Final answer:
After treatment for primary syphilis during pregnancy, monthly lab testing with RPR/FTA is necessary for follow-up.
Step-by-step explanation:
After being diagnosed with primary syphilis during the first trimester of pregnancy and receiving treatment with appropriate antibiotics, the client would require follow-up during the pregnancy. The most suitable option would be monthly lab testing with RPR/FTA. The RPR (Rapid Plasma Reagin) and FTA (Fluorescent Treponemal Antibody Absorption) tests are used to monitor the effectiveness of treatment and to detect any recurrence or persistence of the infection. These tests help in early identification of any potential risk to the fetus and allow for timely intervention.