Question:

If you are pulled over for excessive swerving and your blood alcohol concentration (BAC) is found to be less than the legal limit of 0.08%,

A you can be charged with driving under the influence (DUI).

Explaination

Even if your BAC is less than the legal limit of 0.08%, an officer may charge you with driving under the influence (DUI) if the officer believes that your ability to operate a vehicle safely is impaired by alcohol. [A. Alcohol, Drugs and Driving, Section 6: Dangerous Driving Behaviors, Maryland Driver's Manual]