Mammograms are the gold standard, the baseline test. It will show what a sonogram won't.
A sonogram is a complementary test. If a breast is too dense or unclear for some reason, a doctor will order a sonogram for more information. The problem with sonograms is that they pick up a lot of things — cysts, lesions, etc. — that don't need treatment, but doctors see them and must take the next step to check them out.
Both tests are needed when there is a significant family history or when breast tissue is dense enough to cause the mammogram to be unclear. Your doctor should determine whether extra tests, a sonogram or even an MRI are needed.
When should we start getting mammograms?
Women should start by age 40. If there's a strong family history, they should start earlier. A rule of thumb is to take the age their family member was diagnosed and start 10 years earlier (if mother diagnosed at 40, a daughter should start at 30).
What about self-exams?
Of course, the most important thing is self-exams — this is when we find cancer at its earliest stage. Many women are uncertain what they are looking for, but should get comfortable with their own breasts and know how they usually feel. When something feels different, they should contact their doctor.
For more information on dealing with breast cancer check out breastcancer.org.