all my doc ever did at that stage was check the heartbeat and sometimes the fundus height
i did get have 6 ultrasounds however, one was at 7 or 8 weeks (not by my doc, from a crisis pregnancy center that did my initial test) then one at 12 weeks, then 3 around 20 something weeks (i think it was 21, 24, and 27 or 28 but im not sure 100%) but that was because they thought she might have downs and couldnt get pictures of everything they needed (i also had an amni at the 21 week one) then again at 37 weeks (i think, it was recently) to be sure she was head down cause they couldnt tell
the baby measuring a little small isnt that big of a deal, he/she may not have hit a certain groth spurt yet or maybe your dates were a little off or maybe it was just the positioning, or maybe your baby is just petite...there are lots of reasons
ive actualy measured about a week big (fundus height) until about 28 weeks or so...then i started measuring 2 weeks big lol...but thats because she is long
try not to worry so much, if the baby looked fine on the last ultra sound and the doc isnt concerned i dont think there is too much to worry about...i know it can be hard to do but just try to relax a bit
sorry this went on so long, im tired and rambly lol
~alicia~