Can keflex cause yeast infection when you're pregnant? i'm 23 weeks

It can: It certainly can, but it is one of the less likely antibiotics to do so when compared to drugs that kill a wider variety of bacteria such as Bactrim (sulfamethoxazole and trimethoprim). Taking a probiotic supplement or having yogurt with active cultures in your diet during your course of antibiotic may further reduce this risk slightly. During pregnancy, oral medications for yeast infection are not safe, but vaginal creams are ok.

Dr Jones agreed

Yes, Keflex can: The use of most antibiotics which are used to treat bacterial infections may result in the removal of bacterias from the vaginal environment and allow different types of fungus/yeasts to grow and trigger vaginal symptoms.