Thanks for your query.
The weeks of pregnancy
are definitely calculated from the first day of your last cycle.
However, having just got off birth control, chances are your periods as well as ovulation were erratic and unpredictable.
Hence there is no point trying to date your pregnancy from that angle.
Your sonogram will accurately tell you the exact gestational age
, and it is much better to date your pregnancy from this first ultrasound
It will be more reliable in the long run.