for better guidance.