Should women under 50 add a screening breast ultrasound to their screening mammogram yearly?

No.  A study looked at IF adding a breast ultrasound to a screening mammogram for women less than 50 years old (regardless of their breast cancer risk)  HELPED detect breast cancers. 

Breast cancer was detected at a similar rate across the groups (those with only screening mammogram and those with BOTH screening mammograms and ultrasounds).  5.4 versus 5.5 per 1,000 screens.  So, the additional screening test did not find more breast cancers than screening mammogram alone.

The downside to getting screening ultrasounds in addition to mammogram are unnecessary breast biopsies.  The breast biopsy rate was TWICE as high for the combination screening imaging compared to women who only received screening mammograms. 

Sometimes more testing is not better testing.

Want more information?  https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2728448