how many weeks pregnant are you if gestational sac measuring 7w4d

Nadia

I never quite understood so wanted some expertise if anyone can offer? If my gestational sac measured 7w4d yesterday and was empty, does this mean I am 7w4d pregnant? Surely not, as that would mean i conceived on cd1 of my lmp (7 March). So maybe i am indeed 5w4d? Hence why nothing was seen?....praying..