Ultrasound to determine birth plan!

So to start off with I've had medical problems long before I got pregnant including siezures and high BP. I want to schedule an ultrasound through insurance ( only one I've had in CO was at 21 weeks) to determine approximate size of the baby to determine my birth plan. My mom and little brother almost died at birth because he was too big for her 9lbs 3oz

And with my history of siezures and high BP (havent had any problems while pregnant) if baby is bigger then 8.5 lbs I'd like to schedule a csection. Is it to late to schedule one?! I'll be 35 weeks next week at my dr appt