Hi i think as long as you are close to doubling you are fine, i have stressed myself out sooo many times because I did'nt double in the time frame, and yes i have had some slow starts that have worked out perfectly, try not to worry if it makes you feel better keep having your BTs repeated & get an early US but aim for 5-6weeks so you get the HB you don't want something else to stress about, good luck.