Antibiotics should be taken only if necessary, but because pregnancy increases a woman's risk of developing infections, doctors usually advise taking them even if you aren't sick. This is especially true in the first trimester, when complications often arise from minimal infections. However, if you are already infected with a bacteria that is resistant to common antibiotics, then it may be necessary to start therapy before you know you are pregnant so that the infection will be cleared up before your fetus is affected. Women who are allergic to any drugs in antibiotic family should not take them unless really needed. Some examples of these drugs are penicillin, cephalosporin, erythromycin, and tetracycline. Taking antibiotics during pregnancy can lead to problems such as resistance of bacteria to antibiotics, birth defects, changes in the gut microbiome, and allergies.