It's hard to assign a diagnosis code based on the limited information you've given here. What was the original reason that was given for ordering the test that did not meet medical necessity? Is the patient currently pregnant? 'History of drug use in pregnancy' really doesn't tell us much. In addition, ICD-10 guidelines state that an unspecified drug use codes can only be assigned when it is associated with another disorder:
"As with all other unspecified diagnoses, the codes for unspecified psychoactive substance use (F10.9-, F11.9-, F12.9-, F13.9-, F14.9-, F15.9-, F16.9-, F18.9-, F19.9-) should only be assigned based on provider documentation and when they meet the definition of a reportable diagnosis (see Section III, Reporting Additional Diagnoses). These codes are to be used only when the psychoactive substance use is associated with a physical, mental or behavioral disorder, and such a relationship is documented by the provider."